Sep 2005 Linear Charger for Nickel Cadmium or Nickel Metal

Sep 2005 Linear Charger for Nickel Cadmium or Nickel Metal
DESIGN IDEAS
Linear Charger for Nickel Cadmium
or Nickel Metal Hydride Batteries
by Fran Hoffart
Minimizes Parts Count
Introduction
A Primer on Charging
NiCd/NiMH Batteries
The various methods for charging
Nickel based batteries are categorized
by speed: slow, quick and fast. The
simplest type of charger is a slow
charger, which applies a timer-controlled, relatively low charge current
for about 14 hours. This may be too
long for many portable applications.
For shorter charge times, quick and
fast chargers apply a constant current
while monitoring the battery voltage
and/or temperature to determine
when to terminate or stop the charge
cycle. Charge times typically range
anywhere from 3 to 4 hours (quick
charge) to around 0.75–1.5 hours
(fast charge).
Fast and quick chargers force a
constant charge current and allow
the battery voltage to rise to the level
it requires (within limits) to force this
current. During the charge cycle, the
charger measures the battery voltage
at regular intervals to determine when
to terminate the charge cycle. During
the charge cycle, the battery voltage
Linear Technology Magazine • September 2005
6.5
6.4
BATTERY VOLTAGE (V)
Although rechargeable Lithium Ion
and Lithium Polymer batteries have
lately been the battery of choice in high
performance portable products, the
old workhorse nickel cadmium (NiCd)
and the newer nickel metal hydride
(NiMH) batteries are still important
sources of portable power. Nickel
based batteries are robust, capable
of high discharge rates, good cycle life
and are relatively inexpensive. NiMH
batteries are replacing NiCd in many
applications because of the higher
capacity ratings (40 to 50% higher) and
because of environmental concerns of
the cadmium contained in NiCd cells.
This article covers NiCd/NiMH battery
charging basics, and introduces the
LTC4060 linear battery charger.
–∆V
6.3
6.2
BATTERY
VOLTAGE
6.1
6.0
CHARGE CURRENT
5.9
2A
1A
5.8
5.7
0
10
20
30 40 50 60
CHARGE TIME (MIN)
70
80
Figure 1. Typical charge profile for a
2000mAHr NiMH 4-cell battery pack
charged at a 1C rate.
rises as it accepts charge (see Figure 1).
Near the end of the charge cycle, the
battery voltage begins to rise much
faster, reach a peak, then begins to fall.
When the battery voltage has dropped
a fixed number of mV from the peak
DESIGN IDEAS
Linear Charger for Nickel Cadmium
or Nickel Metal Hydride Batteries
Minimizes Parts Count ......................35
Fran Hoffart
Determine the Real Internal
Resistance of a Battery .....................38
Jim Williams
Digitally Programmable Output
Monolithic Buck Regulator with
Built-In DAC and I2C Interface...........39
Earl Barber
Connect High Impedance Sensors
Directly to an Easy Drive™
Delta Sigma ADC ...............................40
Mark Thoren
Dual Switching Converter Provides
Two Outputs of Any Polarity .............41
Jesus Rosales
Micropower SOT-23 Inverting DC/DC
Converter Extends Battery Life in
Space-Sensitive Applications ............42
Eric Young
OLED Driver with Output Disconnect
and Automatic Burst Mode Improves
Standby Mode Efficiency ...................44
David Kim
(–ΔV), the battery is fully charged and
the charge cycle ends.
The battery has an internal safeguard against overcharge. While the
cell voltage is dropping from its peak,
the battery temperature and internal
pressure quickly rise. If fast charging
continues for a significant amount of
time after full charge is reached, the
battery pressure seal may momentarily open causing gas to vent. This
is not necessarily catastrophic for the
battery, but when a cell vents, some
electrolyte is also released. If venting
occurs often, the cell will eventually
fail. In addition, after venting, the
seal may not close correctly and the
electrolyte can dry out.
Differences Between
NiCd and NiMH Batteries
The open circuit voltage (nominal 1.2V)
and the end-of-life voltage (0.9V to 1V)
are almost identical between the two
battery types, but the charging characteristics differ somewhat. All NiCd cells
can be trickle charged continuously,
but some NiMH cells cannot, and may
be damaged if the trickle charge is
continued after reaching full charge.
Also, the battery voltage profile during
a fast charge cycle differs between the
two battery types.
For NiMH cells, the decrease in
battery voltage (–ΔV) after reaching a
peak is approximately one half that
of NiCd cells, thus making charge
termination based on –ΔV slightly
more difficult. In addition, the NiMH
battery temperature rise during the
charge cycle is higher than NiCd,
and the higher temperature further
reduces the amount of –ΔV that occurs when full charge is reached. For
NiMH cells, –ΔV is almost non-existent at high temperatures for charge
rates less than C/2. (See sidebar for
the definition of “C”). Older batteries
35
DESIGN IDEAS
and cell mismatching further reduce
the already minute drops in battery
voltage.
Other differences between the two
chemistries include higher energy
density and greatly reduced voltage
depression or “memory effect” for NiMH
cells, although NiCd is still preferred
for high current drain applications.
NiCd cells also enjoy lower self-discharge characteristics, but NiMH
technology has room to improve in
this regard, while NiCd technology is
fairly mature.
The LTC4060 NiCd/NiMH
Battery Charger Controller
The LTC4060 is a complete NiCd or
NiMH linear battery charger controller
that provides a constant charge current and charge termination for fast
charging up to four series-connected
cells. Simple to use and requiring a
minimum of external components, the
IC drives an inexpensive external PNP
transistor to provide charge current.
The basic configuration requires only
five external components, although
additional functions are included such
as, NTC input for battery temperature
qualification, adjustable recharge voltage, status outputs capable of driving
an LED and shutdown and pause inputs. Selecting the battery chemistry
and the number of cells to charge is accomplished by strapping pins, and the
charge current is programmed using a
standard value resistor. With adequate
thermal management, charge current
up to 2A is possible, and even higher
current when using an external current sense resistor in parallel with the
internal sense resistor.
What’s Important When
Designing a Charger
Using the LTC4060?
Once the battery chemistry and
number of cells are set, one must
determine the correct charge current.
The LTC4060 is designed for fast
charging nickel-based batteries and
uses –ΔV as the charge termination
method. Battery temperature can
also be monitored to avoid excessive
battery temperature during charging,
and a safety timer shuts down the
36
About Battery Capacity and Charge Current
The correct charge current is always related to a battery’s capacity, or simply “C”. The letter “C” is a term used to indicate the manufacturers stated
battery discharge capacity, which is measured in mA • Hr. For example, a
2000mAHr rated battery can supply a 2000mA load for one hour before the
cell voltage drops to 0.9V or zero capacity. In the same example, charging
the same battery at a C/2 rate would mean charging at 1000mA (1A).
The correct charge current for fast charging NiCd or NiMH batteries is between approximately C/2 and 2C. This current level is needed for the cell
to exhibit the required –ΔV inflection that occurs when the cell reaches full
charge, although charging at 2C may cause excessive battery temperature
rise, especially with small, high capacity NiMH cells. Because of chemical
differences between the two battery chemistries, NiMH cells generate more
heat when fast charging.
near the battery to pause the charge
cycle allowing the battery to cool down
before resuming the charge cycle.
charger if charge termination does
not occur. The typical fast charge voltage profile (the rapid rise, then drop
in battery voltage (–ΔV) near the end
of the charge cycle) only occurs at a
relatively high charge current. If the
charge current is too low, the battery
voltage does not produce the required
drop in battery voltage after reaching
a peak, which is necessary for the
LTC4060 to terminate the charge cycle.
At very low charge current, –ΔV does
not occur at all. On the other hand,
if the charge current is too high, the
battery may become excessively hot
requiring an NTC thermistor located
A Typical LTC4060
Charge Cycle
With sufficient input voltage applied,
no battery connected and the correct charge current, charge time and
thermistor connections in place, the
charger’s output voltage is very close
to the input voltage. Connecting a
discharged battery to the charger pulls
down the charger’s output voltage
below 1.9 • VCELL (VCELL is the total
battery voltage divided by the number
OPTIONAL POWER PATH
COMPONENTS
VIN
9V TO 10V
LED
1µF
LED
1k
1k
4.42k
14
VCC
15
11
CHRG
NTC
LTC4060
13
3
ACPR SENSE
100k
DRIVE
5
OFF ON
INPUTS
ON PAUSE
6
68k
BAT
1
D2
Q2
THERMISTOR
10k
Q1
ICHRG
2
10µF
10V
PAUSE
+
4 CELLS
NiMH
2000mAhr
LTC4060
10
9
4
PROGRAMS
1.5nF
SAFETY TIMER
SHDN
SYSTEM
LOAD
D1
SEL1
PROG
7
158Ω
SEL0
TIMER ARCT
CHEM
12
1.5V
GND
16
8
TOTAL RESISTANCE
PROGRAMS CONSTANT
CHARGE CURRENT
1.18V
590Ω
PROGRAMS RECHARGE
THRESHOLD VOLTAGE
Q1: MJD210 (MUST BE SOLDERED TO GENEROUS AMOUNTS OF COPPER)
Q2: FDN306P
D1: B220A
D2: MBRM120LT3
Figure 2. 4-cell 2A NiMH battery charger with NTC thermistor and power path control
Linear Technology Magazine • September 2005
DESIGN IDEAS
of cells being charged) thus starting
a charge cycle.
If the battery temperature, as
measured by the NTC thermistor, is
outside a 5°C to 45°C window, the
charge cycle pauses and no charge
current flows until an acceptable
temperature is reached. When the
battery temperature is within limits,
the battery voltage is measured and
must be below the max limit.
If VCELL is below 900mV, the charger begins a trickle charge of 20% of the
programmed charge current until the
voltage exceeds 900mV, at which point
the full programmed charge current
begins. Several hundred milliseconds
after the charge cycle begins, if the
battery voltage exceeds 1.95V, the
charge cycle stops. This overvoltage
condition usually means the battery
is defective requiring that the charger
be manually reset by replacing the
battery, toggling the shutdown pin, or
removing and reapplying power.
Once the programmed constant
charge current starts flowing, a period
of time known as “hold-off-time” begins. This hold-off-time ranges from 4
minutes to 15 minutes depending on
the charge current and charge time
settings. During the hold off time,
the –ΔV termination is disabled to
prevent false charge termination. A
battery that is deeply discharged or
has not been charged recently may
exhibit a drop in battery voltage during
the early portion of the charge cycle,
which could be mistaken for a valid
–ΔV termination.
During the charge cycle, the battery voltage slowly rises. When the
battery approaches full charge, the
battery voltage begins to rise faster,
reaches a peak, then begins to drop.
The charger continuously samples the
For further information on any
of the devices mentioned in this
issue of Linear Technology, use
the reader service card or call the
LTC literature service number:
1-800-4-LINEAR
Ask for the pertinent data sheets
and Application Notes.
Linear Technology Magazine • September 2005
battery voltage every 15 to 40 seconds,
depending on charge current and timer
settings. If each sampled voltage reading is less than the previous reading,
for four consecutive readings, and the
total drop in battery voltage exceeds
8mV/cell for NiMH or 16mV/cell for
NiCd, the charge current stops, ending the charge cycle. The open drain
output pin “CHRG”, which was pulled
low during the charge cycle, now be-
The LTC4060 is a complete
NiCd or NiMH linear
battery charger controller
that provides a constant
charge current and charge
termination for fast
charging up to four seriesconnected cells.
comes high impedance.
A user programmable recharge
feature starts a new charge cycle if
the battery voltage drops below a set
voltage level because of self-discharge
or a load on the battery. Also, if a fully
charged battery greater than 1.3V is
connected to the charger, the –ΔV termination detection circuit is enabled
immediately with no hold-off-time,
thus shortening the charge cycle for
a battery that is already close to full
charge.
If the battery reaches approximately
55°C during the charge cycle, the
charger pauses until the temperature
drops to 45°C, then resumes charging
until the –ΔV termination ends the
charge cycle. If no –ΔV termination
takes place, the safety timer stops
the charge cycle. If the timer stops the
charge cycle, it is considered a fault
condition and the charger must be
reset by removing and replacing the
battery, toggling the SHDN pin or toggling the input power to the charger.
Watch Out for These Pitfalls
Don’t connect a load directly to the
battery when charging. The charge
current must remain relatively constant for the –ΔV charge termination
to be effective. Loads with changing
current levels result in small changes
in battery voltage which can trigger
a false –ΔV charge termination. For
applications that require a load, refer
to the power path components shown
in Figure 2. When the input voltage is
present, the load is powered from the
input supply through Schottky diode
D1 and the battery is isolated from
the load. Removing the input voltage
pulls the gate of Q2 low, turning it on
providing a low resistance current path
between the battery and the load.
Minimize the DC resistance between the charger and the battery.
Some battery holders have springs
and contacts that have excessive resistance. The increased resistance in
series with the battery can prevent a
charge cycle from starting because of
a battery overvoltage condition once
the full charge current begins. Poorly
constructed battery holders can also
produce false charge termination if
battery movement generates a premature –ΔV reading.
Unlike Lithium Ion cells that can be
paralleled for increased capacity, NiCd
or NiMH cells should not be paralleled,
especially when fast charging. Interaction between the cells prevents proper
charge termination. If more capacity
is required, select larger cells.
Not all NiCd or NiMH batteries
behave the same when charging.
Manufacturers differ in materials and
construction resulting in somewhat
different charge voltage profiles or
amount of heat generated. A battery
can be designed for general purpose
use, or optimized for high capacity,
fast charge rate, or high temperature
operation. Some batteries may not be
designed for high current (2C) charge
rates resulting in high cell temperature when charging. Also, most new
cells are not completely formed and
require some conditioning before they
reach their rated capacity. Conditioning consists of multiple charge and
discharge cycles.
A thermistor mounted near the battery pack, preferably making contact
with one or more of the cells, is highly
recommended, both as a safety feature
and to increase battery lifetime. Unlike
continued on page 43
37
DESIGN IDEAS
C2
0.1µF
D2
VIN
C4
1µF
D1
SW
VOUT1
–15V
5mA
D
LT3483
CIN
4.7µF
6.3V
1.5M
SHDN
VOUT2
15V
5mA
70
EFFICIENCY (%)
C1
0.1µF
L1
10µF
VIN
2.7V TO
4.2V
75
D3
60
C3
1µF
FB
55
0.01
GND
Figure 3. Compact, high efficiency LCD power supply yields 5mA at ±15V in less than 90mm2.
lead-acid battery as a standby power
supply. Figure 5 shows the LT3483
in a robust step-down backup supply, which uses a small, low profile
1:1 coupled inductor in an inverting
fly-back configuration.
One of the strengths of the
LT3483 is its versatility. It
can be used for inverting
step-up or for inverting stepdown applications.
–5V at 100mA from 12V
The LT3483 can also regulate a negative output voltage that is smaller in
magnitude than the input voltage,
useful for systems that employ a 12V
The LT3483 can be always active,
ready if primary power fails, drawing
only 45µA from the battery. If the
normal power supply fails, the backup
L1A
10µH
•
The LT3483 provides a very compact,
low quiescent current step-up or stepdown DC/DC inverter solution for a
wide input voltage range of 2.5V to 16V
and outputs to –38V, making it a good
fit for a variety of portable or battery
backup applications.
75
70
•
SW
C1
4.7µF
Conclusion
L1B
10µH
VIN
VOUT
–5V
D
LT3483
22pF
511k
FB
SHDN
GND
C2
10µF
C1: TAIYO YUDEN EMK316BJ475ML
C2: TAIYO YUDEN JMK316BJ106ML
L1A, L1B: WURTH 744876100
Figure 5. –5V step-up/step-down converter
LTC4060, continued from page 37
Lithium Ion batteries that exhibit very
little temperature rise when charging,
Nickel based batteries will heat up during the charge cycle, especially NiMH
batteries. Minimizing the length of time
the battery is exposed to elevated temperature extends battery lifetime.
Linear Technology Magazine • September 2005
10
circuit using the LT3483 immediately
delivers up to 100mA at –5V. In the dual
inductor configuration, the LT3483
is also protected against grounding
of the output. A proprietary current
limiting scheme prevents the buildup
of excessive switching currents which
could cause damage to components in
the power path.
Conclusion
NiCd and NiMH batteries are ideal
sources of rechargeable power for
many portable products and backup
applications. This article helps to
familiarize the user with some of the
charging characteristics of nickel
EFFICIENCY (%)
VIN
2.5V TO 16V
1
0.1
LOAD CURRENT (mA)
Figure 4. Efficiency of ±15V
converter at VIN = 3.6V.
C1, C2: TAIYO YUDEN UMK212BJ104KG
C3, C4: TAIYO YUDEN TMK212BJ105KG
D1, D2, D3: PHILIPS PMEG2005EB
L1: MURATA LQH2MCN100
board space. The additional components for the charge pump are offset
by the internal feedback resistor and
integrated Schottky diode. During
shutdown, both the positive and negative loads are disconnected from the
battery, which increases battery run
time. Switching with no load, the circuit draws 135µA from a 3.6V supply.
The advantages offered by this circuit
are low quiescent current, low parts
count, and small footprint.
65
VIN = 5V
65
VIN = 12V
60
55
0.1
1
10
LOAD CURRENT (mA)
100
Figure 6. Efficiency of –5V
step-up/step-down converter
based batteries and how they apply
to the LTC4060 charger. Charging
NiCd and NiMH batteries correctly and
safely is simplified using the LTC4060
linear battery charger controller.
43
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