Part Two - Getting the most from your batteries

Part Two - Getting the most from your batteries
Part Two - Getting the most from your batteries
Prolonging
battery life
BU31 The secrets of battery runtime
BU32
BU33
BU34
BU35
BU36
Battery
applications
- Declining capacity
- Increasing internal resistance
- Elevated self-discharge
- Premature voltage cut-off
Non-correctable battery problems
- High self-discharge
- Low capacity cells
- Cell mismatch
- Shorted cells
- Loss of electrolyte
Memory: Myth or fact?
- How to restore and prolong nickel-based
batteries
- Field results on exercise and recondition
How to prolong lithium-based batteries
- Longevity of high-power lithium-ion
How to restore and prolong lead-based batteries
- The sealed lead-acid (SLA)
- Valve regulated lead-acid (VRLA)
Battery performance as a function of cycling
- Nickel-cadmium
- Nickel-metal-hydride
- Lithium-ion
- What is the best cycling pattern?
BU37 Choosing the right battery for wireless
communications
- What's the best battery for cell phones?
- Counterfeit cell phone batteries
BU38 - What's the best battery for two-way radios?
Choosing the right battery for portable
computing
BU39 - What's the best battery for laptops?
- How to calibrate the battery
Choosing the right battery for industrial
applications
- What's the best battery for video cameras?
BU40 - What's the best battery for still cameras?
- What is the best battery for medical devices?
- What is the best battery for power tools?
What's the best battery for wheeled & stationary
applications?
BU40a - What's the best battery for wheelchairs?
- What's the best battery for the electric bicycle?
- What's the best battery for the electric vehicle?
- What's the best battery for stationary
applications?
Are the Hybrid Cars here to stay?
-What's the best battery for the hybrid car?
-The plug-in hybrid electric vehicle (PHEV)
-The paradox of the hybrid vehicle
-Conclusion
Battery service
BU41 Rapid testing portable batteries
- The load test
- The AC conductance test
- The Cadex QuickTest™
BU42 Rapid testing automotive and stationary
batteries
- Electrochemical Impedance Spectroscopy (EIS)
BU42A - Commercializing Electrochemical Impedance
Spectroscopy
Why do different test methods provide dissimilar
readings?
BU42B - Battery rapid test methods and how they work
- Capacity measurements
- An important need fulfilled
What causes car batteries to fail?
BU42C - Acid stratification, a problem with luxury cars
- The challenge of battery testing
- The tough choice
Starting is easy… but can I steer and brake?
- What is the difference between CCA and RC?
- Capacity measurements, the most
comprehensive battery test
- Early test results on Reserve Capacity
- Summary
BU43 Advanced battery analyzers
- Fixed current analyzers
- Programmable analyzers
- Battery adapters
- Service programs
- Printing
BU44 Computerized battery testing
- Cellular dealers
- Cellular service Centers
- Battery fleet owners
- Manufacturers and pack assemblers
BU45 How to service two-way radio batteries
- The 'green light' lies
- Maintenance of fleet batteries
BU46 How to service cell phone batteries
- Refurbishing, a cost-effective exercise
- Storefront battery service
- Why was storefront service not done earlier?
BU47 How to service laptop batteries
- Battery connection
- Repairing a 'smart' battery
BU48 Increasing battery power by zapping
Battery behavior BU49 Observing batteries in everyday lives
- The personal battery user
- The fleet battery user
- What lack of battery maintenance can do
Comparisons
BU50 Will secondary batteries replace primaries?
- Capacity rating of alkaline cells
- Run-time estimation
- The switch to secondary batteries
BU51 The Cost of Battery Power
- The primary battery
- The secondary battery
- The combustion engine
- The fuel cell
BU52 The fuel cell
- Type of fuel cells
- Applications
- Limitations
BU52A The miniature fuel cell
- Will the fuel cell replace the battery?
BU53 Comparing battery power
- Battery power and the Boeing 747 jumbo jet
- How are newer battery chemistries faring?
Things to come
BU54 The future battery
- What is the ultimate miracle battery?
- Will the fuel cell replace the battery
BU55 Battery Statistics
- Where will commercial batteries come from?
- Up-to-date statistical battery information
The secrets of battery runtime (BU31)
Is the runtime of a portable device directly related to the size of the
battery? The answer should be 'yes' but in reality, the runtime is
governed by other attributes than the specified capacity alone.
This paper examines the cause of unexpected downtime and short
battery service life. We look at four renegades - declining capacity,
increasing internal resistance, elevated self-discharge and premature
voltage cut-off on discharge. We evaluate how these regenerative
deficiencies affect nickel, lead and lithium-based batteries.
Declining capacity
The amount of charge a battery can hold gradually decreases due to
usage and aging. Specified to deliver 100% capacity when new, the
battery should be replaced when the capacity drops to below 80% of the
nominal rating. Some organizations may use different end-capacities as a
minimal acceptable performance threshold.
The energy storage of a battery can be divided into three imaginary
sections consisting of: available energy, the empty zone that can be
refilled, and the unusable part (rock content) that increases with aging.
Figure 1 illustrates these three sections.
Figure 1: Battery charge
capacity.Three imaginary
sections of a battery
consisting of available
energy, empty zone and
rock content.
In nickel-based batteries, the so-called rock content is present in form of
crystalline formation, also known as memory. Restoration is possible
with a full discharge to one volt per cell. However, if no service is done
for four months and longer, a full repair becomes increasingly more
difficult the longer service is withheld. To prevent memory, nickel-based
batteries should be deep-cycled once every one or two months. Nickelcadmium and nickel-metal-hydride batteries are used for two-way
radios, medical instruments and power tools.
Performance degradation of the lead-acid battery is caused by sulfation
and grid corrosion. Sulfation is a thin layer that forms on the negative
cell plate if the battery is being denied a fully saturated charge. Sulfation
can, in part, be corrected with cycling and/or topping charge. The grid
corrosion, which occurs on the positive plate, is caused by over-charge.
Lead-acid batteries are used for larger portable devices and wheeled
applications.
Lithium-ion batteries lose capacity through cell oxidation, a process that
occurs naturally during use and aging. The typical life span of lithium-ion
is 2-3 years under normal use. Cool storage a 40% charge minimizes
aging. An aged lithium-ion cannot be restored with cycling. Lithium-ion is
found in cell phones and mobile computing.
Increasing internal Resistance
The capacity of a battery defines the stored energy - the internal
resistance governs how much energy can be delivered at any given time.
While a good battery is able to provide high current on demand, the
voltage of a battery with elevated resistance collapses under a heavy
load. Although the battery may hold sufficient capacity, the resulting
voltage drop triggers the 'low battery' indicator and the equipment stops
functioning. Heating the battery will momentarily increase the output by
lowering the resistance.
A battery with high internal resistance may still perform adequately on a
low current appliance such as a flashlight, portable CD player or wall
clock. Digital equipment, on the other hand, draw heavy current bursts.
Figure 2 simulates low and high internal resistance with a free-flowing
and restricted tap.
Figure 2: Effects of
internal battery
resistance.A battery with
low internal resistance is
able to provide high
current on demand. With
elevated resistance, the
battery voltage collapses
and the equipment cuts
off.
Nickel-cadmium offers very low internal resistance and delivers high
current on demand. In comparison, nickel-metal-hydride starts with a
slightly higher resistance and the readings increase rapidly after 300 to
400 cycles.
Lithium-ion has a slightly higher internal resistance than nickel-based
batteries. The cobalt system tends to increase the internal resistance as
part of aging whereas the manganese (spinel) maintains the resistance
throughout its life but loses capacity through chemical reaction. Cobalt
and manganese are used for the positive electrodes.
High internal resistance will eventually render the battery useless. The
energy may still be present but can no longer be delivered. This condition
is permanent and cannot be reversed with cycling. Cool storage at a
partial state-of-charged (40%) retards the aging process.
The internal resistance of Lead-acid batteries is very low. The battery
responds well to short current bursts but has difficulty providing a high,
sustained load. Over time, the internal resistance increases through
sulfation and grid corrosion.
Elevated self-discharge
All batteries suffer from self-discharge, of which nickel-based batteries
are among the highest. The loss is asymptotical, meaning that the self-
discharge is highest right after charge and then levels off. nickel-based
batteries lose 10% to 15% of their capacity in the first 24 hours after
charge, then 10% to 15% per month afterwards. One of the best
batteries in terms of self-discharge is Lead-acid; it only self-discharges
5% per month. Unfortunately, this chemistry has the lowest energy
density and is ill suited for portable applications.
lithium-ion self-discharges about 5% in the first 24 hours and 1-2%
afterwards. Adding the protection circuit increases the discharge by
another 3% per month. The protection circuit assures that the voltage
and current on each cell does not exceed a safe limit. Figure 3 illustrates
a battery with high self-discharge.
Figure 3: Effects of high
load impedance.Selfdischarge increases with
age, high cycle count and
elevated temperature.
Discard a battery if the
self-discharge reaches
30% in 24 hours.
The self-discharge on all battery chemistries increase at higher
temperatures. Typically, the rate doubles with every 10°C (18°F). A
noticeable energy loss occurs if a battery is left in a hot vehicle.
Aging and usage also affect self-discharge. nickel-metal-hydride is good
for 300-400 cycles, whereas nickel-cadmium may last over 1000 cycles
before high self-discharge affects the performance. An older nickel-based
battery may lose its energy during the day through self-discharge rather
than actual use. Discard a battery if the self-discharge reaches 30% in 24
hours.
Nothing can be done to reverse this deficiency. Factors that accelerate
self-discharge are damaged separators induced by crystalline formation,
allowing the packs to cook while charging, and high cycle count, which
promotes swelling in the cell. Lead and lithium-based batteries do not
increase the self-discharge with use in the same manner as their nickelbased cousins do.
Premature voltage cut-off
Not all stored battery power can be fully utilized. Some equipment cuts
off before the designated end-of-discharge voltage is reached and
precious battery energy remains unused. Applications demanding high
current bursts push the battery voltage to an early cut-off. This is
especially visible on batteries with elevated internal resistance. The
voltage recovers when the load is removed and the battery appears
normal. Discharging such a battery on a moderate load with a battery
analyzer to the respective end-of-discharge threshold will sometimes
produce residual capacity readings of 30% and higher, jet the battery is
inoperable in the equipment. Figure 4 illustrates high cut-off voltage.
Figure 4: Illustration of
equipment with high cut-off
voltage.Some portable
devices do not utilize all
available battery power and
leave precious energy behind.
High internal battery resistance and the equipment itself are not the only
cause of premature voltage cut-off - warm temperature also plays a role
by lowering the battery voltage. Other reasons are shorted cells in a
multi-cell battery pack and memory on nickel-based batteries.
Non-Correctable Battery Problems (BU32)
Some rechargeable batteries can be restored through external means,
such as applying a full discharge. There are, however, many defects that
cannot be corrected. These include high internal resistance, elevated
self-discharge, electrical short, dry-out, plate corrosion and general
chemical breakdown.
The performance loss of a battery occurs naturally as part of usage and
aging; some is hastened by lack of maintenance, harsh field conditions
and poor charging practices. This paper examines the cause of noncorrectable battery problems and explores ways to minimize these
breakdowns.
High Self-discharge
All batteries are affected by self-discharge. This is not a defect per se,
although improper use enhances the condition. Self-discharge is
asymptotical; the highest loss occurs right after charge, and then tapers
off.
Nickel-based batteries exhibit a relatively high self-discharge. At ambient
temperature, a new nickel-cadmium loses about 10% of its capacity in
the first 24 hours after charge. The self-discharge settles to about 10%
per month afterwards. Higher temperature increases the self-discharge
substantially. As a general guideline, the rate of self-discharge doubles
with every 10°C (18°F) increase in temperature. The self-discharge of
nickel-metal-hydride is about 30% higher than that of nickel-cadmium.
The self-discharge increases after a nickel-based battery has been cycled
for a few hundred times. The battery plates begin to swell and press
more firmly against the separator. Metallic dendrites, which are the
result of crystalline formation (memory), also increase the self-discharge
by marring the separator. Discard a nickel-based battery if the selfdischarge reaches 30% in 24 hours
The self-discharge of the lithium-ion battery is 5% in the first 24 hours
after charge, and then reduces to 1% to 2% per month thereafter. The
safety circuit adds about 3%. High cycle count and aging have little
effect on the self-discharge of lithium-based batteries.
A lead-acid battery self-discharges at only 5% per month or 50% per
year. Repeated deep cycling increases self-discharge.
The percentage of self-discharge can be measured with a battery
analyzer but the procedure takes several hours. Elevated internal battery
resistance often reflects in higher internal battery resistance, a
parameter that can be measured with an impedance meter or the
OhmTest program of the Cadex battery analyzers.
Cell matching
Even with modern manufacturing techniques, the cell capacities cannot
be accurately predicted, especially with nickel-based cells. As part of
manufacturing, each cell is measured and segregated into categories
according to their inherent capacity levels. The high capacity 'A' cells are
commonly sold for special applications at premium prices; the mid-range
'B' cells are used for commercial and industrial applications; and the low-
end 'C' cells are sold at bargain prices. Cycling will not significantly
improve the capacity of the low-end cells. When purchasing rechargeable
batteries at a reduced price, the buyer should be prepared to accept
lower capacity levels.
The cells in a pack should be matched within +/- 2.5%. Tighter
tolerances are required on batteries with high cell count, those delivering
high load currents and packs operating at cold temperatures. If only
slightly off, the cells in a new pack will adapt to each other after a few
charge/discharge cycles. There is a correlation between well-balanced
cells and battery longevity.
Why is cell matching so important? A weak cell holds less capacity and is
discharged more quickly than the strong one. This imbalance may cause
cell reversal on the weak cell if discharged too low. On charge, the weak
cell is ready first and goes into heat-generating overcharge while the
stronger cell still accepts charge and remains cool. In both cases, the
weak cell is at a disadvantage, making it even weaker and contributing
to a more acute cell mismatch.
Quality cells are more consistent in capacity and age more evenly than
the lower quality counterparts. Manufacturers of high-end power tools
choose high quality cells because of durability under heavy load and
temperature extremes. The extra cost pays back on longer lasting packs.
lithium-based cells are by nature closely matched when they come off
the manufacturing line. Tight tolerances are important because all cells
in a pack must reach the full-charge and end-of-discharge voltage
thresholds at a unified time. A built-in protection circuit safeguards
against cells that do not follow a normal voltage pattern.
Shorted Cells
Manufacturers are often unable to explain why some cells develop high
electrical leakage or an electrical short while still relatively new. The
suspected culprit is foreign particles that contaminate the cells during
manufacturing. Another possible cause is rough spots on the plates that
damage the separator. Better manufacturing processes have reduced the
'infant mortality' rate significantly.
Cell reversal caused by deep discharging also contributes to shorted
cells. This may occur if a nickel-based battery is being fully depleted
under a heavy load. nickel-cadmium is designed with some reverse
voltage protection. A high reverse current, however, will produce a
permanent electrical short. Another contributor is marring of the
separator through uncontrolled crystalline formation, also known as
memory.
Applying momentary high-current bursts in an attempt to repair shorted
cells offers limited success. The short may temporarily evaporate but the
damage to the separator material remains. The repaired cell often
exhibits a high self-discharge and the short frequently returns. Replacing
a shorted cell in an aging pack is not recommended unless the new cell is
matched with the others in terms of voltage and capacity.
Loss of Electrolyte
Although sealed, the cells may lose some electrolyte during their life,
especially if venting occurs due to excessive pressure during careless
charging. Once venting has occurred, the spring-loaded vent seal on
nickel-based cells may never properly close again, resulting in a build-up
of white powder around the seal opening. The loss of electrolyte will
eventually lower the battery capacity.
Permeation, or loss of electrolyte in valve regulated lead-acid batteries
(VRLA) is a recurring problem. Overcharging and operating at high
temperatures are the causes. Replenishing lost liquid by adding water
offers limited success. Although some capacity may be regained, the
performance becomes unreliable.
If correctly charged, lithium-ion cell should never generate gases and
cause venting. But in spite of what is said, the lithium-based cells can
build up internal pressure under certain conditions. Some cells include an
electrical switch that disconnects the current flow if the cell pressure
reaches a critical level. Other cells rupture a membrane to release the
gases in a controlled way. lithium-ion-polymer in a pouch cell sometime
grows to the shape of a small balloon because these cells do not include
venting. Ballooning cell are known to damage the housing of the portable
device.
Figure 1: lithium-ion-polymer
cell in a pouch pack. Made
ultra-slim, some cells generate
hydrogen gas during charge
and puff up. The force can
damage the housing of the
portable device
Memory: myth or fact? (BU33)
The word 'memory' was originally derived from 'cyclic memory'; meaning
that a nickel-cadmium battery could remember how much energy was
drawn on preceding discharges. On a longer than scheduled discharge,
the voltage would rapidly drop and the battery would lose power.
Improvements in battery technology have virtually eliminated this
phenomenon.
The problem with nickel-cadmium is not so much the cyclic memory but
the effects of crystalline formation. The active cadmium material is
present in finely divided crystals. In a good cell, these crystals remain
small, obtaining maximum surface area. With memory, the crystals grow
and conceal the active material from the electrolyte. In advanced stages,
the sharp edges of the crystals penetrate the separator, causing high
self-discharge or electrical short.
When introduced in the early 1990s, nickel-metal-hydride was promoted
as being memory-free. Today, we know that this chemistry is also
affected but to a lesser degree than nickel-cadmium. The nickel plate, a
metal that is shared by both chemistries, is partly to blame. While nickelmetal-hydride has only the nickel plate to worry about, nickel-cadmium
also includes the memory-prone cadmium plate. This is a non-scientific
explanation why nickel-cadmium is affected more than nickel-metalhydride.
The stages of crystalline formation of a nickel-cadmium cell are
illustrated in Figure 1. The enlargements show the cadmium plate in a
proper functioning crystal structure, crystalline formation after use (or
abuse) and restoration.
New nickel-cadmium cell. The anode is in
fresh condition. Hexagonal cadmium
hydroxide crystals are about 1 micron in
cross section, exposing large surface area to
the electrolyte for maximum performance.
Cell with crystalline formation. Crystals have
grown to 50 to 100 microns in cross section,
concealing large portions of the active
material from the electrolyte. Jagged edges
and sharp corners may pierce the separator,
leading to increased self-discharge or
electrical short.
Restored cell. After pulsed charge, the
crystals are reduced to 3 to 5 microns, an
almost 100% restoration. Exercise or
recondition are needed if the pulse charge
alone is not effective.
Figure 1: Crystalline formation on nickel-cadmium cell.
Illustration courtesy of the US Army Electronics Command in Fort
Monmouth, NJ, USA.
How to restore and prolong nickel-based batteries
Crystalline formation is most pronounced if a nickel-based battery is left
in the charger for days, or if repeatedly recharged without a periodic full
discharge. Since most applications do not use all energy before recharge,
a periodic discharge to 1 volt per cell (known as exercise) is essential to
prevent memory.
Nickel-cadmium in regular use and on standby mode (sitting in a charger
for operational readiness) should be exercised once per month. Between
these monthly exercise cycles, no further service is needed. No scientific
research is available on the optimal exercise requirements of nickelmetal-hydride. Based on the reduced crystalline buildup, applying a full
discharge once every three months appears right. Because of the shorter
cycle life compared to nickel-cadmium, over-exercising is not
recommended.
Exercise and Recondition - Research has shown that the crystals ingrain
themselves if no exercise is applied to nickel-cadmium for three months
or more. A full restoration with exercise becomes more difficult the
longer service is withheld. In advanced cases 'recondition' is required.
Recondition is a slow, secondary discharge applied below the 1 volt/cell
threshold. During this process, the current must be kept low to minimize
cell reversal. Nickel-cadmium can tolerate a small amount of cell reversal
but caution must be applied to stay within the allowable current limit.
Tests performed by the US Army have shown that a nickel-cadmium cell
needs to be discharged to at least 0.6V to effectively break up the more
resistant crystalline formation. Figure 2 illustrates the battery voltage
during a discharge to 1V/cell, followed by the secondary discharge to
0.4V/cell.
Figure 2:
Exercise and
recondition
features of a
Cadex battery
analyzer.If a
nickel-cadmium
battery has not
been exercised
for three
months or
longer,
recondition is
required to
restore
capacity.
Recondition is a
slow, deep
discharge to
0.4V/cell. If
service is
denied for 6 to
12 months,
recondition
becomes
ineffective.
Figure 3 illustrates the effects of exercise and recondition. Four nickelcadmium batteries afflicted with various degrees of memory are
serviced. The batteries are first fully charged, then discharged to 1V/cell.
The resulting capacities are plotted on a capacity scale of 0 to 120% in
the first column. Additional discharge/charge cycles are applied and the
battery capacities are plotted in the subsequent columns. The green line
represents 'exercise', and the blue line 'recondition'. The exercise and
recondition cycles are applied manually at the discretion of the research
technician.
Figure 3: Effects of exercise and recondition.Four batteries afflicted with
memory are serviced. Battery 'A' improved capacity on exercise alone;
batteries 'B' and 'C' required recondition. The new battery improved
further with recondition.
Battery 'A' responded well to exercise alone and no recondition was
required. This battery may have been in service for only a few months or
has received periodic exercise cycles. Batteries 'B' and 'C' required
recondition to restore performance. Without recondition, these two
batteries would have been discarded.
After service, the restored batteries were returned to full use. When
examined after six months of field service, no noticeable degradation in
the performance was visible. The regained capacity was permanent but
periodic service will be needed to maintain the performance.
Applying the recondition cycle on a new battery (top line on chart)
resulted in a slight capacity gain. This increase is not fully understood,
other than to assume that the battery improved by additional forming.
Another explanation is early presence of memory. Since new batteries
are stored with some charge, the self-discharge that occurs during
storage produces some crystalline formation. Exercising and
reconditioning reverse this effect.
Recondition has its limitations. If no exercise had been applied for 6 to
12 months, permanent damage may have been inflicted. The capacity
may not recover or the pack may suffer from high self-discharge caused
by a marred separator. Older batteries may get worse with recondition.
These packs can be compared to an old man to whom strenuous activity
is harmful. Such batteries must be replaced.
Typically 50%-70% of discarded nickel-cadmium batteries can be
restored when using the exercise and recondition methods of a Cadex
battery analyzer or equivalent. The recovery rate of nickel-metal-hydride
is about 40%. This lower yield is, in part, due to the battery's low cycle
count.
Field results on exercise and recondition
After the Balkan War, the Dutch Army examined how many field batteries
could be restored with a battery analyzer (Cadex). The army was aware
that the packs were used under less than ideal conditions. They had been
sitting in the chargers with only 2-3 hours use per day.
The capacity on some packs had dropped from 100% to 30%. With the
analyzer's recondition function, 9 of 10 batteries were restored to 80%
and higher. The nickel-cadmium batteries were 2-3 years old.
The importance of exercising and reconditioning is emphasized by
another study carried out for the US Navy by GTE Government Systems.
To determine the percentage of batteries needing replacement in the
first year of use, one group of batteries received charge only (no
maintenance), another group was periodically exercised and a third
group received recondition. The batteries studied were used for two-way
radios on US aircraft carriers.
With charge only (charge-and-use), the annual percentage of battery
failure was 45% (Figure 4). With exercise, the failure rate was reduced
to 15%. By far the best results were achieved with recondition. The
failure rate dropped to 5%.
Maintenance
method
Annual % of
batteries
requiring
replacement
Annual battery
cost (US$)
Charge-anduse only
45%
$40,500
Exercise
14%
$13,500
Recondition
5%
$4,500
Figure 4: Replacement rates
of nickel-cadmium batteries.
Exercise and recondition
prolong battery life by three
and nine respectively.
The GTE report concluded that a battery analyzer featuring exercise and
recondition functions costing $2,500US would return its investment in
less than one month on battery savings alone.
Simple Guidelines
•
Do not leave a nickel-based battery in a charger for more than a
few days, even on trickle charge.
•
•
•
Exercise nickel-cadmium every 1 to 2 months and nickel-metalhydride every 3 months. Running the battery down in the
equipment may do this also.
Do not discharge the battery before each recharge. This puts
undue stress on the battery.
Avoid getting the battery too hot during charge. The temperature
should only rise for a short moment at full charge, then cool off.
How to prolong lithium-based batteries (BU34)
Battery research is focusing heavily on lithium chemistries, so much so
that one could presume that all portable devices will be powered with
lithium-ion batteries in the future. In many ways, lithium-ion is superior
to nickel and lead-based chemistries and the applications for lithium-ion
batteries are growing as a result.
Lithium-ion has not yet fully matured and is being improved
continuously. New metal and chemical combinations are being tried
every six months to increase energy density and prolong service life. The
improvements in longevity after each change will not be known for a few
years.
A lithium-ion battery provides 300-500 discharge/charge cycles. The
battery prefers a partial rather than a full discharge. Frequent full
discharges should be avoided when possible. Instead, charge the battery
more often or use a larger battery. There is no concern of memory when
applying unscheduled charges.
Although lithium-ion is memory-free in terms of performance
deterioration, batteries with fuel gauges exhibit what engineers refer to
as "digital memory". Here is the reason: Short discharges with
subsequent recharges do not provide the periodic calibration needed to
synchronize the fuel gauge with the battery's state-of-charge. A
deliberate full discharge and recharge every 30 charges corrects this
problem. Letting the battery run down to the cut-off point in the
equipment will do this. If ignored, the fuel gauge will become
increasingly less accurate. (Read more in 'Choosing the right battery for
portable computing', Part Two.)
Aging of lithium-ion is an issue that is often ignored. A lithium-ion
battery in use typically lasts between 2-3 years. The capacity loss
manifests itself in increased internal resistance caused by oxidation.
Eventually, the cell resistance reaches a point where the pack can no
longer deliver the stored energy although the battery may still have
ample charge. For this reason, an aged battery can be kept longer in
applications that draw low current as opposed to a function that
demands heavy loads. Increasing internal resistance with cycle life and
age is typical for cobalt-based lithium-ion, a system that is used for cell
phones, cameras and laptops because of high energy density. The lower
energy dense manganese-based lithium-ion, also known as spinel,
maintains the internal resistance through its life but loses capacity due
to chemical decompositions. Spinel is primarily used for power tools.
The speed by which lithium-ion ages is governed by temperature and
state-of-charge. Figure 1 illustrates the capacity loss as a function of
these two parameters.
Figure 1: Permanent capacity loss of lithium-ion as a function of
temperature and charge level.
High charge levels and elevated temperatures hasten permanent capacity
loss. Improvements in chemistry have increased the storage
performance of lithium-ion batteries.
The mentioning of limited service life on lithium-ion has caused concern
in the battery industry and I will need to add some clarifications. Let me
explain:
If someone asks how long we humans live, we would soon find out that
the longevity varies according to life style and living conditions that exist
in different countries. Similar conditions exist with the batteries, lithiumion in particular. Since BatteryUniversity bases its information on the
feedback from users as opposed to scientific information derived from a
research lab, longevity results may differ from manufacturer'
specifications. Let's briefly look at the various living conditions of the
lithium-ion battery.
The worst condition is keeping a fully charged battery at elevated
temperatures, which is the case with running laptop batteries. If used on
main power, the battery inside a laptop will only last for 12-18 months. I
must hasten to explain that the pack does not die suddenly but begins
with reduced run-times.
The voltage level to which the cells are charged also plays an important
role to longevity. For safety reasons, most lithium-ion cannot exceed
4.20 volts per cell. While a higher voltage boosts capacity, the
disadvantage is lower cycle life. Figure 2 shows the cycle life as a
function of charge voltage.
Figure 2: Effects on
cycle life at different
float charge levels
(Choi et al., 2002)
Higher charge
voltages boost
capacity but lower
cycle life.
There are no remedies to restore lithium-ion once worn out. A
momentary improvement in performance is noticeable when heating up
the battery. This lowers the internal resistance momentarily but the
condition reverts back to its former state when the temperature drops.
Cold temperature will increase the internal resistance.
If possible, store the battery in a cool place at about a 40% state-ofcharge. Some reserve charge is needed to keep the battery and its
protection circuit operational during prolonged storage. Avoid keeping
the battery at full charge and high temperature. This is the case when
placing a cell phone or spare battery in a hot car. Running a laptop
computer on the mains has a similar temperature problem. While the
battery is kept fully charged, the inside temperature during operation
rises to 45°C (113°F).
Removing the battery from the laptop when running on fixed power
protects the battery from heat. With the concern of the battery
overheating and causing fire, a spokesperson for the U.S. Consumer
Product Safety Commission advises to eject the battery of affected
laptops and to run the machines on a power cord. It should be noted that
on a power outage, unsaved works will be lost.
The question is often asked, should the laptop be disconnected from the
main when not in use? Under normal circumstances, it should not matter
with lithium-ion. Once the battery is fully charged, no further charge is
applied. However, there is always the concern is malfunction of the AC
adapter, the laptop or the battery.
A large number of lithium-ion batteries for cell phones are being
discarded under the warranty return policy. Some failed batteries are
sent to service centers or the manufacturer, where they are refurbished.
Studies show that 80%-90% of the returned batteries can be repaired
and returned to service.
Some lithium-ion batteries fail due to excessive low discharge. If
discharged below 2.5 volts per cell, the internal safety circuit opens and
the battery appears dead. A charge with the original charger is no longer
possible. Some battery analyzers (Cadex) feature a boost function that
reactivates the protection circuit of a failed battery and enables a
recharge. However, if the cell voltage has fallen below 1.5V/cell and has
remained in that state for a few months, a recharge should be avoided
because of safety concerns. To prevent failure, never store the battery
fully discharged. Apply some charge before storage, and then charge
fully before use.
All personal computers (and some other electronic devices) contain a
battery for memory back up. This battery is commonly a small nonrechargeable lithium cell, which provides a small current when the device
is turned off. The PC uses the battery to retain certain information when
the power is off. These are the BIOS settings, current date and time, as
well as resource assignment for Plug and Play systems. Storage does
shorten the service life of the backup battery to a few years. Some say 12 years. By keeping the computer connected to the main, albeit turned
off, a battery on the PC motherboards should be good for 5-7 years. A PC
should give the advanced warning when battery gets low. A dead backup battery will wipe out the volatile memory and erase certain settings.
After battery is replaced, the PC should again be operational.
Longevity of high-power lithium-ion
Generally speaking, batteries live longer if treated in a gentle manner.
High charge voltages, excessive charge rate and extreme load conditions
will have a negative effect and shorten the battery life. This also applies
to high current rate lithium-ion batteries.
Not only is it better to charge lithium-ion battery at a slower charge rate,
high discharge rates also contribute the extra wear and tear. Figure 3
shows the cycle life as a function of charge and discharge rates. Observe
the good laboratory performance if the battery is charged and discharged
at 1C. (A 0.5C charge and discharge would further improve this rating.)
Figure 3:
Longevity
of
lithiumion as a
function
of charge
and
discharge
rates.
A
moderate
charge
and
discharge
puts less
stress on
the
battery,
resulting
in a
longer
cycle life.
Battery experts agree that the life of lithium-ion depends on other
factors than charge and discharge rates. Even though incremental
improvements can be achieved with careful use of the battery, our
environment and the services required are not always conducive to
achieve optimal battery life. The longevity of a battery is often a direct
result of the environmental stresses applied.
Simple Guidelines
•
Avoid frequent full discharges because this puts additional strain
on the battery. Several partial discharges with frequent recharges
are better for lithium-ion than one deep one. Recharging a
partially charged lithium-ion does not cause harm because there is
no memory. (In this respect, lithium-ion differs from nickel-based
batteries.) Short battery life in a laptop is mainly cause by heat
rather than charge / discharge patterns.
•
Batteries with fuel gauge (laptops) should be calibrated by
applying a deliberate full discharge once every 30 charges.
Running the pack down in the equipment does this. If ignored, the
fuel gauge will become increasingly less accurate and in some
cases cut off the device prematurely.
•
Keep the lithium-ion battery cool. Avoid a hot car. For prolonged
storage, keep the battery at a 40% charge level.
•
Consider removing the battery from a laptop when running on
fixed power. (Some laptop manufacturers are concerned about
dust and moisture accumulating inside the battery casing.)
•
Avoid purchasing spare lithium-ion batteries for later use. Observe
manufacturing dates. Do not buy old stock, even if sold at
clearance prices.
•
If you have a spare lithium-ion battery, use one to the fullest and
keep the other cool by placing it in the refrigerator. Do not freeze
the battery. For best results, store the battery at 40% state-ofcharge.
How to restore and prolong lead-acid batteries (BU35)
The sealed lead-acid battery is designed with a low over-voltage
potential to prohibit the battery from reaching its gas-generating state
during charge. This prevents water depletion of the sealed system.
Consequently, these batteries will never get fully charged and some
sulfation will develop over time.
Finding the ideal charge voltage threshold is critical and any level is a
compromise. A voltage limit above 2.40 volts per cell produces good
battery performance but shortens the service life due to grid corrosion
on the positive plate. The corrosion is permanent. A voltage below the
2.40V/cell threshold strains the battery less but the capacity is low and
sulfation sets in over time on the negative plate.
Driven by diverse applications, two sealed lead-acid types have emerged.
They are the sealed lead acid (SLA), and the valve regulated lead acid
(VRLA). Technically, both batteries are the same. Engineers may argue
that the word 'sealed lead acid' is a misnomer because no lead acid
battery can be totally sealed.
The SLA has a typical capacity range of 0.2Ah to 30Ah and powers
personal UPS units, local emergency lighting and wheelchairs. The VRLA
battery is used for large stationary applications for power backup. We
are looking at methods to restore and prolong these two battery systems
separately.
The sealed lead-acid (SLA)
SLA batteries with mild sulfation can be restored but the work is time
consuming and the results are mixed. Reasonably good results are
achieved by applying a charge on top of a charge. This is done by fully
charging an SLA battery, then removing it for a 24 to 48 hour rest period
and applying a charge again The process is repeated several times and
the capacity is checked with a final full discharge and recharge.
Another method of improving performance is by applying an equalizing
charge, in which the charge voltage threshold is increased by about
100mV, typically from 2.40V to 2.50V. This procedure should last no
longer than one to two hours and must be carried out at moderate room
temperature. A careless equalize charge could cause the cells to heat up
and induce venting due to excessive pressure. Observe the battery
during the service.
The cylindrical SLA, made by Hawker and sold under the Cyclone name,
requires slightly higher voltages to reverse sulfation. An adjustable
power supply works best for the service. Set the current limit to the
lowest practical setting and observe the battery voltage and temperature
during charge. Initially, the cell voltage may rise to 5V, absorbing only a
small amount of current. In about two hours, the small charging current
converts the large sulfate crystals back into active material. The internal
cell resistance decreases and the cell starts to clamp the voltage. At
around 2.30V, the cell accepts charge. If the sulfation is advanced, this
remedy does not work and the cell needs replacing.
Sealed lead-acid batteries are commonly rated at a 20-hour discharge.
Even at such a slow rate, a capacity of 100% is difficult to achieve. For
practical reasons, most battery analyzers use a 5-hour discharge when
servicing these batteries. This produces 80% to 90% of the rated
capacity. SLA batteries are normally overrated and manufacturers are
aware of this practice.
Cycling an SLA on a battery analyzer may provide capacity readings that
decrease with each additional cycle. A battery may start off at a marginal
88%, then go to 86%, 84% and 83%. This phenomenon can be corrected
by increasing the charge voltage threshold from 2.40V to 2.45V and
perhaps even 2.50V. Always consider the manufacturer's recommended
settings. Cyclone batteries require slightly higher voltage settings than
the plastic version.
Avoid setting the charge voltage threshold too high. In an extreme case,
the limiting voltage may never be reached, especially when charging at
elevated temperatures. The battery continues charging at full current
and the pack gets hot. Heat lowers the battery voltage and works against
a further voltage raise. If no temperature sensing is available to
terminate the charge, a thermal runaway can be the result.
The recovery rate of SLA batteries is a low 15%. Other than reverse
sulfation, there is little one can do to improve SLA. Because the SLA has a
relatively short cycle life, many fail due to wear-out.
Valve regulated lead-acid (VRLA)
The charge voltage setting on VRLA is generally lower than SLA. Heat is a
killer of VRLA. Many stationary batteries are kept in shelters with no air
conditioning. Every 8°C (15°F) rise in temperature cuts the battery life in
half. A VRLA battery, which would last for 10 years at 25°C (77°F), will
only be good for 5 years if operated at 33°C (95°F). Once damaged by
heat, no remedy exists to improve capacity.
The cell voltages of a VRLA battery must be harmonized as close as
possible. Applying an equalizing charge every 6 months brings all cells to
similar voltage levels. This is done by increasing the cell voltage to
2.50V/cell for about 2 hours. During the service, the battery must be
kept cool and careful observation is needed. Limit cell venting. Most
VRLA vent at 0.3 Bar (5 psi). Not only does escaping hydrogen deplete
the electrolyte, it is highly flammable.
Water permeation, or loss of electrolyte, is a concern with sealed lead
acid batteries. Adding water may help to restore capacity but a long-term
fix is uncertain. The battery becomes unreliable and requires high
maintenance.
Simple Guidelines
•
•
•
•
Always store lead-acid in a charged condition. Never allow the
open cell voltage to drop much below 2.10V. Apply a topping
charge every six months or when recommended.
Avoid repeated deep discharges. Charge more often. Use a larger
battery to reduce the depth of discharge.
Prevent sulfation and grid corrosion by choosing the correct
charge and float voltages. If possible, allow a fully saturated
charge of 14h.
To reverse sulfation, raise the charge voltage above 2.40V/cell for
a few hours.
•
Avoid operating lead-acid at elevated ambient temperatures.
Note: Wheelchair batteries don't last as long as golf cart batteries
because of sulfation. The theory goes that a golf cart battery gets
a full 14 hours charge whereas a wheelchair only gets 7 hours
while the user sleeps.
Battery performance as a function of cycling (BU36)
As part of ongoing research to find the most durable battery system,
Cadex has performed life cycle tests on nickel-cadmium, nickel-metalhydride and lithium-ion batteries. All tests were carried out on the Cadex
7000 Series battery analyzers in the test labs of Cadex, Vancouver,
Canada. The batteries tested received an initial full-charge, and then
underwent a regime of continued discharge/charge cycles. The internal
resistance was measured with Cadex's OhmTest™ method, and the selfdischarge was obtained from time-to-time by reading the capacity loss
incurred during a 48-hour rest period. The test program involved 53 cell
phone batteries, of which one per chemistry was chosen for the charts
below.
Nickel-cadmium
In terms of life cycling, standard nickel Cadmium is the most enduring
battery. Figure 1 illustrates the capacity, internal resistance and selfdischarge of a 7.2V, 900mA nickel-cadmium battery with standard cells.
Due to time constraints, the test was terminated after 2300 cycles. The
capacity remained steady, the internal resistance stayed flat at 75mW
and the self-discharge was stable. This battery receives a grade 'A' for
almost perfect performance. It should be noted that nickel-cadmium has
a moderate energy density, requires periodic full discharges and contains
toxic metals.
Figure 1: Cycle
performance of standard
nickel-cadmium.
7.2V, 900mAh This
battery deserves an 'A'
for almost perfect
performance in terms of
stable capacity, internal
resistance and selfdischarge over many
cycles.
The ultra-high capacity nickel-cadmium offers up to 60% higher in
energy density compared to the standard version at the expense of
reduced cycle life. In Figure 2, we observe a steady drop of capacity
during the 2000 cycles delivered. At the same time, the internal
resistance rises slightly. A more serious degradation is the increase of
self-discharge after 1000 cycles.
Figure 2: Cycle
performance of ultra-high
nickel-cadmium.
6V, 700mAhThis battery
offers higher energy
density than the standard
version at the expense of
reduced cycle life.
Nickel-metal-hydride
Figure 3 examines nickel-metal-hydride. We observe good performance
at first but past 300-cycles, the readings starts to deteriorate rapidly.
One can observe the swift increase in internal resistance and selfdischarge after cycle count 700. nickel-metal hydride has a higher energy
density than nickel-cadmium and does not contain toxic metals. Some
argue that nickel-metal-hydride is an interim step to lithium-ion.
Figure 3: Cycle
performance of nickelmetal-hydride
6V, 950mAh.This battery
offers good performance
at first but past 300
cycles, the capacity,
internal resistance and
self-discharge start to
deteriorate rapidly.
Lithium-ion
In Figure 4 we examine the capacity and internal resistance of a lithiumion battery. A gentle and predictable capacity drop is observed over 1000
cycles and the internal resistance increases only slightly. Because of low
readings, self-discharge has been omitted on this test. lithium-ion offers
the highest energy density of the above-mentioned chemistries and
contains no toxic metals. Limited discharge current, the need for safety
circuits and aging are negative attributes of this battery.
Figure 4: Cycle
performance of lithiumion.
3.6V, 500mAlithium-ion
offers good capacity and
steady internal resistance
over 1000 cycles. Selfdischarge was omitted
because of low readings.
When conducting battery tests in a laboratory, it should be noted that
the performance in a protected environment is commonly superior to
that in field use. Elements of stress and inconsistency present in
everyday use cannot be simulated accurately in the lab. Here are some of
the reasons why:
Under a full cycle program, as conducted in this test, nickel-based
batteries are not affected by crystalline formation (memory). Memory
shortens battery life in everyday use if not properly maintained. Applying
a full discharge/charge cycle once a month solves this problem. Nickelcadmium is more prone to memory than nickel-metal-hydride.
Lithium-ion benefited from a controlled life cycle test because the aspect
of aging plays a less significant role. The service life of lithium-ion in real
life is a combination of cycle count and aging. All batteries are affected
by aging in various degrees.
Another reason why life the lab cycling produced very positive readings
is the controlled temperature environment in which the tests were
carried out. In true life, the batteries meet much harsher treatment and
are often exposed to heat. Furthermore, the batteries in our test were
charged with a well-defined charge algorithm. Overcharge was
minimized and damaging heat buildup prevented. Low-cost consumer
chargers do not always service the battery optimally.
The type of load with which the batteries are discharged also plays a
role. The above test consisted of an even DC discharge. Digital equipment
loads the battery with heavy current bursts. Tests have shown reduced
cycle life when a battery is discharged with sharp current pulses as
opposed to DC, even though the delivered end-energy is the same. Cell
phones, laptop computers digital cameras are devices that draw such
heavy current spikes.
In some other aspects, however, a lab test may be harder on the battery
than actual field use. In our test, each cycle applied a full discharge. The
nickel-based packs were drained to 1.0 volt and lithium-ion to 3.0 volts
per cell. In typical field use, the discharge before re-charge is normally
shallower. A partial discharge puts less strain on the battery, which
benefits lithium-ion and to some extent also nickel-metal-hydride.
Nickel-cadmium is least affected by delivering full cycles. Manufacturers
normally specify the cycle life of lithium-ion at an 80% depth-ofdischarge.
What is the best cycling pattern?
I often get asked by the readers, "how deep can a battery be discharged
and still achieve maximum service life?" There are no definite answers.
Batteries are like us humans. Suppose we ate all the vegetables our
mother heaped on our plates and do our daily exercise, would we life
longer? Perhaps. But by how much, no one will know. Batteries lose
capacity as part of aging, cycling and exposure to heat. Nickel-cadmium
also loses capacity due to lack of exercise because of memory.
To maximize service life, satellite batteries are kept at a cool
temperature and undergo a very shallow discharge of only 10% before
recharge. Nickel-based batteries in space also receive a periodic full
discharge. This regime allows ten of thousands of cycles. Closer to earth,
the ideal charge/discharge patterns cannot be scheduled; neither is the
temperature always perfect. As a result, a replacement will be required
sooner or later.
If possible, do not discharge lithium-based batteries too deeply. Instead,
recharge more often. Allow a nickel-cadmium battery to fully discharge
once every 30 cycles or so. This also applies to nickel-metal-hydride but
to a lesser extent. Exact data as to how often a nickel-based battery
should be discharged is not available. Neither do we know low long a
lithium-ion will last under different depth-of-discharge regimes.
Manufacturers typically specify lithium-ion at an 80% depth-ofdischarge.
Choosing the right battery for wireless communications
(BU37)
Research has brought about a variety of battery chemistries, each
offering distinct advantages but none providing a fully satisfactory
solution. With today's variety of battery types, better choices can be
made to suit specific user applications. This paper talks about the
recommended battery chemistry for cell phones and two-way radios in
terms of energy density, durability and price.
What's the best battery for cell phones?
Early cell phones were powered with nickel-based batteries but most
newer phones are now equipped with lithium-ion. This chemistry is
lightweight, offers high energy density and lasts long enough to span the
typical life of the product. Lithium-ion contains no toxic metals.
To obtain thin geometry, some cell phone manufacturers switched to
lithium-ion-polymer.
This satisfied consumer requests for slim designs. In the meantime,
technological advancements also made low profile lithium-ion possible.
lithium-ion packs are now available in 3 mm, a profile that suits most
designs. lithium-ion has the advantage of lower manufacturing cost,
better performance and longer cycle life than the polymer version.
Lithium-ion is a low maintenance battery. No periodic discharge is
needed and charging can be done at random. A random charge means
that the battery does not need to be fully depleted before recharge. In
fact, it is better to recharge before the battery gets too low. Full
discharges put an unnecessary strain on the battery. A recharge on a
partially charged battery does not cause memory because there is none.
Charging lithium-ion is simpler and cleaner than nickel-based batteries
but the chargers require tighter tolerances. lithium-ion cannot absorb
overcharge and no trickle charge is applied on full charge. This allows
lithium-ion to be kept in the chargers until used. Some chargers apply a
topping charge every week or so to replenish the capacity lost through
self-discharge while the battery sits idle in the charger. Repeated
insertion into the charger or cradle does not damage the battery though
overcharge. If the battery is full, no charge is applied. The battery
voltage determines the need to charge.
On the negative side, lithium?ion gradually loses charge acceptance as
part of aging, even if not used. lithium?ion batteries should not be stored
for long periods but be rotated like perishable food. The buyer should be
aware of the manufacturing date when purchasing a replacement
battery. Aging affects battery chemistries at different degrees.
Counterfeit cell phone batteries (clone batteries)
In the search for low-cost battery replacements, consumers may
inadvertently purchase clone cell phone batteries that do not include an
approved protection circuit. Lithium-ion packs require a protection circuit
to shut off the power source if the charger malfunctions and keep on
charging, or if the pack is put under undue stress (electrical short).
Overheating and 'venting with flame' can be the result of such strain.
(See photos of an exploded cell phone with clone battery on charge.)
Photos of a cell phone with a clone
battery
that exploded while left on charge in
a car
Cell phone manufacturers strongly advise customers to replace the
battery with an approved brand. Failing to do so may void the warranty.
Counterfeit cell phone batteries have become visible since the beginning
of 2003 when the world was being flooded with cheap replacement
batteries from Asia.
Cell phone manufacturers act out of genuine concern for safety rather
than using scare tactics to persuade customers to buy their own
accessories. They do not object to third party suppliers in offering
batteries and chargers as long as the products are well built, safe and
functioning. The buyer can often not distinguish between an original and
a counterfeit battery because the label may appear bona fide.
Caution should also be exercised in purchasing counterfeit chargers.
Some units do not terminate the battery correctly and rely on the
battery's internal protection circuit to cut off the power when fully
charged. Precise full-charge termination and a working protection circuit
are needed for the safe use of the lithium-ion battery.
What's the best battery for two-way radios?
Most two-way radios use nickel-cadmium. These batteries are durable
and forgiving if abused. But nickel-cadmium batteries have only
moderate energy density and are environmentally unfriendly.
Environmental agencies have been discouraging its use, especially in
Europe. The recommended alternative is nickel-metal-hydride, a battery
that has higher energy density and contains no toxic metals. nickelmetal-hydride has been tested in two-way radios for a number of years
but the results are mixed. Shorter than expected service life is the major
drawback.
For two-way radios, nickel-metal-hydride has a cycle life, which is half
that of standard nickel-cadmium. nickel-metal-hydride prefers a
moderate discharge current of 0.5C or less. A two-way radio, on the
other hand, draws a discharge current of about 1.5A when transmitting
at 4W of power. High discharge loads and sharp pulse currents shorten
battery life.
To compare the longevity of nickel-metal-hydride under different load
condition, a test was carried out in which batteries of the same type
were discharged with a DC and digital load. In both tests, the batteries
were discharged to 1.04 volts per cell. The DC load was a steady 500mA;
the digital load simulated the Global System for Mobile Communications
(GSM) at 1.65 ampere peak for 12 ms every 100 ms with 270 mA
standby. (Note that the GSM pulse for voice is about 550 ms every 4.5
ms).
With the DC discharge, nickel-metal-hydride wore out gradually,
providing an above average service life. At 700 cycles, the battery still
provided 80% capacity. By contrast, the same battery type faded more
rapidly with a digital discharge and the 80% capacity threshold was
reached after only 300 cycles. This phenomenon indicates that the
kinetic characteristics for nickel-metal-hydride deteriorate more rapidly
with a digital than analog load. Although the test was simulating a GSM
cell phone, Tetra and other digital two-way radios have similar loading.
Let's briefly compare the characteristics of nickel-cadmium and nickelmetal-hydride. nickel-cadmium has the advantage of maintaining steady
high capacity and low internal resistance through most of its service life.
nickel-metal-hydride, on the other hand, starts with good capacity and
low internal resistance but the resistance increases after a few hundred
cycles, causing the voltage to drop on a load. Even though the energy
may still be present, the battery cannot deliver the high current during
transmit and the message cuts off. The radio becomes unreliable.
Nickel-based batteries are high in maintenance. Periodic discharge cycles
are needed to prevent crystalline formation on the cell plates, also
known as memory. nickel-cadmium is more receptive to memory than
nickel-metal-hydride because both nickel and cadmium plates are
affected by memory.
Nickel-cadmium should be exercised once ever 1 to 2 months, whereas
nickel-metal-hydride can get by with a deliberate full discharge once
every 3 months. Without proper maintenance, the advantage of nickelcadmium over nickel-metal-hydride in terms of cycle life cannot be
realized.
Lithium-ion has been tested for two-way radios and the results are
positive. Substituting lithium-ion with nickel-based will require chargers
specifically suited for this chemistry. While nickel-cadmium and nickelmetal-hydride can often share the same charger, lithium-ion uses a
different charge algorithm. There is also a cost premium for lithium-ion.
Future two-way radios will undoubtedly be fitted with lithium-ion.
Choosing the right battery for portable computing (BU38)
Laptops are known to be tough hosts on their batteries. The host
demands a stream of uninterrupted power but offers a poor working
environment in return. As a result, the battery cannot provide the
promised runtime and the service cuts short, often with little notice. In
this paper we address the unhappy marriage between the host and
battery, and examine what causes a battery to deteriorate faster than in
other portable devices.
What's the best battery for laptops?
Batteries for laptops have a unique challenge - they must be small and
lightweight. In fact, the laptop battery should be invisible to the user and
deliver enough power to endure a five-hour flight from Toronto to
Vancouver. In reality, a typical laptop battery provides only about 90
minutes of service. Many users complain of much shorter runtimes.
Computer manufacturers are hesitant to add a larger battery because of
increased size and weight. A recent survey indicated that, given the
option of larger size and more weight for longer runtimes, most users
would settle for what is being offered today. For better or worse, we
have learned to accept the short runtime of a laptop.
The energy density of modern batteries improves by about 10% per year.
However, the benefit of better battery performance is eaten up by higher
power requirements of laptops. This results in the same runtime but
more powerful laptops.
Figure 1: Net
runtime.
The energy
density of
modern
batteries
increases by
about 10% per
year. This gain
is compensated
by the demand
for better laptop
performance.
The runtime
remains the
same.
During the last few years, batteries have improved in terms of energy
density. But any benefit in better battery performance is being eaten up
by the higher power requirements of the laptops. This trend is continuing
and the net effect will be the same runtimes but more powerful laptops.
Most laptops are powered by lithium-ion. This chemistry has a high
energy density and is lightweight. There is no immediate breakthrough
on the horizon of a miracle battery that would provide more power than
the current electro-chemical battery.
Fuel cells, when available, will offer a continued stream of power by
allowing the exchange of fuel cartridges when empty. Unfortunately,
commercial fuel cells for laptops and other portable devices are still
several years away. Power handling, size and cost remain the biggest
hurdles. The early fuel cells will function more like a portable charger
than a battery replacement. The fuel cells currently in use have the
difficulty in providing spontaneous high power on demand.
The runtime of a laptop battery is based on the activity of the computer.
The basic housekeeping, which the computer needs to stay alive, draws
less power than, for example, reading, writing, computations and
searching for files. Manufacturers prefer using idle time when specifying
runtime.
A battery in a laptop ages more quickly than in other applications
because of heat. During use, the inside temperature of a laptop rises to
45°C (113°F). The combination of high temperature and full state-ofcharge promotes cell oxidation, a condition that cannot be reversedonce
present. The battery's life expectancy when operating at high
temperature is half compared to running at a more moderate 20°C
(68°F) or lower. Leaving the laptop in a parked car under the hot sun will
also aggravate the situation. All batteries suffer permanent capacity loss
as part of elevated temperatures but lithium-ion is affected more than
other batteries.
Some Japanese computer manufacturers have introduced a number of
sub-notebooks in which the battery is mounted externally, forming part
of the housing. This design improves battery life because the battery is
kept at room temperature. Some models carry several different battery
sizes to accommodate a range of user demands.
Lithium-ion is well suited for laptop users who continually switch from
fixed power to battery use. This user pattern is typical for those in the
sales, service and medical field. Here is the reason why:
With nickel-based batteries, the charger applies a full charge each time
the portable device is connected to fixed power. The battery is put on
charge until a signal is received indicating that the battery is full. This
signal is in form of a voltage change or rising temperature. Because of
the sluggish response, permanent capacity loss occurs caused by
overcharge and elevated temperature. Lithium-ion only receives charge if
the voltage is low.
How to calibrate the battery
Most laptop batteries are 'smart'; meaning that they know how much
energy is left. Such a feature has definite benefits but the readings are
often inaccurate. A laptop may indicate 30 minutes of remaining runtime
when suddenly the screen goes dark. Here is the reason why:
With use and time, a tracking error occurs between the chemical battery
and the digital sensing circuit. The most ideal use of the 'smart' battery,
as far as fuel-gauge accuracy is concerned, is a full charge followed by a
full discharge at a constant current. In such a case, the tracking error
would be less than 1% per cycle. In real life, however, a battery may be
discharged for only a few minutes and the load may vary widely. Long
storage also contributes to errors because the circuit cannot accurately
compensate for self-discharge. Eventually, the true capacity of the
battery no longer synchronizes with the fuel gauge and a full charge and
deliberate full discharge will be needed to 're-learn' or calibrate the
battery.
There are no standards to tell what constitutes a fully charged and fully
discharged battery. Lithium-ion packs are considered fully charged when
the limiting voltage (4.20V/cell) is reached and the saturation current
has decreased to 3% of the nominal value (50mA on a 1700mAh cell).
Some chargers use 5% and 8% as 'ready' criteria.) A full discharge
occurs when the cell reaches 3V/cell or lower. At this voltage level, the
battery has a remaining capacity of 3 to 10%. Modern batteries adjust to
a lower cut-off voltage on high load currents and include temperature
compensation.
To calibrate a battery, a full charge and discharge is necessary. One
without the other does not constitute a calibration. A problem arises if
the battery is recharged after a brief use without providing the
opportunity of a full discharge. A forced discharge to "Low Battery" may
be needed from time to time.
What happens if no battery calibration is done? Can such a battery be
used in confidence? Most 'smart' battery chargers obey the dictates of
the chemical cells rather than that of the electronic circuit. In this case,
the battery will fully charge regardless of the fuel gauge setting and
function normally but the digital readout will become increasingly more
inaccurate. If not corrected, the fuel gauge simply becomes a nuisance.
Cadex Electronics manufactures 'smart' chargers and battery analyzers
that are capable of calibrating a 'smart' battery.
Choosing the right battery for industrial applications (BU39)
Industrial applications have unique power needs and the choice of
battery is important. While consumer products demand high energy
density to obtain slim and elegant designs, industry focuses on durability
and reliability. Industrial batteries are commonly bulkier than those used
in consumer products but achieve a longer service life.
Batteries are electro-chemical devices that convert higher-level active
materials into an alternate state during discharge. The speed of such
transaction determines the load characteristics of a battery. Also referred
to as concentration polarization, the nickel and lithium-based batteries
are superior to lead-based batteries in reaction speed. This attribute
reflects in good load characteristics.
Discharge loads range from a low and steady current flow of a flashlight
to intermittent high current bursts in a power tool, to sharp current
pulses on digital communications equipment, laptops and cameras. In
this paper we evaluate how the various battery chemistries perform in a
given application.
What's the best battery for video cameras?
Nickel-cadmium batteries continue to power a large percentage of
professional cameras. This battery provided reliable service and performs
well at low temperature. nickel-cadmium is one of the most enduring
batteries in terms of service life but has only moderate energy density
and needs a periodic full discharge.
The need for longer runtimes is causing a switch to nickel-metal-hydride.
This battery offers up to 50% more energy than nickel-cadmium.
However, the high current spikes drawn by digital cameras have a
negative affect and the nickel-metal-hydride battery suffers from short
service life.
There is a trend towards lithium-ion. Among rechargeables, this
chemistry has the highest energy density and is lightweight. A steep
price tag and the inability to provide high currents are negatives.
The 18650 cylindrical lithium-ion cell offers the most economical power
source. "18" defines the cell's diameter in millimeters and "650" the
length. No other lithium-ion cell, including prismatic or polymer types,
offers a similar low cost-per-watt ratio.
Over the years, several cell versions of 18650 cells with different Ah
ratings have emerged, ranging from 1.8Ah to well above 2Ah. The cells
with moderate capacities offer better temperature performance, enable
higher currents and provide a longer service life than the souped up
versions.
The typical 18650 for industrial use is rated at 2Ah at 3.60 volts. Four
cells are connected in series to obtain the roughly 15 volts needed for the
cameras. Paralleling the cells increases the current handling by about 2A
per cell. Three cells in parallel would provide about 6A of continuous
power. Four cells in series and three in parallel is a practical limit for the
18650 system.
Lithium-ion requires a protection circuit to provide safe operations under
all circumstances. Each cell in series is protected against voltage peaks
and dips. In addition, the protection circuit limits each cell to a current
about 2A. Even if paralleled, the current of a lithium-ion pack is not high
enough to drive digital cameras requiring 10 to 15A peak current. Tests
conducted at Cadex Electronics have shown that the 18650 allows short
current peaks above the 2A/cell limit. This would allow the use of
lithium-ion on digital cameras, provided the current bursts are limited to
only a few seconds.
What's the best battery for still cameras?
The power requirement of a professional digital camera is sporadic in
nature. Much battery power is needed to take snapshots, some with a
powerful flash. To view the photo, the backlit color display draws
additional power. Transmitting a high-resolution image over the air
depletes another portion of the energy reserve.
Most non-professional cameras use a primary lithium battery. This
battery type provides the highest energy density but cannot be
recharged. This is a major drawback for professional use. Rechargeable
batteries are the answer and lithium-ion fits the bill but faces similar
challenges to the video cameras.
What is the best battery for medical devices?
One of the most energy-hungry portable medical devices is the heart
defibrillator. The battery draws in excess of 10 amperes during
preparation stages. Several shocks may be needed to get the patient's
heart going again. The battery must not hamper the best possible patient
care.
Most defibrillators are powered by nickel-cadmium. nickel-metal-hydride
is also being used but there is concern of short service life. In a recent
study, however, it was observed that a defibrillator battery cycles far
less than expected. Instead of the anticipated 200?cycles after two years
of seemingly heavy use, less than 60 cycles had been delivered on the
battery examined. 'Smart' battery technology makes such information
possible. With fewer cycles needed, the switch to higher energy-dense
batteries becomes a practical alternative.
Sealed lead-acid batteries are often used to power defibrillators intended
for standby mode. Although bulky and heavy, the Lead-acid has a low
self-discharge and can be kept in prolonged ready mode without the
need to recharge. Lead-acid performs well on high current spurts. During
the rest periods the battery disperses the depleted acid concentrations
back into the electrode plate. Lead-acid would not be suitable for a
sustained high load.
The medical industry is moving towards lithium-ion. The robust and
economical 18650 cells make this possible. The short but high current
spurts needed for defibrillators are still a challenge. Paralleling the cells
and adding current-limiting circuits that allow short spikes of high
current will help overcome this hurdle.
What is the best battery for power tools?
Power tools require up to 50 amperes of current and operate in an
unfriendly environment. The tool must perform at sub zero temperatures
and endure in high heat. The batteries must also withstand shock and
vibration.
Most power tools are equipped with nickel-cadmium batteries. nickelmetal-hydride has been tried with limited success. Longevity is a
problem but new designs have improved. lithium-ion is too delicate and
could not provide the high amperage. Lead-acid is too bulky and lacks
persistent power delivery. The power tool has simply no suitable
alternatives to the rugged and hard-working nickel-cadmium.
In an attempt to pack more energy into power tools, the battery voltage
is increased. Because of heavy current and application at low
temperatures, cell matching is important. Cell matching becomes more
critical as the number of cell connected in series increases. A weak cell
holds less capacity and is discharged more quickly than the strong ones.
This imbalance causes cell reversal on the weak cell if the battery is
discharged at high current below 1V/cell. An electrical short occurs in
the weak cell if exposed to reverse current and the pack needs to be
replaced. The higher the battery voltage, the more likely will a weak cell
get damaged.
What's the best battery for wheeled and stationary
applications? (BU40)
Consumer products have benefited the most from the advancements in
battery technology. The size and weight reductions achieved for the
high-end cell phones, PDA's and laptops have not trickled down to
batteries for wheeled and stationary applications in an expected fashion.
Only marginal improvements have been gained on larger batteries. One
of the reasons for the apparent lack in progress is the loyalty to the
classic sealed lead-acid battery.
The wheeled and stationary industries have several reasons for their
unwillingness to change: [1] lead-acid is mature and inexpensive. [2]
The low energy density is no major drawback because the battery is
either on wheels or is stationary. [3] The limited cycle life can, to some
extent, be compensated by using larger batteries. Unlike portable
devices, most wheeled and stationary batteries are replaced due to age
rather than wear out effect induced by high cycle count.
What's the best battery for wheelchairs?
Wheelchairs and scooters are almost exclusively powered by sealed leadacid batteries. Regular car batteries are sometimes used for cost
reasons. There is, however, a danger of spillage if overturned. Neither
are regular car batteries designed for deep cycling. The demanding
cycling regiments of wheelchairs and scooters cause an undue strain on
these batteries and shorten their lives. nickel-based batteries would be
lighter than lead-acid but are more expensive and maintenance prone.
Lithium-ion would simply be too delicate, not to mention the high cost.
A new generation of wheelchair is being developed that is able to 'stand
up' and climb chairs. These high-tech devices use gyroscopes for
balancing. To obtain the extra power needed to run its internal computer
and electric motors without adding too much weight, nickel-based
batteries are used. The two-wheeled Segway scooter being touted to
solve city transportations problems also uses nickel-based batteries.
What's the best battery for the electric bicycle?
Anyone serious about the electric bicycle would use nickel-based
batteries. Sealed lead-acid is simply too heavy and does not provide the
cycle count needed to satisfy daily use. In addition, lead-acid requires a
long charge time of 10 hours and more. Lithium-ion would simply be too
expensive and delicate. The lack of a suitable battery that is light,
durable and inexpensive is, in my opinion, delaying the public acceptance
of the electric bicycle.
What's the best battery for the electric vehicle?
The electric vehicle will gain public acceptance as soon as a battery
emerges that is inexpensive and provides 10 years of reliable service.
The high cost and limited cycle life of the batteries used in hybrid
vehicles negate the savings achieved in burning less fuel. The benefits
are more environmental in nature rather than in cost savings. Higher fuel
prices could change this equilibrium.
nickel and lithium-based batteries have been tried but both chemistries
have problems with durability and stability. lithium-ion has an advantage
in weight but this gain is offset by a high price. Similarly, nickel-metalhydride used for the hybrid vehicle is expensive and requires forced aircooling. No battery manufacturer is willing to commit to a 10-year
warranty. After excursions into new battery chemistries, design
engineers always come back to the old but proven lead-acid.
The fuel cell may still be two decades away before offering a viable
alternative for cars. An executive from Ford stated recently that the fuel
cell may never be feasible to replace the internal combustion engine.
Cost and longevity remain major drawbacks.
Since the invention in 1839 by Sir William Grove, the advancements in
the fuel cell have been slow. Much attention was then placed on
improving the internal combustion engine. It was not until the Gemini
and Apollo programs in the 1960s that the fuel cell was used to provide
power and water in space. During the 1990s, renewed activities took
place and the fuel cell stocks soared. Unlike the rapid developments in
microelectronics, which generated income in its early stages, fuel cell
research continues to depend on government grants and public
investors. It is our hope that one day the fuel cell will become a viable
option to the polluting combustion engine.
What's the best battery for stationary applications?
Until the mid 1970s, most stationary batteries were flooded lead-acid.
The Valve Regulated Lead Acid (VRLA) allowed batteries to be installed in
smaller confinements because the cells could be stacked and mounted in
any position. Although VRLA are less durable than flooded lead-acid,
simple mounting and lower cost make them the preferred battery system
for small and medium sized installations. Most UPS systems repeater
stations for cell phones use VRLA. Large installations, such as internet
hubs, hospitals, banks and airports still use the flooded lead-acid.
Heat is the main killer of batteries. Many outdoor installations for
communication systems lack proper venting, not to mention air
conditioning. Instead of the expected 10-year service life, the batteries
need replacement after 2 to 5 years. Many batteries in the field are in
such bad conditions that they could only provide power for a short time,
should a major power outage occur. Stationary batteries are often
installed and forgotten.
A Canadian manufacturer of lithium-polymer batteries is taking
advantage of the heat problem. They offer lithium-polymer for standby
applications, a battery that needs heat to operate. The dry lithiumpolymer lacks conductivity at ambient temperature and must be heated.
The battery includes heating elements to keep its core temperature at
60°C (140°F). The mains provide the energy for heating. On a power
outage, the battery must also provide power for heating the core. To
conserve energy, the battery is well insulated. Unlike the VRLA, the high
ambient heat does not shorten the lithium-polymer battery. The high cost
remains a drawback and only a few lithium-polymer batteries are used
for stationary applications today.
Flooded nickel-cadmium batteries have been used for many years in
applications that must endure hot and cold temperatures. This battery
system is substantially more expensive that Lead-acid but the improved
longevity makes up for the higher investment cost. The flooded nickelcadmium batteries are non-sintered and do not suffer from memory. It
should be noted that only the sintered sealed nickel-cadmium cells are
affected by memory and need regular discharges
Are the Hybrid Cars here to stay?(BU40a)
The hybrid car is not new - Ferdinand Porsche designed the series-hybrid
vehicle in 1898. Called the Lohner-Porsche carriage, the hybrid function
served as an electrical transmission rather than power boost. With Mr.
Porsche in the driver's seat, the car broke several Austrian speed
records, including the Exelberg Rally in 1901. Another example of an
early hybrid was the 1915 Woods Motor Vehicle built in Chicago. The car
used a four-cylinder internal combustion engine and an electric motor.
Below 15 mph (25 km/h), the electric motor propelled the vehicle; at
higher speeds, the gasoline engine kicked in to take the vehicle up to a
top speed of 35 mph (55 km/h). As part of the Federal Clean Car
Incentive Program, Victor Wouk installed a hybrid drive train in a 1972
GM Buick Skylark but the EPA canceled the program in 1976. Meanwhile,
Honda and Toyota have made strong headways by commercializing
attractive and fuel-efficient hybrid cars.
The hybrid electric vehicle (HEV) conserves fuel by using an electric
motor that assists the internal-combustion engine (IC) on acceleration
and harnesses kinetic energy during breaking. Furthermore, the IC motor
turns off at stops and during slow travel. When full power is required,
both the IC engine and the electric motors engage simultaneously to get
maximum boost. This power-sharing scheme offers two advantages; it
calls for a smaller IC engine and improves acceleration because the
electric motor has excellent torque characteristics.
Most HEVs use a mechanical drive train from the IC engine to the wheels.
In this respect, the HEV is similar to an ordinary vehicle with crankshaft,
clutch and transmission, with the difference of having an electric motor
and a battery. This design is known as a parallel configuration. Most upand-coming plug-in HEVs use the serial configuration in which the
wheels are powered by one or several electric motors. Instead of a
mechanical link, the IC engine energizes a generator to produce
electricity for the motor(s). Similar to a laptop or a cell phone, the driver
plugs the car into the AC outlet for an overnight charge. The typical
driving range with a full charge is 20 miles or 32 km. On long trips, the
IC engine engages to provide continuous power for the electric motors.
What's the best battery for the hybrid car?
The early HEV models used lead acid batteries because there was no
alternative. Today, Honda and Toyota employ nickel-metal-hydride
(NiMH). This chemistry is lighter and environmentally friendlier than
lead-based systems. The battery consists of cylindrical cells that are
connected in series to attain several hundred volts. The cell strings are
suspended in mid air to allow air-cooling. Figure 1 shows a
demonstration pack of an early Toyota hybrid car battery.
Figure 1: Nickelmetal-hydride
battery of a Toyota
hybrid car.
The cells (orange
color) are
supported to allow
forced air-cooling.
The battery is
placed behind the
back seat.
Courtesy of the Toyota
Museum, Nagaya, Japan
One of the critical battery requirements for hybrid applications is
longevity. Rechargeable batteries for consumer products typically last for
two to three years. This short service life is no major drawback with cell
phones, laptops and digital cameras because the devices get obsolete
quickly. At $2,000 to $3,000 per battery pack, the replacement cost of an
HEV battery would constitute a major expense.
Most batteries for HEV are guaranteed for eight years. To meet this long
service life, the cells are optimized for longevity and not size and weight,
as is the case with portable applications. Since the battery runs on
wheels, the increased weight and size is not too critical.
A NiMH for an HEV can be charged and discharged 1,000 times if done at
an 80% depth-of-discharge. In a hybrid vehicle, a full discharge occurs
seldom except if the owner lives on a mountain and requires all available
battery power to commute home. Such a routine would add stress to the
battery and the life would be shortened. In most other application, the
hybrid car only uses 10% of the rated battery capacity. This allows
thousands of charge/discharge cycles. Batteries in satellites use a
similar system in which the battery discharges less than 10% during a
satellite night. NASA achieves this by over-sizing the battery.
One of the limitations of NiMH is moderate energy conversion efficiency.
This translates to the battery getting hot on charge and discharge. The
charge efficiency is best at 50-70% state-of-charge. Above 70% the
battery cannot absorb the charge well and much of the charging energy
is lost in heat. Operating a battery with a partial charge requires a larger
mass that lowers the energy-to-weight ratio and efficiency.
The Japanese car manufacturers have tried several battery chemistries,
including going back to lead acid. Today, the focus is on lithium-ion. The
cobalt-based lithium-ion is one of the first chemistries in the lithium
family and offers a very high energy density. Unfortunately, this battery
system cannot deliver high currents and is restricted to portable
applications.
HEV manufacturers are experimenting with manganese (spinel) and
phosphate versions. These lithium-ion systems offer an extremely low
internal resistance, deliver high load currents and accept rapid charge.
Unlike the cobalt version, the resistance stays low throughout the life of
the battery. To verify the characteristic of manganese-based lithium-ion,
a research lab applied 30,000 discharge/charge cycles over a period of
seven years. Although the capacity dropped from 100% to 20%, the cell
retained its low internal resistance. The drawback of manganese and
phosphate is lower energy density but these systems provide 20% more
capacity per weight than NiMH and three times more than lead acid.
Figure 2 illustrates the energy densities of the lead, nickel and lithiumion systems.
(See also http://www.batteryuniversity.com/partone-5A.htm) It should
be noted that lithium-ion systems have the potential of higher energy
densities but at the cost of lower safety and reduced cycle life.
Figure 2: Energy
densities of
common battery
chemistries.
Lithium-cobalt
enjoys the highest
energy density.
Manganese and
phosphate systems
are thermally more
stable and deliver
higher load
currents than
cobalt.
The Lithium-ion systems are promising candidates for both the HEV and
plug-in HEV but require more research. Here are some of the roadblocks
that need to be removed:
Durability: The buyer requests a warranty of ten years and more.
Currently, the battery manufacturer for hybrid electric vehicles can only
give eight years on NiMH. The longevity of lithium-ion has not yet been
proven and honoring eight years will be a challenge.
Cost: If the $2,000 to $3,000 replacement cost of a nickel-metal-hydride
pack is prohibitive, lithium-ion will be higher. These systems are more
expensive to produce than most other chemistries but have the potential
for price reductions through improved manufacturing methods. NiMH has
reached the low cost plateau and cannot be reduced further because of
high nickel prices.
Safety: Manganese and phosphate-based lithium-ion batteries are
inherently safer than cobalt. Cobalt gets thermally unstable at a
moderate temperature of 150°C (300°F). Manganese and phosphate cells
can reach 250°C (480°F) before becoming unsafe. In spite of the
increased thermal stability, the battery requires expensive protection
circuits to supervise the cell voltages and limit the current in fail
conditions. The safety circuit will also need to compensate for cell
mismatch that occurs naturally with age. The recent reliability problems
with lithium-ion batteries in portable devices may delay entry into the
HEV market.
Availability: Manufacturers of manganese and phosphate cells can hardly
keep up with the demand. A rapid increase of lithium for HEV batteries
would put a squeeze on battery production. With 7 kg (15 lb) of lithium
per battery, there is talk of raw material shortages. Most of the known
supplies of lithium are in South America, Argentina, Chile and Bolivia.
The plug-in hybrid electric vehicle (PHEV)
Imagine a plug-in electric vehicle that can go 20 miles (32 km) with a
single charge from the electrical outlet at home. There is no pollution and
the neighbors won't hear you coming and going because the vehicle is
totally silent. With the absence of gas tax, the road system is yours to
use for free. Or is it?
As good as this may sound, the savings will be small or non-existent
because of the battery. Dr. Menahem Anderman, a leading expert on
advanced automobile batteries, says that we still have no suitable
battery for the plug-in HEV and that the reliability of lithium-ion
technology for automotive applications has not yet been proven. Unlike
the ordinary HEV that operates on shallow charges and discharges, the
plug-in HEV is in charge depletion mode that requires deep discharges.
To obtain an acceptable driving range, the PHEV battery will need to be
five times larger than the HEV battery. With an estimated life span of
1000 full charge and discharge cycles, the battery would need to be
replaced every three years. At an estimated $10,000 per battery
replacement, the anticipated cost savings would be quickly exhausted.
Modern cars do more than provide transportation; they also include
auxiliary devices for safety, comfort and pleasure. The most basic of
these auxiliaries are the headlights and windshield wipers. Most buyers
would also want heating and air-conditioning systems. These amenities
are taken for granted in gasoline-powered vehicles and will need to be
used sparingly in a PHEV.
Analysts give another 10 years before a viable plug-in HEV will be
available. The promise of a clean-burning fuel cell car is still vivid in our
memory. Analysts now estimate 20 years before the fuel cell is ready for
mass-produced cars. There are rumors that the fuel cell may never make
it into an ordinary car. If this is true, a dream will go down in history
with the steam-powered airplane of the mid 1800s that was simply too
cumbersome to fly.
The paradox of the hybrid vehicle
At the Advanced Automotive Battery Conference in Hawaii, a delegate
member challenged a maker of HEVs with the claim that a German diesel
car can get better fuel economy than the hybrid. The presiding speaker,
being a trained salesman, flatly denied this notion. There is some truth to
his claim, however. On the highway, the diesel car is indeed more fuelefficient but the HEV has the advantage in city driving. Power boost for
fast acceleration and regenerative breaking are advantages that the
German diesel does not offer.
Someone then asked, "What would happen if the HEV depletes its
batteries while driving up a long mountain pass? Will the car have
enough power?" The answer was that the car would make it with the IC
engine alone but the maneuverability would be restraint. To compensate
for this eventuality, some HEV manufacturers offer SUVs featuring a fullsized IC motor of 250 hp and an electrical motor at 150 hp; 400 hp in
total. Such a vehicle would surly find buyers, especially if the
government provides grant money for being 'green.' It's unfortunate that
the buyers of a small car or the commuters taking public transport won't
qualify for such a handout.
Conclusion
We anticipate that lithium-ion will eventually replace nickel-metalhydride in hybrid electric vehicles but short service life, high
manufacturing costs and safety issues will stand in its way today. We
need to remind ourselves that the automotive market can only tolerate a
marginal cost increase for a new battery technology. In terms of added
capacity, lithium-ion offers only a 20% increase in energy density per
weight over nickel-based systems. The nickel-metal-hydride has proven
to work well in current HEVs and a new chemistry would need to offer
definite advantages over present systems to find buyers.
Toyota, Honda and Ford are leading in HEV technology. Other major
automakers are expected to offer competitive models by 2010. Currently,
Panasonic EV Energy and Sanyo supply over 90% of the HEV batteries.
Both companies are also developing lithium-ion batteries.
While Japan and Korea are focusing on manganese systems, the USA is
experimenting with phosphate, the chemistry that made the A123
Systems famous. Europe is relying on clean-burning diesel. These
engines are so clean that they won't even stain a tissue that is placed on
the exhaust pipe. BMW is working on a zero emission hydrogen car.
Time will tell who will be the winner in the race for cleaner, more fuelsavvy vehicles and longer-living cars. In terms of longevity, the diesel
would be the winner today. We hope that future batteries will one-day
have the endurance to match or exceed the robust diesel engine.
References: Menahem Anderman, Status and Prospect of Battery Technologies for Hybrid Electric
Vehicles,
including Plug-in Hybrid Electric Vehicles, January 2007.
Rapid-testing of batteries (BU41)
When studying the characteristics relating to battery state-of-health
(SoH) and state-of-charge (SoC), some interesting and disturbing effects
can be observed - the properties are cumbersome and not linear. Worst
of all, the parameters are unique for every battery type. This inherent
complexity makes it difficult to create a formula for rapid testing that
works for all batteries.
In spite of these seemingly insurmountable odds, battery rapid testing is
possible. But the questions are asked, how accurate will the test results
be and how will the system adapt to different battery types. Instrument
cost and ease-of-use are also concerns. This paper evaluates currently
used methods, which include the load test, AC conductance test and the
six-point test developed by Cadex.
The load test
The load test provides important battery information consisting of open
battery voltage, voltage under load and internal resistance. nickel-based
batteries should always indicate an open terminal voltage of about
1.1V/cell, even if empty. The electro-chemical reaction of the different
metals in the cell generates this voltage potential. A depressed voltage
may indicate high self-discharge or a partial electrical short.
A lead-based battery must always have a charge and the open terminal
voltage should read 2.0V/cell and higher. If below 2 volts, a sulfation
layer builds up that makes a recharge difficult, if impossible. An open
terminal voltage of 2.10V/cell indicates that the battery is roughly 50%
charged.
The voltage of a lithium-based battery can, to some extent, indicate SoC.
A fully charged cell reads about 4.0V/cell and a partially charged cell
measures between 3.0 and 4.0V/cell.
The load test applies a momentary load, during which the voltage is
measured. Voltage over current equals the resistance. More accurate
results are obtained by applying a two-stage load. Figure 1 illustrates the
voltage pattern of such a two-stage load test.
Figure 1:
DC load
test.
The DC
load test
measures
the
battery's
internal
resistance
by reading
the
voltage
drops of
two loads
of
different
strength.
A large
drop
indicates
high
resistance.
The AC conductance test
An alternative method of measuring the internal battery resistance is the
AC conductance test. An alternating current of 50 to 1000 Hertz is
applied to the battery terminals. The battery's reactance causes a phase
shift between voltage and current, which reveals the condition of the
battery. AC conductance works best on single cells. Figure 2
demonstrates the relation of voltage and current on a battery.
Figure 2:
AC load
test.
The AC
method
measures
the phase
shift
between
voltage
and
current.
The
battery's
reactance
and/or
voltage
deflections
are used
to
calculate
the
impedance
Some AC resistance meters evaluate only the load factor and disregard
the phase shift information. This technique behaves similar to the pulse
method in that the AC voltage is superimposed on the battery's DC
voltage and acts as brief charge and discharge pulses. The amplitude of
the ripple is utilized to calculate the internal battery resistance.
There are some discrepancies in the resistance readings between the
'load test' and 'AC conductance test'. The differences are more apparent
on marginal than on good batteries. So which reading is correct? In many
aspects, the AC conductance is superior to the load test, however, one
single frequency cannot provide enough data to evaluate the battery
adequately. Multi-frequency devices are being developed but their
complexity rises exponentially with the number of frequencies used.
Resistance measurement, as a whole, provides only a rough sketch of the
battery's performance because various battery conditions affect the
readings. For example, a battery that has just been charged shows a
higher resistance reading than one that has rested for a few hours. An
empty or nearly empty battery also exhibits elevated internal resistance.
To obtain reliable readings, a battery must be at least 50% charged.
Temperature further affects the internal resistance readings. A hot
battery reads a lower resistance than one at ambient temperature or one
that is cold. In addition, the chemistry, the number of cells connected in
series and the current rating (size in mAh) of a battery influence the
results. Many batteries also contain a protection circuit that further
distorts the readings.
The Cadex QuickTest™
Cadex Electronics has developed a method to measure the state-ofhealth (SoH) of a battery in 3 minutes. QuickTest™ uses a patentpending inference algorithm to fuse data from 6 variables, which are:
capacity, internal resistance, self-discharge, charge acceptance,
discharge capabilities and mobility of electrolyte. The data is combined
with a trend-learning algorithm to provide an accurate SoH reading in
percent. Figure 3 illustrates general structure of such a network.
Figure 3: General structure of the
Cadex QuickTest™
Multiple variables are fed to the
micro controller, 'fuzzified' and
processed through parallel logic.
The information is averaged and
weighted according to the
battery application.
QuickTest™ is built into the Cadex C7000-Series battery analyzers and
services nickel, lithium and lead-based batteries for two-way radios, cell
phones, laptops, scanners and medical devices. The analyzers are userprogrammable and also perform battery priming, reconditioning, fastcharging, life-testing and boosting functions.
QuickTest™ uses battery specific matrices that are obtained with the
analyzer's trend learning process. The ability to learn allows adapting to
new batteries in the field. The matrices are stored in the battery adapters
and automatically configure the analyzer to the correct battery setting.
The adapters commonly include the matrix at time of purchase. If
missing, the matrix can be added in the field by scanning two or more
batteries on the analyzer's Learn program. The required charge level to
perform QuickTest™ is 20-90%. If outside this range, the analyzer
automatically applies a brief charge or discharge.
What is the definition of state-of-health and when should a battery be
replaced? SoH reveals the overall battery conditions based on the above
mentioned variables, which are capacity, internal resistance, selfdischarge, charge acceptance, discharge capabilities and mobility of
electrolyte. If any of these variables provide marginal readings, the end
result will be affected. A battery may have a good capacity but the
internal resistance is high. In this case, the end SoH reading will be
lowered accordingly. Similar demerit points are added if the battery has
high self-discharge or exhibits other chemical deficiencies. The battery
should be replaced if the SoH falls below 80%.
Rapid testing automotive and stationary batteries (BU42
Portable batteries for cell phones, laptops and cameras may be rapidtested by applying a number of load pulses while observing the
relationship between voltage and current. Ohm's Law is used to calculate
the internal resistance. Comparing the readings against a table of values
estimates the battery's state-of-health.
This load pulse method does not work well for larger batteries and AC
conductance is commonly used. An AC voltage is applied to the battery
terminals that floats as a ripple on top of the battery's DC voltage and
charges and discharges the battery alternatively.
AC conductance has been incorporated into a number of hand-held
testers to check batteries for vehicular and stationary batteries. To offer
simple and low-cost units, these testers load the battery with pulses
rather than injecting sinusoidal signals. The pulses are commonly not
voltage controlled and the thermal battery voltage* may be surpassed.
The thermal voltage threshold of a lead-acid battery is 25mV per cell.
Exceeding this voltage is similar to over-driving an audio amplifier.
Amplified noise and distortion is the result.
AC conductance provides accurate readings, provided the battery is fully
charged, has rested or has been briefly discharged prior to taking the
reading. AC conductance tends to become unreliable on low charge and
sometimes fails a good battery. At other times, a faulty battery may pass
as good. The correlation to the battery's state-of-charge is a common
complaint by users. AC conductance works best in identifying batteries
with definite deficiencies.
AC conductance is non-invasive, quick and the test instruments are
relatively inexpensive. There are, however, some fundamental problems.
Most commercial testers use only one frequency, which is commonly
below 100 Hertz. Multi-frequency systems would be more accurate but
require complex data interpretation software and expensive hardware.
In this paper we focus on Electrochemical Impedance Spectroscopy
(EIS), a method that overcomes some of the shortcomings of AC
conductance.
________________
* Batteries are non-linear systems. The equations, which govern the battery's response becomes
linear below 25mV/cell at 25°C. This voltage is called the battery thermal voltage.
Electrochemical Impedance Spectroscopy (EIS)
EIS evaluates the electrochemical characteristics of a battery by applying
an AC potential at varying frequencies and measuring the current
response of the electrochemical cell. The frequency may vary from about
100 micro Hertz (µHz) to 100 kilo Hertz (kHz). 100µHz is a very low
frequency that takes more than two hours to complete one full cycle. In
comparison, 100kHz completes 100,000 cycles in one second.
Applying various frequencies can be envisioned as going through
different layers of the battery and examining its characteristics at all
levels. Similar to tuning the dial on a broadcast radio, in which individual
stations offer various types of music, so also does the battery provide
different information at varying frequencies.
Battery resistance consists of three types, which are: pure resistance,
inductance and capacitance. Figure 1 illustrates the classic Randles
model,which represents a typical battery
Figure 1: Randles model of a lead acid
battery. The overall battery resistance
consists of pure Ohmic resistance,
inductance and capacitance. There are
many other models
Capacitance is responsible for the capacitor effect; and the inductance is
accountable for the so-called magnetic field, or coil effect. The voltage on
a capacitor lags behind the current. On a magnetic coil, on the other
hand, the current lags behind the voltage. When applying a sine wave to
a battery, the reactive resistance produces a phase shift between voltage
and current. This information is used to evaluate the battery.
EIS has been used for a number of years to perform in-flight analysis of
satellite batteries, as well as estimating grid corrosion and water loss on
aviation and stationary batteries. EIS gives the ability to observe the
kinetic reaction of the electrodes and allows analyzing changes of
analyze changes that occur in everyday battery usage. Increases in
impedance readings hint at minute intrusion of corrosion and other
deficiencies. Impedance studies using the EIS methods have been carried
out on lead-acid, nickel-cadmium, nickel-metal-hydride and lithium-ion
batteries. Best results are obtained on a single cell.
One of the difficulties of EIS is data interpretation. It is easy to amass a
large amount of data; making practical use of it is more difficult.
Analyzing the information is further complicated by the fact that the
readings are not universal and do not apply equally to all battery makes
and types. Rather, each battery type generates its own set of signatures.
Without well-defined reference readings and software to interpret the
results, gathering information has little meaning for the ordinary person.
Modern technology can help by storing characteristic settings of a given
battery type in the test instrument. Advanced digital signal processors
are able to carry out millions of instructions per second. Software
translates the data into a single reading. EIS has the potential of
becoming a viable alternative to AC conductance in checking automotive,
traction and stationary batteries. Noteworthy advancements are being
made in his field.
Commercializing Electrochemical Impedance Spectroscopy
Cadex is developing a battery rapid test method incorporating EIS based
techniques. Trademarked Spectro™, the system injects sinusoidal signals
at multiple frequencies. The signals are voltage controlled and remain
below the thermal battery voltage.
Spectro™ is being tested on randomly sampled automotive batteries of
various states-of-health conditions. Automotive batteries serve the
purpose well because of easy availability. To demonstrate the accuracy,
we tested six typical automotive batteries (A, B, C, D, E, and F) with
various state-of-health conditions. The batteries are flooded lead acid of
the same model.
Prior to testing, the batteries were fully charged and the actual Cold
Cranking Ampere (CCA) reading was established using standards
developed under SAE J537. The batteries were then re-tested using the
AC conductance and Spectro™ methods. The Spectro™ approximations
were conducted using model-specific matrices.
Figure 2: Comparison readings of CCA and Spectro™ using battery-specific
matrices. The blue markers compare readings with AC conductance. Spectro™
follows the CCA measurements very closely.
Batteries arrive for testing in all conditions, including low state-of-charge
(SoC). With AC conductance, the charge level affects the CCA readings to
such a degree that the test results may become meaningless. To
demonstrate SoC immunity of Spectro™, Spectro was used to estimate
CCA at different charge levels. The results are shown in Figure 3.
Figure 3: CCA
rapid-tests at
various SoC.
Spectro™
provides
robust
readings
from 40100% SoC.
The AC
conductance
readings are
strongly
affected by
the charge
level.
Ideally, the line should be perfectly horizontal. Spectro™ departs only
moderately within the 40-100% SoC range. In comparison, the CCA
approximations using AC conductance show a strong departure from the
horizontal line, caused by the charge level.
Although early test results conducted with the Spectro™ based
technology demonstrate strong advantages over existing test methods,
the electrical requirements and complexities are demanding. Injecting
multi-frequency sinusoidal signals at controlled levels and processing
reams of data will add cost.
Research is continuing to include a broad range of battery sizes and
chemistries, and to reduce the test time from two minutes to about 20
seconds per battery test. Patents for Spectro™ have been applied for.
Why do different test methods provide dissimilar readings?
(BU42A)
During the last 20 years, three basic battery rapid test methods have
emerged: DC load, AC conductance and multi-frequency electro-chemical
impedance spectroscopy (EIS). All methods are resistance based, a
characteristic that reveals the battery's ability to deliver load current.
Internal resistance provides useful information in detecting problems
and indicating when a battery should be replaced. However, the battery
often drops below the critical 80% level set by IEEE before the condition
can effectively be detected. Neither does resistance alone provide a
linear correlation to the battery's capacity. Rather, the increase of cell
resistance relates to aging.
When measuring the internal resistance of brand new VRLA cells from
the same batch, variations of 8% between cells are common.
Manufacturing process and materials used contribute to the
discrepancies. Rather than relying on an absolute resistance reading,
service technicians are asked to take a snapshot of the cell resistances
when the battery is installed and then measure the subtle changes as the
cells age. A 25% increase in resistance over the baseline indicates a
performance drop from 100% to about 80%. Battery manufacturers
honor warranty replacements if the internal resistance increases by
50%.
Before analyzing the different test methods, let's briefly brush up on
internal resistance and impedance, terms that are often used incorrectly
when addressing the conductivity of a battery.
Resistance is purely resistive and has no reactance. There is no trailing
phase shift because the voltage and current are in unison. A heating
element is such a pure resistive load. It works equally well with direct
current (DC) and alternating current (AC).
Most electrical loads, including the battery, contain a component of
reactance. The reactive part of the load varies with frequency. For
example, the capacitive reactance of a capacitor decreases with rising
frequency. A capacitor is an insulator to DC and no current can pass
through. The inductor, on the other hand, acts in the opposite way and
its reactance increases with rising frequency. DC presents an electrical
short. A battery combines ohmic resistance, as well as capacitive and
inductive reactance. The term impedance represents all three types.
The battery may be viewed as a set of electrical elements. Figure 1
illustrates Randles' basic lead-acid battery model in terms of resistors
and a capacitor (R1, R2 and C). The inductive reactance is commonly
omitted because it plays a negligible role in a battery at low frequency.
Figure 1: Randles model of a lead
acid battery.
The overall battery resistance
consists of pure ohmic resistance,
as well as inductive and capacitive
reactance. The values of these
components are different for every
battery tested.
Battery rapid test methods and how they work
Let's now look at the different battery test methods and evaluate their
strengths and limitations. It is important to know that each method
provides a different internal resistance reading when measured on the
same battery. Neither reading is right or wrong. For example, a cell may
read higher resistance readings with the DC load method than with a
1000-hertz AC signal. This simply implies that the battery performs
better on an AC than DC load. Manufacturers accept all variations as long
as the readings are taken with the same type of instrument.
DC load method: The pure ohmic measurement is one of the oldest and
most reliable test methods. The instrument applies a load lasting a few
seconds. The load current ranges from 25-70 amperes, depending on
battery size. The drop in voltage divided by the current provides the
resistance value. The readings are very accurate and repeatable.
Manufacturers claim resistance readings in the 10 micro-ohm range.
During the test, the unit heats up and some cooling will be needed
between measurements on continuous use.
The DC load blends
R1 and R2 of the
Randles model into
one combined
resistor and ignores
the capacitor. C is a
very important
component of a
battery and
represents 1.5
farads per 100 Ah
cell capacity.
Figure 2:DC load method.
The true integrity of the
Randles model cannot be
seen. R1 and R2 appear as
one ohmic value.
AC conductance method: Instead of a DC load, the instrument injects an
AC signal into the battery. A frequency of between 80-100 hertz is
chosen to minimize the reactance. At this frequency, the inductive and
capacitive reactance converges, resulting in a minimal voltage lag.
Manufacturers of AC conductance equipment claim battery resistance
readings to the 50 micro-ohm range. AC conductance gained momentum
in 1992; the instruments are small and do not heat up during use.
The single frequency
technology sees the
components of the
Randles model as
one complex
impedance, called
the modulus of Z.
The majority of the
contribution is
coming from the
conductance of the
first resistor.
Figure 3: AC conductance
method.
The individual
components of the
Randles model cannot be
distinguished and appear
as a blur.
Multi-frequency electro-chemical impedance spectroscopy (EIS): Cadex
Electronics has developed a rapid-test method based on EIS. Called
Spectro™, the instrument injects 24 excitation frequencies ranging from
20-2000 Hertz. The sinusoidal signals are regulated at 10mV/cell to
remain within the thermal battery voltage of lead acid. This allows
consistent readings for small and large batteries..
With multifrequency
impedance
Spectroscopy, all
three resistance
values of the
Randles model can
be established.
A patented process
evaluates the fine
nuances between
each frequency to
enable an in-depth
battery analysis.
Figure 4: Spectro™
method.
R1, R2 and C can be
measured separately,
enabling the estimation
of battery conductivity
and capacity
Spectro™ is the most complex of the three methods. The 20-second test
processes 40 million transactions. The instrument is capable of reading
to a very low micro-ohms level. With stored matrices as reference,
Spectro™ is capable of providing battery capacity in Ah, conductivity
(CCA) and state-of-charge.
The EIS concept is not new. In the past, EIS systems were hooked up to
dedicated computers and diverse laboratory equipment. Trained
electrochemists were required to interpret the data. Advancements in
data analysis automated this process and high-speed signal processors
shrunk the technology into a handheld device.
Capacity measurements
DC load and AC conductance have one major limitation in that these
methods cannot measure capacity. With the growing demand of auxiliary
power on cars and trucks and the need to assess performance of
stationary batteries non-invasively, testers are needed that can estimate
battery capacity. Cadex has succeeded in doing this with car batteries.
The company is working on applying this technology to stationary
batteries.
Figure 5 reveals the reserve capacity (RC) readings of 24 car batteries,
arranged from low to high on the horizontal axis. The batteries were first
tested according to the SAE J537 standard, which includes a full charge,
a rest period and a 25A discharge to 1.75V/cell during which the reserve
capacity was measured (black diamonds). The tests were then repeated
with Spectro™ (purple squares) using battery-specific matrices. The
derived results approach laboratory standards, as the chart reveals
Figure 5: Reserve capacity of 24 batteries with a model-specific matrix.
The black diamonds show capacity readings derived by a 25A discharge;
the purple squares represent the Spectro™ readings.
Some people claim a close relationship between battery conductivity
(ohmic values) and capacity. Others say that internal ohmic readings are
of little practical use and have no relation to capacity. To demonstrate
the relationship between resistance and capacity, Cadex Electronics has
carried out an extensive test involving 175 automotive batteries in which
the cold cranking amps (CCA) were compared with the RC readings. CCA
represents the conductivity of the battery and is closely related with the
internal resistance.
Figure 6 shows the test results. The CCA readings are plotted on the
vertical Y-axis and the RC on the horizontal X-axis. For ease of reading,
the batteries are plotted as a percentage of their nominal value and are
arranged from low-to-high on the X-axis.
Figure 6: CCA as a function of reserve capacity (RC).
Internal resistance (represented by CCA) and capacity do not follow the
red line closely and fail to provide accurate capacity readings.
Note: The CCA and RC readings were obtained according to SAE J537 standards. CCA is defined as
a discharge of a fully charged battery at -18°C at the CCA-rated current. If the voltage remains at
or above 7.2V after 30 seconds, the battery passes. The RC is based on a full charge, rest period
and a discharge at 25A to 1.75V/cell.
If the internal resistance (CCA) were linear with capacity, then the blue diamonds would be in
close proximity of the red reference line. In reality, CCA and RC wander off left and right. For
example, the 90% CCA battery produces an RC of only 38%, whereas the 71% CCA delivers a
whopping 112% capacity (green dotted line).
An important need is fulfilled
Cadex has packaged the EIS technology into an elegant hand-held tester
that is currently being beta-tested in the USA, Canada, Europe and Japan.
(Please visit http://www.cadex.com/prod_testers_ca12.asp)
Being able to obtain battery capacity makes the EIS technology one of
the most sought-after test systems for automotive, marine, aviation,
defense, wheeled mobility, traction and UPS batteries. Capacity fading
due to aging and other deficiencies can be tracked and a timely
replacement scheduled.
____________________________
Created: July 2004, Last edited: October 2004
What causes car batteries to fail? (BU42B)
Driving habits rather than battery defect may be the cause
A German manufacturer of luxury cars reveals that of 400 car batteries
returned under warranty, 200 are working well and have no problem.
Low charge and acid stratification are the most common causes of the
apparent failure. The car manufacturer says that the problem is more
common on large luxury cars offering power-hungry auxiliary options
than on the more basic models.
In Japan, battery failure is the largest complaint among new car owners.
The average car is only driven
13 km (8 miles) per day and mostly in a congested city. As a result, the
batteries will never get fully charged and sulfation occurs. The batteries
in Japanese cars are small and only provide enough power to crank the
engine and perform some rudimentary functions. North America may be
shielded from these battery problems, in part because of long distance
driving.
Good battery performance is important because problems during the
warranty period tarnish customer satisfaction. Any service requirement
during that time is recorded and the number is published in trade
magazines. This data is of great interest among prospective car buyers
throughout the world.
Battery malfunction is seldom caused by a factory defect; driving habits
are the more common culprits. The heavy auxiliary power drawn during a
short distance driven never allows the periodic fully saturated charge
that is so important for the longevity of a lead acid battery. According to
a leading European manufacturer of car batteries, factory defects
amounts to less than 7 percent.
Acid stratification, a problem with luxury cars
A common cause of battery failure is acid stratification. The electrolyte
on a stratified battery concentrates on the bottom, causing the upper
half of the cell to be acid poor. This effect is similar to a cup of coffee in
which the sugar collects on the bottom when the waitress forgets to
bring the stirring spoon. Batteries tend to stratify if kept at low charge
(below 80%) and never have the opportunity to receive a full charge.
Short distance driving while running windshield wiper and electric
heaters contributes to this. Acid stratification reduces the overall
performance of the battery.
Figure 1 illustrates a normal battery in which the acid is equally
distributed form top to bottom. This battery provides good performance
because the correct acid concentration surrounds the plates. Figure 2
shows a stratified battery in which the acid concentration is light on top
and heavy on the bottom. A light acid limits plate activation, promotes
corrosion and reduces performance. High acid concentration on the
bottom, on the other hand, artificially raises the open circuit voltage. The
battery appears fully charged but provides a low CCA. High acid
concentration also promotes sulfation and decreases the already low
conductivity further. If unchecked, such a condition will eventually lead
to battery failure.
Figure 1: Normal battery
The acid is equally distributed from the
top to the bottom in the cell and
provides maximum CCA and capacity.
Figure 2: Stratified battery
The acid concentration is light on top
and heavy on the bottom. High acid
concentration artificially raises the
open circuit voltage. The battery
appears fully charged but has a low
CCA. Excessive acid concentration
induces sulfation on the lower half of
the plates.
Allowing the battery to rest for a few days, applying a shaking motion or
tipping the unit over tends to correct the problem. A topping charge by
which the 12-volt battery is brought up to 16 volts for one to two hours
also reverses the acid stratification. The topping charge also reduces
sulfation caused by high acid concentration. Careful attention is needed
to keep the battery from heating up and losing excessive electrolyte
through hydrogen gassing. Always charge the battery in a well-ventilated
room. Accumulation of hydrogen gas can lead to an explosion. Hydrogen
is odorless and can only be detected with measuring devices.
The challenge of battery testing
During the last 20 years, battery testing lagged behind other
technologies. The reason: the battery is a very difficult animal to test,
short of applying a full charge, discharge and recharge. The battery
behaves similar to us humans. We still don't know why we perform
better on certain days than others.
Even by using highly accurate charge and discharge equipment, lead acid
batteries produce disturbingly high capacity fluctuations on repetitive
measurements. To demonstrate the variations, Cadex tested 91 car
batteries with diverse performance levels (Figure 3). We first prepared
the batteries by giving them a full charge and a 24-hour rest period. We
then measured the capacity by applying a 25A discharge to 10.50V or
1.75V/cell (black diamonds).
This procedure was repeated for a second time and the resulting
capacities were plotted (purple squared). This produced a whooping +/15% variation in capacity readings across the full population. Some
batteries had higher readings the second time; others were lower. Other
chemistries appear to be more consistent in capacity readings than lead
acid.
Figure 3: Capacity fluctuations. Capacities of 91 car batteries measured
with a conventional discharge method show a fluctuation of +/-15%.
From the beginning, load testers have been the standard test method for
car batteries. The year 1992 brought us AC conductance, a method that
simplified battery testing. Now we are experimenting with multi-model
electrochemical impedance spectroscopy (EIS) in a portable version at
an affordable price.
Getting a fast and dependable assessment of a failing battery is difficult.
Most battery testers in use only take cold cranking amps (CCA) and
voltage readings. Capacity, the most important measurement of a
battery, is unavailable. While taking the CCA reading alone is relatively
simple, measuring the capacity is very complex and instruments offering
this feature are expensive.
The Spectro CA-12 by Cadex Electronics is the first in a series of high-end
battery testers capable of measuring capacity, CCA and state-of-charge
(SoC) in a single, non-invasive test. The technology is based on multimodel electrochemical impedance spectroscopy (EIS). The system injects
24 excitation frequencies ranging from 20 to 2000 Hertz. The sinusoidal
signals are regulated at 10mV/cell to remain within the thermal battery
voltage of lead acid. This achieves stable readings for small and large
batteries.
During the 30-second test, over 40 million transactions are completed. A
patented algorithm analyses the data and the final results are displayed
in capacity, CCA and state-of-charge. (For more information, please visit
http://www.cadex.com/prod_testers_ca12.asp.
EIS is very complex and until recently required dedicated computers and
expensive laboratory equipment, not to mention chemists and engineers
to interpret the readings. The hardware of a full EIS system is commonly
mounted on racks and the installation runs into tens of thousands of
dollars.
The tough choice
No battery tester solves all problems. Entry-level testers are low cost,
simple to use and capable of servicing a broad range of batteries.
However, these units only provide a rough indication of the battery
condition. A lab test at Cadex demonstrates that a battery tester based
on EIS is four times more accurate in detecting weak batteries than AC
conductance. Conventional testers often misjudge the battery on account
of low state-of-charge. Many batteries are replaced when they should
have been recharged, while others are given a clean bill of health when it
should have been replaced.
Acid stratification is difficult to measure, even with the EIS technology.
Non-invasive testers simply take a snapshot, average the measurements
and spit out the results. Stratified batteries tend to show higher state-ofcharge readings because of elevated voltage. On preliminary tests, the
Spectro CA-12 also shows slightly higher CCA and capacity readings than
normal. After letting the battery rest, the capacity tends to normalize.
This may be due to diffusion effects in the stratified as a result of resting.
Little information is available on how long a stratified battery needs to
rest to improve the condition, other than to note that higher
temperatures will hasten the diffusion process.
Ideally, a battery tester should indicate the level of acid stratification;
sulfation, surface charge and other such condition and display how to
correct the problem. This feature is not yet possible. Much research is
being done in finding a solution that offers a more complete battery
evaluation without the need for a full discharge. The knowledge gained
on lead acid batteries can then be applied to other battery systems, such
as traction, military, marine, aviation and stationary batteries.
Starting is easy… but can I steer and brake? (BU42C)
A look at emerging technologies capable of estimating battery reserve
capacity
AC conductance testing was introduced in 1992 as a new way of
measuring the cold cranking amps (CCA) of a car battery. This noninvasive method was hailed as a major breakthrough and, to a large
degree, eliminates load testing to measure battery performance. The test
only takes a few seconds; the readings are displayed in digital numbers
and a message spells out the condition of the battery. There are no
sparks at the battery terminals and the instrument remains cool.
But single frequency AC conductance has limitations. It does not
measure CCA according to SAE standards but offers an approximation
relating to the battery's power output capability. This relative power
figure often varies with state-of-charge and other battery conditions. At
times, a good battery fails and a faulty one passes by error. But the most
significant drawback is its inability to read the reserve capacity (RC).
Despite these shortcomings, AC conductance has become an accepted
standard for predicting battery life and determining when to replace an
old battery before it becomes a nuisance.
What is the difference between CCA and RC?
A good battery needs high CCA and high capacity readings but these
attributes reflect differently depending on the application. A high CCA
reading assures good battery conductivity and provides strong cranking
ability. High CCA goes hand-in-hand with a low internal battery
resistance. Figure 1 compares high CCA with a large, open tap that
allows unrestricted flow.
Figure 1: Battery with high CCA and
100% reserve capacity.
A high CCA battery can be compared
to a large, open tap that allows
unrestricted flow.
Reserve capacity governs the amount of energy the battery can store. A
new battery is rated at a nominal capacity of 100%. As the battery ages,
the reserve capacity drops and the battery eventually needs replacing
when the reserve capacity falls below 70%. The RC reading always refers
to a fully charged battery; the state-of-charge (SoC) should not affect the
readings when measured with a rapid-tester.
A battery may provide a good CCA reading and start a car well but be low
on reserve capacity. This battery would be run down in no time when
drawing auxiliary power. Figure 2 illustrates such a battery. The socalled 'rock content' that builds up as the battery ages is permanent and
cannot be reversed.
Figure 2: Battery with high CCA but
low reserve capacity.
The cranking on this battery is good
but running on auxiliary power will
drain the battery quickly.
Figure 3 illustrates a battery with good reserve capacity but low CCA.
This battery has a difficult task turning the starter and needs replacing
even though it could be used for low load applications.
Figure 3: Battery with low CCA but
high reserve capacity.
The low CCA of this battery provides
poor cranking although the reserve
capacity is high.
Capacity measurements, the most comprehensive battery test
With increased demand for auxiliary power on vehicles, measuring
energy reserve is more relevant than CCA. The slogan goes: "Starting is
easy… but can I steer and brake?" Modern battery testers must adapt to
this new requirement and also include RC measurement. European car
manufacturers place heavy emphasis on reserve capacity, while in North
America CCA is still the accepted standard to assess battery
performance. Most modern battery testers also provide state-of-charge
readings (SoC).
Measuring reserve capacity is more complex than CCA. Many methods
have been tried, including multi-frequency conductance, but most have
limitations. One of the main obstacles is processing large volumes of
data received when scanning a battery with multiple frequencies.
Collecting the data is easy; making practical use of the information is the
problem. The cost of high-speed microprocessors and processing
difficulties has put the price tag on such battery testers out of range. Put
changes are coming.
Cadex Electronics has developed a method that enables the processing of
a large volume of data received through multi-model electro-chemical
impedance spectroscopy (EIS). Trademarked Spectro™, the system
injects 24 excitation frequencies ranging from 20 to 2000 Hertz. The
signals are regulated at 10mV to remain within the thermal battery
voltage of lead acid. This permits stable readings for small and large
batteries. The test takes 20 seconds, during which about 40 million
transactions are completed.
Normally, EIS requires dedicated equipment and a computer to analyze
the obtained data. To permit such analyses in a hand held unit, highspeed digital signal processing is used. In 2005, the Spectro™ invention
received a patent (US patent 6,778,913, Jörn Tinnemeyer).
Spectro™ has primarily been demonstrated on 12V lead-acid batteries,
automotive in particular. The large pool of available car batteries
provides an excellent platform to verify the technology. The same
technology can also be used on nickel and lithium-based batteries.
On the strength of our invention, Cadex has developed a battery rapidtester (CA-12) for automotive batteries. One of the strongest features of
Spectro™ is its ability to reveal CCA, reserve capacity and state-of-charge
on a single measurement. Displaying RC has been on the wish list of
battery manufacturers and service centers for many years. In fact, this
will be the first time such information can be obtained non-invasively
with a commercial hand-held tester. Figure 4 shows the suggested
display format.
Figure 4: Displays
CCA, RC and SoC.
During the 20second test time,
the digital signal
processor
completes 40
million
transactions.
The battery needs to be charged for testing. The typical test band is 50%
to 100% SoC. Early tests provide stable results over a wide temperature
range. There is good immunity to electrical noise. Parasitic loads of up to
30A have been tried without notable side effects. Furthermore, Spectro™
appears to be less sensitive to surface charge than single frequency AC
conductance and the CCA readings are more consistent. The tester
tolerates some acid stratification but chemical additives may affect the
readings. Figure 5 illustrates the CA-12 tester.
Figure 5: Rendering of the Cadex CA12 battery rapid-tester.
The test results are available in most
global standards. The RC can be
shown as a percentage of the nominal
capacity or in discharge time.
Early test results on Reserve Capacity
Verifying the accuracy and repeatability of a new invention takes much
time and effort. To test Spectro™, Cadex assembled a test bed of 91 car
batteries with diverse performance levels. The preparation consisted of a
fully saturated charge, followed by a 24-hour rest period and a 25A
discharge to 10.50V (1.75V/cell), during which the reserve capacity was
measured. This procedure produced an astonishing +/-15% variation in
capacity readings across the full population. When comparing the
capacity obtained through a conventional discharge and by non-invasive
means, one must take into account the vulnerability of lead acid in
producing varied readings even when using highly accurate charges and
load banks.
Figure 6 compares the reserve capacities of 38 randomly chosen car
batteries. The black diamonds show the reserve capacity derived through
a full discharge; the purple squares represent Spectro™ estimations
using a generic matrix.
Figure 6: RC
comparison
of 38
batteries
with a
generic
matrix.
The black
diamonds
show the
RC obtained
with a full
discharge;
the purple
squares
represent
Spectro™
estimations.
How can the RC readings be further improved? Best results are achieved
by sorting the batteries according to architecture and CCA rating. We
developed a model specific matrix and tested a group of same-model
batteries. Figure 7 shows the reserve capacity readings derived through
a conventional full discharge and Spectro™. With specific matrices, the
readings approach laboratory standards in terms of accuracy.
Figure 7:
RC
comparison
of 24
batteries
with a
modelspecific
matrix.
The purple
squares
(Spectro™)
followthe
black
diamonds
very
closely.
Specific
matrices
approach
reading
within
laboratory
standards.
Although the test results in Figure 6, and in particular Figure 7, look very
encouraging, we need to be reminded that Spectro™ is not a universal
battery tester capable of measuring any battery that comes along. It
cannot be compared to a photocopier that duplicates any document or
flat object by simply pressing the copy button. Rather, Spectro™ needs a
battery specific matrix as a reference. To a large extent, the quality of
the matrix governs the accuracy. The matrices are stored in the tester
and need selecting together with the Ah and/or CCA rating. We are
currently making gains in establishing generic matrices that may be used
for CCA and RC measurements.
Price is another issue. Because of added complexity and higher parts
count compared to single frequency AC conductance, the Spectro™
technology will command a higher price. We are not competing directly
with currently available battery testers; rather, we offer a solution for
those needing a better technology because the present method may be
insufficient.
Summary
Technology has advanced to a point where measuring battery
performance through non-invasive means will become the acceptable
standard. Applying a full discharge for the purpose of obtaining the
reserve capacity is impractical and stresses the battery. Multi-model
electrochemical impedance spectroscopy with improved data processing
algorithms will bring this task one step closer to reality.
Multi-frequencies EIS not only makes RC estimations possible; it also
improves the CCA readings. Rather than providing a reference numbers
relating to battery conductivity, EIS can give actual CCA equivalents. The
technology also improves state-of-charge estimations. Typical
applications include verifications of battery warranty returns, assessing
the state-of-life of stationary batteries and checking the capacity for
batteries in defense and marine applications. EIS is also an indispensable
tool in examining batteries for wheelchair, golf carts, robots, boats and
forklifts.
___________________
Created: June, 2005
Advanced battery analyzers (BU43)
How are batteries checked and serviced? This article describes the
advancements of the modern battery analyzer and explains how these
instruments are used in the industry. While organizations such as public
safety have been using battery analyzers for the last two decades to
restore and prolong nickel-cadmium batteries, analyzers have made their
way also into the cell phone, portable computing, medical and defense
markets. The early models were impractical and did not adapt well to
changing battery chemistries. In addition, the analyzers provided limited
service and did not offer the quick test results and restoration
capabilities customers demand today.
The last few years have brought a rebirth of the battery analyzer. With
the move from the high-maintenance nickel-based batteries to the
maintenance-free lithium-based packs, the duty of a battery analyzer is
changing from life-extending cycling to rapid testing and boosting.
Fixed current analyzers
There are two basic types of battery analyzers: the fixed current and
programmable versions. Fixed current units are the lower priced of the
two, and charge and discharge a battery at a preset current of about
600mA. Smaller batteries get serviced reasonably fast but larger
batteries are slow. The service time of an 1800mAh battery is three times
that of a 600mAh pack. The capacity readout is in mAh and reflects the
length of discharge. The fixed-current analyzers are the predecessors of
the programmable units.
Programmable analyzers
The programmable analyzers allow servicing the battery against preset
parameters. The charge and discharge currents are adjusted according to
the battery rating, and the voltage is set to flag batteries with incorrect
voltages. These analyzers provide more accurate readings and enable
higher battery throughput than fixed current units. In addition,
programmable analyzers are better suited to service new battery
systems and have proven to be more effective in restoring weak
batteries. The Cadex C7000-Series are such programmable battery
analyzers.
Battery adapters
Interfacing the batteries has always been a challenge with battery
analyzers. Technicians have invented contraptions with springs and
levers so complicated that only they themselves are able to operate.
Everybody else stays away from them of fear.
Cadex solved the battery interface issue with the custom adapters for
common batteries and the universal adapters for specialty packs. The
custom adapters are the easiest to use and provide the most accurate
test results. User-programmable cables accommodate larger batteries or
assist when no adapter is on hand. Smaller batteries can be serviced with
the Cadex FlexArm™. Two contact probes mounted on flexible arms
provide the connection when lowered to the battery terminals. Magnetic
guides keep the battery in position and a temperature sensor safeguards
the battery. Figure 1 illustrates the Cadex FlexArm™.
Figure 1: Cadex FlexArm™.
Snapped into the Cadex 7000-Series
battery analyzers, the FlexArm™
establishes contact by lowering the arms
to the battery. Magnetic guides keep the
battery in position. The FlexArm™ stores
up to 10 battery types, each of which can
be given a unique name.
The Cadex adapters contain a memory chip that configures the analyzer
to the correct setting. Each adapter stores 10 battery configuration codes
to service 10 different battery types. The parameters can be edited with
a few keystrokes on the analyzer's keypad.
Service programs
Advanced battery analyzers are capable of evaluating battery conditions
and implementing corrective service to restore weak performance. The
Cadex system, for example, automatically applies a recondition cycle to
nickel-based packs if a user-selected target capacity cannot be reached.
Other programs include Prime to prepare a new battery for field use,
Charge to allow fast-charge and Custom to apply unique cycles composed
of charge, discharge, recondition, trickle charge or any combination,
including rest periods and repeats.
Many modern analyzers also offer battery rapid test programs. This often
requires entering the battery voltage and rating (in mAh). To obtain
accurate readings, a battery-specific matrix may also be required. The
Cadex QuickTest™ stores the matrix in the battery adapter, together with
the configuration code. Installing the adapter sets the analyzer to the
correct parameters, transparent to the user.
With the Cadex system, the matrix is commonly included when
purchasing the adapter. If missing, scanning several batteries with
various state-of-health conditions creates the matrix. The test time is 3
minutes and requires a charge level of 20-90%. If outside this range, the
analyzer automatically applies a brief charge or discharge.
Many batteries are discarded, even when restoration is possible. Cell
phone dealers have confirmed that 80-90% of returned mobile phone
batteries can be repaired with a battery analyzer. However, most dealers
are not equipped to handle the influx of warranty batteries and the packs
are returned to the manufacturers for replacement or are discarded.
Rapid test enables checking the battery while the customer waits. Minor
battery problems can be corrected on the spot.
A typical failure of lithium-ion batteries is excessive low discharge. If
discharged below 2.5 volts per cell, the internal safety circuit deactivates
and the battery no longer accepts charge with a regular charger. An
excessive low discharge can occur if the battery is not recharged for
some time after a full discharge through extensive use.
The Cadex battery analyzers feature Boost, a program, which reactivates
batteries that appear dead. Boost works by applying a gentle current to
the battery to re-energize the safety circuit and raise the cell voltage.
After reaching the operating voltage, the battery can be charged and
tested normally. Boosted batteries perform flawlessly as long as a repeat
deep discharge is avoided.
Printing
Most analyzers are capable of printing service reports and battery labels.
This feature simplifies maintenance, especially in a fleet environment
where the operators must observe periodic service requirements. Printed
reports also benefit customer service staff and engineers.
Figure 2: Label printer.
The label printer automatically
spits out a label with each
battery serviced. The labels
contain the service date; service
due date, battery capacity and
internal battery resistance.
Labeling the batteries with service date and test results is self-governing
in the sense that the user only picks a properly labeled battery and has
recently been serviced. Batteries with past due service date are
segregated for service. With such a system, the user has full confidence
that the battery will last through the shift, with energy to spare. Weak
batteries are weeded out.
_________________________
Created: February 2003
Computerized battery testing (BU44)
Keeping track of batteries can be difficult, especially when encountering
continuously changing battery types and observing periodic maintenance
needs. To assist, several manufacturers of battery analyzers are offering
software to enable PC interface. While a PC is helpful for battery service,
the available software is often not refined enough to satisfy most market
requirements.
Describing the features of software packages is difficult, if not outright
boring. For this reason, this paper takes BatteryShop™ by Cadex as the
example of a fully functioning, multi-tasking battery maintenance
system. Developed during the last 10 years, BatteryShop™ is a mature
product that meets today's battery service requirements.
BatteryShop™ works in conjunction with Cadex C7000 Series battery
analyzers. Although the analyzers work as stand-alone units, the
software overrides the analyzers when connected to a PC. BatteryShop™
is equally proficient in supporting one analyzer or a fully extended
system of 120 units.
To simplify the service of batteries, BatteryShop™ includes a database of
over 2000 batteries for wireless communications, portable computing,
medical, broadcast and defense. The listing includes the battery
configuration codes, known as C-Codes. The newer battery listings also
contain the QuickTest™ matrix to enable rapid testing. The user can
extend the library by adding new models or downloading the updates
from www.cadex.com. The Internet allows sharing the C-Codes and
QuickTest™ matrices with the global battery community. In addition,
battery manufacturers can prepare and publish the service parameters
before releasing a new battery.
To service a battery, the user selects the battery model from the
database and clicks the mouse. The PC sets the analyzer to the correct
parameters, ready to service the battery. Programming the analyzer by
scanning the battery model is also possible. Figure 1 illustrates a typical
setup with two analyzers.
Figure 1: BatteryShop™
provides a simple, yet
powerful PC-interface to
control and monitor
Cadex battery analyzers.
The Internet allows the
use of common test
parameters and sharing
of test results. The
software accommodates
up to 128 battery
analyzers.
Software is commonly written to accommodate a broad range of
applications, even though a customer may never use more than one
function. BatteryShop™ is designed to service cell phone batteries at
storefronts, check batteries in large repair centers, assist in the
scheduled maintenance of fleet batteries, and tend to engineers in
research labs. Here are a few examples how the PC software can be
used:
Cellular dealers
When testing a cell phone battery at point-of-sale with BatteryShop™,
the service clerk selects the battery from the database, clicks the mouse
and connects the pack. To simplify the selection, photo images of the
pack can also be shown on the monitor. The service programs range from
QuickTest™ to check a battery in three minutes, Boost to wake up a
seemingly dead battery, Prime to prepare a new battery and verify its
performance, Charge to fast-charge, and Auto to exercise and recondition
a battery. Systems such as these prevent the liberal replacement of
batteries returned by frivolous customers who complain about
reoccurring problems with a handset.
Battery software can also be integrated into rapid test stations capable
of examining the basic functions of a complete cell phone at storefronts.
Willtek Communications offers such a system. With the use of these test
instruments, only handsets with genuine problems are sent in for repair.
Cell phone dealers have indicated that less that 20% of cell phones
brought in for service have real problems with the handset or the
battery. The cost savings by pre-testing the handsets and batteries are
considerable.
Cellular service Centers
Not all batteries are tested at point-of-sale. Many cell phone
manufactures use strategically placed service centers to repair handsets
and batteries. A large service center may repair as many as 50,000 units
per month. The database for such an organization can be tailored to
include only those batteries that are being handled by that organization.
The test results can be used for statistical analyses or sent to a central
location for evaluation. Enabling access to vital test information allows
battery manufacturers to correct recurring battery problems quickly and
effectively.
Battery fleet owners
Service software is especially helpful in tracking the maintenance
requirements of fleet batteries. All packs are first marked with a unique
battery ID number. The ID number is printed on a label in bar code
format and permanently attached to the battery. BatteryShop™
generates these labels. To perform the scheduled maintenance, the user
scans the battery ID and the PC automatically configures to the analyzer
to the correct setting. On completion of the service, the battery test
results are stored in the database under the assigned battery ID number.
All references to the battery in terms of vendor information, purchase
date, custodian, maintenance schedules, performance history and
planned replacement are available with a click of the mouse.
Manufacturers and pack assemblers
Battery service software assists battery manufacturers and pack
assemblers in terms of life cycle testing, batch checking for quality
control and verifying warranty claims. Chemistry, voltage and current
setting can be entered manually through the PC. Charge and discharge
voltages are displayed in real-time graphics. The graphs also include
battery temperature and internal resistance readings that are recoded
during charge and discharge cycles. All test data can be stored for future
reference.
_________________________
Created: February 2003
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