null  null
Nickel-Metal Hydride
Page 1 of 36
Application Manual
Mobility is increasingly viewed as an essential attribute of today’s lifestyles, both
personal and professional. Advanced electronic devices such as cellular phones and
portable computers now permit people on the go to operate more effectively than was
possible in home and office-bound environments of a generation ago. But the price of
mobility has been increasing demands and dependence on portable power sources.
Fortunately, with the development of new nickel-metal hydride (NiMH) battery options,
improvements in electronics have now been matched by significant improvements in the
batteries that power them. Nickel-metal hydride battery cells provide more power (in
equivalently sized packages) than nickel-cadmium (NiCd) cells while also eliminating
some of the concerns over use of heavy metals in the cells.
This manual provides an introduction to this exciting new battery technology while
presenting recommendations for use of nickel-metal hydride cells that will provide
optimum results in battery-powered products.
Advantages of the Nickel-Metal Hydride Cell
The three major benefits of the nickel-metal hydride cells to designers of portable
electrical and electronic products are:
Improved energy density (up to 40 percent greater than nickel-cadmium cells) which
can be translated into either longer run times from existing batteries or reductions in
the space necessary for the battery.
Elimination of the constraints on cell manufacture, usage, and disposal imposed
because of concerns over cadmium toxicity.
Simplified incorporation into products currently using nickel cadmium cells because of
the many design similarities between the two chemistries.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 2 of 36
Typical Applications
The nickel-metal hydride cell is currently finding widespread application in those highend portable electronic products where battery performance parameters, notably run
time, are a major consideration in the purchase decision. First adoption of the nickelmetal hydride cell has occurred in two markets, cellular phones and portable computers,
which are growing dramatically thanks to significant reductions in weight and volume
coupled with major improvements in performance. Examples of the range of products
currently powered by nickel-metal hydride batteries are shown in Figure 1.
Penetration of the nickel-metal hydride cell technology has been strongest in premium
electronic products that require premium performance. As production volumes increase
and the cell technology and production processes mature, nickel-metal hydride cells are
expected to compete aggressively with nickel-cadmium cells in most markets with the
possible exceptions of the very high discharge rate and high temperature specialty
Comparison of NiMH and NiCd Cells
Nickel-metal hydride cells are essentially an extension of the proven sealed nickelcadmium cell technology with the substitution of a hydrogen-absorbing negative
electrode for the cadmium-based electrode. While this substitution increases the cell
electrical capacity (measured in ampere-hours) for a given weight and volume and
eliminates the cadmium which raises toxicity concerns, the remainder of the nickelmetal hydride cell is quite similar to the nickel-cadmium product. Many application
parameters are little changed between the two cell types, and replacement of nickelcadmium cells in a battery with nickel-metal hydride cells usually involves few significant
design issues. Table 1 compares key design features between the two cell chemistries.
Table 1 - Summary Comparison of Nickel-Metal Hydride Application Features.
Application Feature
Comparison of Nickel-Metal Hydride to NickelCadmium Batteries
Nominal Voltage
Same (1.25V)
Discharge Capacity
NiMH up to 40% greater than NiCd
Discharge Profile
Discharge Cutoff Voltages
High Rate Discharge Capability
Effectively the same rates
High Temperature (>35oC) Discharge Capability
NiMH slightly better than standard NiCd cells
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 3 of 36
Charging Process
Generally similar; multiple-step constant current
with overcharge control recommended for fast
charging NiMH
Charge Termination Techniques
Generally similar but NiMH transitions are more
subtle. Backup temperature termination
Operating Temperature Limits
Similar, but with NiMH, cold temperature charge
limit is 15oC.
Self-Discharge Rate
NiMH slightly higher than NiCd
Cycle Life
Generally similar, but NiMH is more application
Mechanical Fit
Mechanical Properties
Selection of Sizes/Shapes/Capacities
NiMH product line more limited
Handling Issues
Environmental Issues
Reduced with NiMH because of elimination of
cadmium toxicity concerns.
The nickel-metal hydride cell chemistry is a hybrid of the proven positive electrode
chemistry of the sealed nickel-cadmium cell with the energy storage features of metal
alloys developed for advanced hydrogen energy storage concepts. This heritage in a
positive-limited cell design results in batteries providing enhanced capacities while
retaining the well-characterized electrical and physical design features of the sealed
nickel-cadmium cell design.
The electrochemistry of the nickel-metal hydride cell is generally represented by the
following charge and discharge reactions:
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 4 of 36
At the negative electrode, in the presence of the alloy and with an electrical potential
applied, the water in the electrolyte is decomposed into hydrogen atoms, which are
absorbed into the alloy, and hydroxyl ions as indicated below.
Alloy + H2O + e` ⇔ Alloy (H) + OH`
At the positive electrode, the charge reaction is based on the oxidation of nickel
hydroxide just as it is in the nickel-cadmium couple.
Ni(OH)2 + OH` ⇔ NiOOH + H2O + e`
At the negative electrode, the hydrogen is desorbed and combines with a hydroxyl ion
to form water while also contributing an electron to the circuit.
Alloy (H) + OH` ⇔ Alloy + H2O + e`
At the positive electrode, nickel oxyhydroxide is reduced to its lower valence state,
nickel hydroxide.
NiOOH + H2O + e` ⇔ Ni(OH)2 + OH`
Cell Components
Nickel-metal hydride cells, with the exception of the negative electrode, use the same
general types of components as the sealed nickel-cadmium cell.
Negative Electrode
The basic concept of the nickel-metal hydride cell negative electrode emanated from
research on the storage of hydrogen for use as an alternative energy source in the
1970s. Certain metallic alloys were observed to form hydrides that could capture (and
release) hydrogen in volumes up to nearly a thousand times their own volume. By
careful selection of the alloy constituents and proportions, the thermodynamics could be
balanced to permit the absorption and release process to proceed at room temperatures
and pressures. The general result is shown schematically in Figure 2 where the much
smaller hydrogen atom is shown absorbed into the interstices of a bimetallic alloy
crystal structure.
Two general classes of metallic alloys have been identified as possessing
characteristics desirable for battery cell use. These are rare earth/nickel alloys generally
based around LaNi5 (the so-called AB5 class of alloys) and alloys consisting primarily of
titanium and zirconium (designated as AB2 alloys). In both cases, some fraction of the
base metals is often replaced with other metallic elements. The AB5 formulation
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 5 of 36
appears to offer the best set of features for commercial nickel-metal hydride cell
The metal hydride electrode has a theoretical capacity approximately 40 percent higher
than the cadmium electrode in a nickel-cadmium couple. As a result, nickel-metal
hydride cells provide energy densities that are 20-40 percent higher than the equivalent
nickel-cadmium cell.
Figure 2.
Schematic of Metal-Alloy Crystal Structure Within Nickel-Metal Hydride Negative Electrode
Positive Electrode
The nickel-metal hydride positive electrode design draws heavily on experience with
nickel-cadmium electrodes. Electrodes that are economical and rugged exhibiting
excellent high-rate performance, long cycle life, and good capacity include pasted and
sintered-type positive electrodes.
The balance between the positive and negative electrodes is adjusted so that the cell is
always positive-limited as illustrated in Figure 3. This means that the negative electrode
possesses a greater capacity than the positive. The positive will reach full capacity first
as the cell is charged. It then will generate oxygen gas that diffuses to the negative
electrode where it is recombined. This oxygen cycle is a highly efficient way of handling
moderate overcharge currents.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 6 of 36
Figure 3.
Relative Electrode Balances for Nickel-Metal Hydride Cell During Discharge/Charge/Overcharge
The electrolyte used in the nickel-metal hydride cell is alkaline, a dilute solution of
potassium hydroxide containing other minor constituents to enhance cell performance.
The baseline material for the separator, which provides electrical isolation between the
electrodes while still allowing efficient ionic diffusion between them, is a nylon blend
similar to that currently used in many nickel-cadmium cells.
Cell Construction
The nickel-metal hydride couple lends itself to the wound construction shown in Figure
4, which is similar to that used by present-day cylindrical nickel-cadmium cells. The
basic components consist of the positive and negative electrodes insulated by
separators. The sandwiched electrodes are wound together and inserted into a metallic
can that is sealed after injection of a small amount of electrolyte.
In variation of this design, nickel-metal hydride cells are also being produced in
prismatic versions such as that illustrated in Figure 5. The prismatic cells may fit more
easily into volume-critical applications.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 7 of 36
Figure 4.
Schematic of Cylindrical Cell Construction
The general internal construction of the prismatic cell is similar to the cylindrical cell
except the single positive and negative electrodes are now replaced by multiple
electrode sets. Thus the trade-off for improved packaging in select applications is
increased complexity in cell assembly with the corresponding increases in production
Both cylindrical and prismatic nickel-metal hydride cells are typically two-piece sealed
designs with metallic cases and tops that are electrically insulated from each other. The
case serves, as the negative terminal for the cell while the top is the positive terminal.
Figure 5.
Schematic of Prismatic Cell Construction
Some finished cell designs may use a plastic insulating wrapper shrunk over the case to
provide electrical isolation between cells in typical battery applications.
Nickel-metal hydride cells contain a resealable safety vent built into the top, as
illustrated in Figure 6. The nickel-metal hydride cell is designed so the oxygen
recombination cycle described earlier is capable of recombining gases formed during
overcharge under normal operating conditions, thus maintaining pressure equilibrium
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 8 of 36
within the cell. However, in cases of charger failure or improper cell/charger design for
the operating environment, it is possible that oxygen, or even hydrogen, will be
generated faster than it can be recombined. In such cases the safety vent will open to
reduce the pressure and prevent cell rupture. The vent reseals once the pressure is
Figure 6.
Schematic of Resealable Vent Mechanism
The discharge behavior of the nickel-metal hydride cell is generally well suited to the
needs of today’s electronic products - especially those requiring a stable voltage for
extended periods of operations.
Definitions of Capacity
The principal battery parameter of interest to a product designer is usually the run time
available under a specified equipment use profile. While establishing actual run times in
the product is vital prior to final adoption of a design; battery screening and initial design
are often performed using rated capacities. Designers should thoroughly understand the
conditions under which a cell rating is established and the impact of differences in rating
conditions on projected performance.
The standard cell rating, often abbreviated as C, is the capacity obtained from a new,
but thoroughly conditioned cell subjected to a constant-current discharge at room
temperature after being optimally charged. Since cell capacity varies inversely with the
discharge rate, capacity ratings depend on the discharge rate used. For nickel-metal
hydride cells, the rated capacity is normally determined at a discharge rate that fully
depletes the cell in five hours.
The published C value may reflect either an average or minimum value for all cells.
Typically nickel-cadmium cells are rated based on minimum values while nickel-metal
hydride cells are rated on average values. The difference between the two values may
be significant (~ 10 percent) depending on the variability in the manufacturing process.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 9 of 36
Many charge and discharge parameters are normalized by the C rate since cell
performance within a family of varying cell sizes and capacities is often identical when
compared on the C basis.
Equivalent Circuit
For purposes of electrical analysis of the battery cell, the Thevenin equivalent discharge
circuit shown in Figure 7 is often used. This models the circuit as a series combination
of a voltage source (Eo), a series resistance (Rh = the effective instantaneous
resistance), and the parallel combination of a capacitor (Cp = the effective parallel
capacitance) and a resistor (Rd = the effective delayed resistance).
Figure 7.
Equivalent Discharge Circuit for a Nickel-Metal Hydride Cell
Eo = effective cell no-load voltage
Re = (Rh + Rd) = total effective internal resistance
Rh = effective instantaneous resistance
Rd = effective delayed resistance
Cp = effective parallel capacitance
E = cell termination voltage
For steady state purposes, the cell voltage at a given current is Eo - iRe, where Re, the
effective internal resistance, is the sum of Rh and Rd. The transient response is shown
in Figure 8 where the initial voltage drops immediately to Eo - iReh and then transfers
exponentially (with a time constant = Cp *Rd) to the steady-state voltage. Obviously the
process reverses when the load is reduced or removed. For many applications the
steady-state voltage is adequate for describing cell performance since the time constant
for most cells is small: usually less than 3 percent of the discharge time. When
compared to a nickel-cadmium cell, the steady-state voltage for the nickel-metal hydride
cell will be reduced since, although the instantaneous resistance is comparable, the
delayed resistance will be on the order of 10 percent higher.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 10 of 36
Figure 8.
Example of Transient Voltage Profile for a Nickel-Metal Hydride Cell
Voltage During Discharge
The discharge voltage profile, in addition to the transient effects discussed above, is
affected by environmental conditions, notably discharge temperature and discharge
rate. However, under most conditions the voltage curve retains the flat plateau desirable
for electronics applications.
Shape of Discharge Curve
A typical discharge profile for a cell discharged at the 5-hour rate (the 0.2C rate) is
shown in Figure 9. The initial drop from an open-circuit voltage of approximately 1.4
volts to the 1.2 volt plateau occurs rapidly.
Figure 9.
Typical Discharge Voltage Profile for a Nickel-Metal Hydride Cell
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 11 of 36
Then, as with nickel-cadmium cells, the nickel-metal hydride cell exhibits a sharp "knee"
at the end of the discharge where the voltage drops quickly.
As can be seen by the flatness of the plateau and the symmetry of the curve, the midpoint voltage (MPV - the voltage when 50 percent of the available capacity is
discharged) provides a useful approximation to average voltage throughout the
Environmental Effects
The principal environmental influences on the location and shape of the voltage profile
are the discharge temperature and discharge rate.
As indicated in Figure 10, small variations from room temperature (± 10oC) do not
appreciably affect the nickel-metal hydride cell voltage profile. However major
excursions, especially lower temperatures, will reduce the mid-point voltage while
maintaining the general shape of the voltage profile.
Figure 10.
Mid-Point Voltage Variation with Temperature
Discharge Rate
The effect of discharge rate on voltage profile is shown in Figure 11. There is no
significant effect on the shape of the discharge curves for rates under 1C; for rates over
1C; both the beginning and ending transients consume a larger portion of the discharge
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 12 of 36
Figure 11.
Voltage Profile Variation with Discharge Rate
Discharge Capacity Behavior
As with the voltage profile, the capacity available during a discharge is dramatically
affected by the cell temperature during discharge and the rate of discharge. The
capacity is also heavily influenced by the operating history of the cell, i.e. the recent
charge/discharge/storage history of the cell. Obviously a cell can only discharge the
capacity which has been returned to it from the previous charge cycle less whatever is
lost to self discharge. Charging/charge return issues are discussed in the next section
while storage and self-discharge is addressed in a later section.
Effect of Temperature
The primary effects of cell temperature on dischargeable capacity, assuming adequate
charging, are at lower temperatures (<0oC) as shown in Figure 12. Use of nickel metal
hydride cells in cold environments may force significant capacity derating from roomtemperature values.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 13 of 36
Figure 12.
Variation of Discharge Capacity with Temperature
Effect of Discharge Rate
Figure 13 illustrates the influence of discharge rate on total capacity available. There is
no significant effect on capacity for discharge rates below 1C. At the discharge rates
above 1C and below the current maximum discharge rate of 4C, significant reductions
in voltage delivery occur. This voltage reduction may also result in capacity reduction
depending on the choice of discharge termination voltage as discussed on page 11.
Figure 13.
Effect of Discharge Rate on Capacity
Discharge Application Considerations
In general, the discharge behavior of nickel-metal hydride cells closely follows that of
similar nickel-cadmium cells used in the same environment. Thus much of the design
expertise gathered for nickel-cadmium cells is directly applicable to nickel-metal hydride
cells. Discussed below are some specific issues often raised by designers using nickelmetal hydride cells. As the nickel-metal hydride experience base builds, additional
information that will help designers optimize the use of nickel-metal hydride cells is
becoming available. For this reason, close consultation with the factory during the
design effort is encouraged.
State-of-Charge Measurement
A major issue for users of portable electronics is the run time left before they need to
recharge their batteries. Users of portable computers, in particular, expect some form of
"fuel gauge" to help them determine when they need to save their work. A variety of
schemes for measuring state-of-charge have been suggested. In general, experience
with nickel-metal hydride cells indicates that, due to the flatness of the voltage plateau
under normal discharge rates, voltage sensing cannot be used to accurately determine
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 14 of 36
To date, the only form of state-of-charge sensing found to consistently give reasonable
results is coulometry—comparing the electrical flows during charge and discharge to
indicate the capacity remaining. Many devices already have the electronics available to
perform sophisticated tracking of charge flows including estimation of self-discharge
losses. Some off-the-shelf charging circuitry includes this form of charge tracking as
part of the package. With careful initial calibration and appropriate compensation for
environmental conditions, predictions accurate within 5 to 10 percent of actual capacity
can be obtained.
Memory/Voltage Depression
The issue of "memory" or voltage depression has been a concern for many designers of
devices, using nickel-cadmium cells. In some applications where nickel-cadmium cells
are routinely partially discharged, a depression in the discharge voltage profile of
approximately 150 mV per cell has been reported when the discharge extends from the
routinely discharged to rarely discharged zones. While the severity of this problem in
nickel-cadmium cells is open to differing interpretations, the source of the effect is
generally agreed to be in the structure of the cadmium electrode. With the elimination of
cadmium in the nickel-metal hydride cell, memory is no longer a concern.
Discharge Termination
To prevent the potential for irreversible harm to the cell caused by cell reversal in
discharge, removal of the load from the cell(s) prior to total discharge is highly
recommended. The typical voltage profile for a cell carried through a total discharge
involves a dual plateau voltage profile as indicated in Figure 14. The voltage plateaus
are caused by the discharge of first the positive electrode and then the residual capacity
in the negative. At the point both electrodes are reversed, substantial hydrogen gas
evolution occurs, which may result in cell venting as well as irreversible structural
damage to the electrodes. It should be noted that the nickel-metal hydride cell, because
it uses a negative electrode that absorbs hydrogen, might actually be somewhat less
susceptible to long-term damage from cell reversal than the sealed nickel-cadmium cell.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 15 of 36
Figure 14.
Nickel-Metal Hydride Cell Polarity Reversal Voltage Profile
The key to avoiding harm to the cell is to terminate the discharge at the point where
essentially all capacity has been obtained from the cell, but prior to reaching the second
plateau where damage may occur. Two issues complicate the selection of the proper
voltage for discharge termination: high-rate discharges and multiple-cell effects in
Voltage Cutoff at High Rates
Normally discharge cutoff is based on voltage drops with a value of 0.9 volts per cell (75
percent of the 1.2 volt per cell nominal mid-point voltage) often being used. As can be
seen in Figure 11, 0.9 volts is an excellent value for most medium to long-term
discharge applications (<1C).
However, again as seen in Figure 11, with high drain-rate usage (1-4C), the change in
shape in the voltage curve with the more rounded "knee" to the curve means that an
arbitrary 0.9V/cell cutoff may be premature, leaving a significant fraction of the cell
capacity untapped. For this reason, a better choice for voltage cutoff in high-rate
applications is 75 percent of the mid-point voltage at that discharge rate. Note, however,
that this choice of end-of-discharge voltage (EODV) is dictated only by considerations of
preventing damage to the cell. There may be end-application justification for selection of
a higher voltage cutoff with the resulting sacrifice of some potential additional capacity.
Discharge Termination in Batteries
Normal manufacturing variation produces a range of capacities for battery cells. As
these cells are combined in batteries, the effects of cell capacity variations are amplified
by the number of cells in the battery. Use of termination voltage based on a simple
multiple of 0.9V/cell times the number of cells may result in a weaker cell being driven
into reverse significantly before the battery reaches the termination voltage. Both
charging techniques that minimize the amount of overcharge applied to the cell and
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 16 of 36
frequent repetitive discharging of the battery may exacerbate the problem. The result
may be premature battery failure due to the damage caused by reversal of the weak
cell. Experience indicates selection of the EODV by the following formula provides
acceptable margin to minimize battery failure from repeated cell polarity reversal:
EODV= [(MPV-150mV)(n-1)]-200mV
Where MVP is the single-cell mid-point voltage at the given discharge rate and n is the
number of cells in the battery.
Selection of the proper discharge termination voltage, especially for large batteries or
complicated application profiles, should be done in consultation with the cell
Proper charging of nickel-metal hydride cells is the key to satisfaction with their
performance in any product. A successful charging scheme balances the need for
quick, thorough charging with the need to minimize overcharging, a key factor in
prolonging life. In addition, a selected charging scheme should be economical and
reliable in use.
In general, the nickel-metal hydride cell appears to be more sensitive to charging
conditions than the nickel-cadmium cell. It also has yet to develop the volume of
operational data that guides design of nickel-cadmium chargers. For these reasons,
charging strategies should be selected and charging parameters established in
consultation with the cell manufacturer. One advantage today’s application designers do
have in developing chargers for nickel-metal hydride cells is the increasing availability of
packaged charger circuits.
Charging Summary
The keys to successful charging of nickel-metal hydride cells are:
Use a three-step charging strategy to speed return to service while minimizing
excessive overcharge.
Design for more subtle indications of entry into overcharge.
Use redundant fast-charge termination techniques.
Provide fail-safe charge-termination backup (thermal fuse, etc.).
When these guidelines are followed, nickel-metal hydride cells can be quickly and
reliably charged while maximizing cycle life.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 17 of 36
Cell Behavior During Charge
Unlike discharge performance where the behavior of nickel-metal hydride cells and
traditional nickel-cadmium cells is very similar, there are significant differences in
behavior on charge between the two cell types that relate to basic electrochemical
differences. Specifically nickel-cadmium cells are endothermic on charge while nickelmetal hydride cells are exothermic. This difference is manifested in the
interrelationships among voltage, pressure, and temperature as discussed below.
Voltage, Pressure, Temperature Interrelationships
Figure 15 sketches typical behavior of a nickel-metal hydride cell being charged at the C
rate. These curves both indicate why charge control is important and illustrate some of
the cell characteristics used to determine when charge control should be applied.
The voltage spikes up on initial charging then continues to rise gradually through
charging until full charge is achieved. Then as the cell reaches overcharge, the voltage
peaks and then gradually trends down.
Since the charge process is exothermic, heat is being released throughout charging
giving a positive slope to the temperature curve. When the cell reaches overcharge
where the bulk of the electrical energy input to the cell is converted to heat, the cell
temperature increases dramatically.
Cell pressure, which increases somewhat during the charge process, also rises
dramatically in overcharge as greater quantities of gas are generated at the C rate than
the cell can recombine. Without a safety vent, uncontrolled charging at this rate could
result in physical damage to the cell.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 18 of 36
Figure 15.
Nickel-Metal Hydride Cell Charging Characteristics
Charge Acceptance at Temperature
The effect of temperature on charging efficiency (the increase in cell capacity per unit of
charge input) is one area of difference between nickel-metal hydride and nickelcadmium cells. Specifically charge acceptance in the nickel-metal hydride cell (as
shown in Figure 16) decreases monotonically with rising temperature beginning below
20°C and continuing through the upper limits of normal cell operation. This contrasts
with the nickel-cadmium cell, which has a peak in charge acceptance in the vicinity of
room temperature. With either cell type, the drop in charge acceptance at higher
temperatures remains a significant concern to product designers who are mounting the
cells in close proximity to heat sources or in compartments with limited cooling or
Rate Effect on Charge Acceptance
Figure 17 indicates that the charge acceptance efficiency for the nickel-metal hydride
cell is improved as the charging rate is increased.
Overcharge Detection
Determining when overcharge has occurred is critical to charging schemes that
minimize the amount of time spent at high charge rates in overcharge. In turn, these
efficient charging techniques are a key to maximizing cell life, as will be discussed later.
Primary charge control schemes typically depend on sensing either the dramatic rise in
cell temperature illustrated in Figure 18 or the peak in voltage show in Figure 19.
Figure 16.
Effect of Charge Temperature on Discharge Capacity
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 19 of 36
Figure 17.
Effect of Charge Rate on Charge Acceptance
Charge control based on temperature sensing is the most reliable approach to
determining appropriate amounts of charge for the nickel-metal hydride cell.
Temperature-based techniques are thus recommended over voltage-sensing control
techniques for the primary charge control mechanism.
Recommended Charging Rates
Today’s trend to faster charge times requires higher charge rates than the 0.1 to 0.3C
rates often recommended for many nickel-cadmium charging systems. Both Figures 18
and 19 indicate that fast-charge rates serve to accentuate the slope changes used to
trigger both the temperature and voltage-related charge terminations. A charge rate of
1C is recommended for restoring a discharge cell to full capacity. For charging schemes
that then rely on a timed "topping’ charge to ensure complete charge, a rate of 0.1C
appears to balance adequate charge input with minimum adverse effects in overcharge.
Finally a maintenance (or trickle) charge rate of 0.025C (C/40) is adequate to counter
self-discharge and maintain cell capacity.
Effective Charging Strategies
Products using nickel-metal hydride cells often make use of the sophistication of today’s
chip-level packaged charging systems to tailor the charging profile to fast capacity
recovery while minimizing overcharge stress. Two general classes of strategies have
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 20 of 36
Figure 18.
Temperature Profiles During Charge
Figure 19.
Voltage Profiles During Charge
Two-Stage—This approach uses a timer to switch from the initial charge rate to the
maintenance charge rate. Because there is no sensing of the cell’s transition into
overcharge, the charge rate must be kept low (0.1C) to minimize overcharge-related
impact on cell performance and life. Charge durations are typically set at 16 to 24
hours to ensure full recharge in cases of complete discharge. Although economical,
since this scheme makes no allowance for the degree of discharge or for
environmental conditions, its use is rarely recommended for typical nickel-metal
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 21 of 36
hydride applications.
Three-Stage—Here a fast charge restores approximately 90 percent of the
discharged capacity, an intermediate timed charge completes the charge and restores
full capacity, then a maintenance charge provides a continuous trickle current to
balance the cells and compensate for self-discharge. The fast charge (with currents in
the 1C range) is typically switched to the intermediate charge using a temperaturesensing technique, which triggers at the onset of overcharge. The intermediate
charge normally consists of a 0.1C charge for a timed duration selected based on
battery pack configuration. This intermediate-charge replaces the need to fast-charge
deeply into the overcharge region to ensure that the cell has received a full charge.
Three-step charging, such as illustrated in Figure 20, requires greater charger
complexity (to incorporate a second switch point and third charge rate), but reduces
cell exposure to life-limiting overcharge.
Charging System Redundancy
Because of the sensitivity of cell life to overcharge history and the greater subtlety of
some of the overcharge transitions, charge termination redundancy in charger design is
recommended. This applies to both built-in redundant charge control techniques and
fail-safe charge termination techniques such as thermal fusing. Both of these
considerations are discussed in more detail in the cell and battery design sections.
Temperature-Based Charge Control
Use of charge control based on the temperature rise accompanying the transition of the
cell to overcharge is generally recommended because of its reliability (when compared
to voltage peak sensing techniques) in sensing overcharge. However, temperature
sensing is typically more expensive to implement than voltage sensing since it requires
additional sensors. The exothermic nature of the nickel-metal hydride charge process
(as illustrated in Figure 18) results in increasing temperature throughout charging. This
requires care in selection of setpoints to avoid premature charge termination.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 22 of 36
Figure 20.
Recommended Charge Regime for Nickel-Metal Hydride Cells
Charge switching based on the change in slope of the temperature profile eliminates
much of the influence of the external environment and can be a very effective technique
for early detection of overcharge in a three-step charging scheme.
The simple form of temperature-based switching is to use an absolute increment in
temperature from the start of charging e.g. a 20°C increase in cell temperature from
onset of charge. The chosen ∆T has to account for both normal temperature gain during
charge and the spike at overcharge. Selection of the proper temperature increment can
be greatly influenced by the environment surrounding the cell. Thus it should be done
based on bench testing of the cell in the application and done after consultation with the
cell manufacturer.
Maximum Temperature
Charge switching based on the absolute cell temperature (as opposed to temperature
increment) is subject to varying use patterns—Alaska or the Sahara—and is
recommended only as a fail-safe strategy to avoid destructive heating in case of failure
of the primary switching strategy.
Voltage-Based Charge Control
Charge control based on voltage changes is attractive because it can be accomplished
using only existing leads to the battery, eliminating the expense and complexity of
additional temperature-sensing leads to the cell. However, the voltage peak typically
occurs later in the overcharge process, the voltage overcharge is not as distinct as that
seen with temperature, and the voltage behavior may change with cycling. For these
reasons, most product designers choose to use voltage-sensing techniques only as
backups to temperature-based control.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 23 of 36
Despite the concerns voiced above, Figure 19 does indicate a significant knee to the
voltage early in overcharge when charging at the 1C rate. Sensing this slope change in
a dV/dt system can provide an effective economical approach to detecting early entry to
Sensing the absolute voltage rise, if carefully performed, can be a useful charge control
strategy. It can be most easily utilized if cells are usually fully discharged prior to
recharge. This approach is subject to the same caveats mentioned previously regarding
consultation and bench-level verification.
Since the voltage does peak during overcharge, switching on the voltage decrease is
feasible. This eliminates the concerns faced in both voltage and temperature increment
methods about determining the increment that ensures charge return without excessive
Charge control through the absolute value of the voltage is relatively imprecise and
unsuited for primary charge-control techniques. It can be used as a redundant control
technique in, for example, a dV/dt scheme.
Time-Based Charge Control
Timer-controlled charging systems are the simplest and most economical of all charging
strategies. However, to avoid adverse effects on cell life and performance, charging
rates must be limited to 0.1C, which constrains time-based charging to those products
where overnight return of charge is acceptable. In typical application scenarios where
the degree of discharge varies widely, a charging system using time as the primary
control variable will either undercharge or overcharge the battery. However, time-based
redundant charge termination and/or time-based control of intermediate charging
(topping charge) in a three-step system are often key elements of an integrated chargecontrol strategy.
Environmental Influences on Charging Strategy
The discussions above are most pertinent for devices operating in the room-ambient
range. Designers of products predominantly operating at either temperature extreme
should consult closely with their cell suppliers in designing their charging system.
High Temperature
Although high-temperature performance (in the 40 to 55°C range) is equivalent or even
slightly better than the standard nickel-cadmium product, charging of nickel-metal
hydride cells in high-temperature environments requires careful attention for two
reasons: (1) the selection of setpoints, for both temperature and voltage-sensing
systems, can be affected if the cells are already at elevated temperatures prior to
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 24 of 36
starting charge; and (2) charge duration may have to be extended due to the charge
acceptance inefficiencies illustrated in Figure 16.
Low Temperature
Even though low temperature charge acceptance is better for the nickel-metal hydride
cell than for nickel-cadmium cells, designers must ensure that low temperatures do not
adversely affect their charge-control scheme. The charge time increases at lower
temperatures so charge durations must be carefully considered to provide adequate
low-temperature charging while avoiding excessive charge at normal temperatures.
Charge rates must also be reduced at low temperatures. An upper limit of 0.1C is
recommended below 15°C. Charging below 0°C is not advisable. Consult the factory for
more details on low-temperature charging.
Available Battery Charging Systems
Traditionally, application designers tailored their charging system to their application.
With the rapid evolution of chip-based charging circuitry, designers can now use
standardized designs providing a sophisticated charging scheme while allowing the
designer wide latitude in selecting charge parameters. Such systems are available from
a variety of sources including both cell manufacturers and integrated-circuit design
houses, in forms ranging from basic chip to complete charger packages.
Essentially all rechargeable battery cells gradually discharge over time whether they are
used or not. This capacity loss is typically due to slow parasitic reactions occurring
within the cell. As such, the loss rate (self-discharge rate) is a function of the cell
chemistry and the temperature environment experienced by the cell. Due to the
temperature sensitivity of the self-discharge reactions, relatively small differences in
storage temperature may result in large differences in self-discharging rate. Extended
storage with a load connected not only speeds the discharge process, but may also
cause chemical changes after the cell is discharged, which may be difficult or
impossible to reverse.
Cell and battery storage issues of concern to most application designers relate either to
the speed with which the cells lose their capacity after being charged or the ability of the
cells to charge and discharge "normally" after storage for some period of time. In both
situations, general guidelines developed for nickel-cadmium cells will work acceptably
for nickel-metal hydride cells.
Retained Capacity
Figure 21 illustrates the amount of capacity available from nickel-metal hydride cells
after standing for a given number of days in four different thermal environments. The
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 25 of 36
common rule of thumb for nickel-cadmium cells that a 10°C increase in storage
temperature halves the time required for a cell to self-discharge to a given level remains
approximately correct for nickel-metal hydride cells.
Figure 21.
Self-Discharge Characteristics for Nickel-Metal Hydride Cells
Recommended Storage Conditions
Storage recommendations for nickel-metal hydride cells parallel those for nickelcadmium cells:
Store at the lowest feasible temperatures (-20°C to 30°C being the generally
recommended storage temperatures).
Store cells/batteries open-circuit to eliminate loaded storage effects (see next page).
Store in a clean, dry, protected environment to minimize physical damage to batteries.
Use good inventory practices (first in, first out) to reduce time cells spend in storage.
Capacity Recovery After Storage
In normal practice, stored cells will provide full capacity on the first discharge after
removal from storage and charging with standard methods. Cells stored for an extended
period or at elevated temperatures may require more than one cycle to attain prestorage capacities. Consultation with the manufacturer is recommended if prolonged
storage and rapid restoration of capacity is planned.
Loaded Storage
Cells and batteries intended for storage for extended periods of time (past the point
where they are fully discharged) should be removed from their load. In particular, many
portable electronic devices place a very low-level drain requirement on their batteries
even when in the "off" position. These micro-current loads may be sustaining volatile
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 26 of 36
memory, powering sense circuits or even maintaining switch positions. Such loads
should be eliminated when storing devices for protracted periods.
When nickel-metal hydride cells are stored under load, small quantities of electrolyte
can ultimately begin to seep around the seals or through the vent. This creep leakage
may result in the formation of crystals of potassium carbonate, which detract
cosmetically from the appearance of the cell. In extreme cases, creep leakage can
result in corrosion of cells, batteries, or the adjoining componetry. Although such
occurrences are rare, positive methods of electrically isolating the cell, such as an
insulating tape over the positive terminal or removal from the product are suggested for
applications requiring extended storage of cells.
A key determinant of the economic and practical feasibility of using nickel-metal hydride
cells and batteries in portable electronic applications is the cell’s cycle life: the ability of
the nickel-metal hydride cell to deliver acceptable capacity on a repetitive basis. Nickelmetal hydride cell cycle life has received intensive development attention with the result
that operational life expectations are now competitive with those for nickel-cadmium
Limiting Mechanisms
The life of any battery cell is determined by a combination of abrupt failure events and
gradual cell deterioration. With the nickel-metal hydride cell, abrupt failures, typically
mechanical events resulting in the cell either shorting or going open-circuit, are
relatively rare and randomly distributed. Cell deterioration can take two forms:
Oxidation of the negative active material that increases cell internal resistance
resulting in reduction of available voltage from the cell (MPV depression). This also
affects the balance between electrodes within the cell and may possibly result in
reduced gas recombination, increased pressure, and ultimately, cell venting.
Deterioration of the positive active material results in less active material being
available for reaction with the consequent loss of capacity.
Both phenomena result in a loss of usable capacity, but pose differing design issues.
Mid-point voltage depression requires that the application design be able to adapt to
variations in supply voltage from cycle to cycle. Capacity reduction simply requires that
initial cell selection be sized to provide adequate capacity at end-of-life for the desired
number of cells.
The actual mechanism that will determine cell life may vary depending on application
parameters and the cell characteristics. Development work has reduced oxidation in the
negative electrode reducing the depression in MPV as the cell ages.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 27 of 36
Factors Affecting Life
The way the nickel-metal hydride cell is designed into an application can have dramatic
effects on the life of the cell. This is especially true of the design of the charging circuitry
for the application to ensure adequate return of charge while minimizing overcharge. In
fact, effective control of overcharge exposure, time and charge rate is the way of
enhancing cell life.
Charge Regime
In general, tailoring the charge regime to the application use scenario is even more
important with nickel-metal hydride cells than with nickel-cadmium cells because of the
increased subtlety of the voltage and temperature indications of full charge and the
greater sensitivity of cell life to overcharge history.
Degree of Overcharge
Establishing the appropriate degree of overcharge for a battery-powered application is
dependent on the usage scenario. Some overcharge of the battery is vital to ensure that
all cells are fully charged and balanced, but maintenance of full charge currents for
extended periods once the cell has reached full charge can reduce life. The three-step
charge process works to minimize some of the overcharge stress. Details of the
charging process and the application context should be carefully reviewed with the cell
manufacturer to ensure maximum cell life for the specific application.
Exposure to High Temperatures
In general, higher temperatures accelerate chemical reactions including those, which
contribute, to the aging process within the battery cell. High temperatures are a
particular concern in the charging process as charge acceptance is reduced. Sensing
the transition from charge to overcharge is also more difficult at higher temperatures.
Although early data indicate that nickel-metal hydride cells may tolerate hightemperature charging better than standard nickel-cadmium cells, close consultation with
the cell manufacturer is encouraged to select a charging strategy that meets operational
requirements while maximizing cell life.
Cell Reversal
Discharge of nickel-metal hydride batteries to the degree that some or all of the cells go
into reverse can shorten cell life, especially if this overdischarge is repeated routinely.
Prolonged Storage under Load
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 28 of 36
Maintaining a load on a cell (or battery) past the point of full discharge may eventually
cause irreversible changes in the cell chemistry and promote life-limiting phenomena
such as creep leakage.
Incorporation of nickel-metal hydride cells into applications is generally straightforward,
particularly for designers accustomed to designing with nickel-cadmium cells. Primary
differences between the two cell chemistries are:
Nickel-metal hydride cells offer higher energy densities.
Environmental and occupational health issues relating to cadmium are eliminated with
nickel-metal hydride cells.
More care is required in design of nickel-metal hydride charging systems.
Since nickel-metal hydride cells may emit hydrogen in heavy overcharge or
overdischarge, both charge-control redundancy and location of the battery package in
the product deserve careful scrutiny.
Nickel-metal hydride cells have yet to offer the wealth of sizes and design variations found in
the mature nickel-cadmium line.
Capacity Guide
A convenient aid to early analysis of battery systems is the cell selection guide shown in
Figure 22.
Figure 22.
Nickel-Metal Hydride Cell Selection Guide
This chart allows estimation of the run times available from specified cell sizes when
exposed to a given constant discharge rate. Included on the chart are nickel-metal
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 29 of 36
hydride cell sizes available from the manufacturer at the publication date. Other sizes
are being added rapidly; consult the manufacturer for an updated capacity guide
covering existing offerings. Note that comparison information is also provided for one
size of nickel-cadmium cell to allow estimation of the actual performance increment
achieved with nickel-metal hydride cells.
Typical use for the capacity guide is to enter the guide with a given discharge rate. The
intersection of that discharge rate with the performance line for each cell size then
indicates the amount of run time nominally available from that cell. The values provided
by this guide should be used for planning purposes only; final cell selection should be
based on actual discharge times obtained from testing under realistic application
Materials of Construction
The materials of construction for the nickel-metal hydride cell external surfaces are, like
the nickel-cadmium cell, largely comprised of nickel-plated steel, and therefore, are
resistant to attack by most environmental agents.
Nickel-metal hydride cells will operate satisfactorily in any orientation.
Environmental Suitability
The nickel-metal hydride cell is designed to operate effectively in all environments
normally experienced by portable electronic equipment. Application designers intending
to use nickel-metal hydride cells in especially adverse environments should consult
closely with the cell manufacturer to ensure design suitability.
Like most other battery cells, nickel-metal hydride cells are most comfortably applied in
a near-room-temperature environment (-25°C); however, with careful attention to design
parameters, they can be successfully utilized when exposed to a much wider range of
Nickel-metal hydride cells can be successfully applies in temperatures from 0 to 50°C
with appropriate derating of capacity at both the high and low ends of the range. Design
charging systems to return capacity in high or low temperature environments without
damaging overcharge requires special attention.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 30 of 36
Cells are best stored in temperatures from 0 to 30°C although storage for limited
periods of time at higher temperatures is feasible.
Shock and Vibration
Expect nickel-metal hydride cells to easily withstand the normal shock and vibration
loads experienced by portable electronic equipment in day-to-day handling and
shipping. Consult with the cell manufacturer regarding applications required operation in
more intense shock and vibration environments.
Ventilation and Isolation
The primary gas emitted from the nickel-metal hydride cell when subjected to excessive
overcharge is hydrogen as opposed to oxygen for the nickel-cadmium cell. Although
venting of gas to the outside environment should not occur in a properly designed
application, isolation of the battery compartment from other electronics (especially
mechanical switches that might generate sparks) and provision of adequate ventilation
to the compartment are required to eliminate concerns regarding possible hydrogen
Isolation of the battery from heat-generating componetry and ventilation around the
battery will also reduce thermal stress on the battery and ease design of appropriate
charging systems.
Since the exterior of the nickel-metal hydride cell is nearly identical to that of the nickelcadmium cell, all termination procedures accepted for the nickel-cadmium cell apply
equally well to the nickel-metal hydride cell. The recommendation against use of
mechanical (pressure) contacts in favor of welded terminations, especially to nickelmetal hydride cells. The prohibition against soldering directly to the cell to prevent heat
damage to plastic seal components also applies.
Other Selections Considerations
To date, applications for nickel-metal hydride cells have been focused on electronics
that have nominal drain rates of 2C or less. As a result, cell internal current-carrying
components such as tabs and current collectors have not been designed for high
currents such as found in portable tools and appliances. Although there appear to be no
intrinsic constraints on discharge rates imposed by cell chemistry, existing cell designs
are for applications with maximum currents of less than 4C.
Nickel-metal hydride cells are versatile performers easily adapted to most application
demands. Existing design libraries for nickel-cadmium cells can usually be easily
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 31 of 36
modified to incorporate nickel-metal hydride cells instead. Economical off-the-shelf
designs can be tailored to the specific voltage, space, and termination requirements of
an application.
Figure 23 illustrates a typical battery installation within a representative application,
while Figure 24 diagrams many of the components recommended for a nickel-metal
Figure 23.
Installation Within Typical Application (Notebook Computer)
Packaging Considerations
Nickel-metal hydride batteries are generally packaged in two forms:
Hard plastic cases are recommended for applications requiring the end-user to handle
the battery. These cases offer greater protection against handling damage and shock
and vibrations stresses. But depending on the design, thermal management may be
more difficult within the hard case.
Figure 24.
Elements of Battery Assembly
Injection molding of hard cases requires a substantial investment for mold construction
and is thus best suited for high volumes.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 32 of 36
Lighter shrink-wrapped plastic packaging may be used when routine battery removal is
not expected. These packs, as illustrated in Figure 24, usually consist of the cell
assembly with insulators covering the exposed terminals. Plastic shrink tubing then
covers the whole pack. Shrink-wrapped batteries have acceptable mechanical integrity
for assembly, and when properly secured, withstand normal portable-product shock and
vibration levels. Shrink packaging provides ample opportunity for hydrogen to diffuse
and for internally generated heat to dissipate. Additional insulation from heat may be
needed at the tangent points within the cell stacks (where they shrink material directly
contacts the cell).
Either type of packaging must maintain adequate ventilation to the individual cells while
providing room for cell interconnections, battery terminations, and requisite charge
control sensors.
Battery shapes can be adjusted to fit application constraints. Among the most popular
battery shapes are the following:
Sticks—the terminal of one cells butts against the base of the next cell forming a long,
slender battery.
Linear—the cells are placed side by side in a straight line.
Paired—cells are arranged in two (or more) symmetric rows.
Nested—the cells of one row are nested within the indentations formed by the adjacent row.
Materials used in the assembly of nickel-metal hydride batteries must withstand the high
temperature environment that accompanies venting of the cell. Because of the
exothermic nature of the charging process, should cells vent in overcharge, the vented
gases will be largely high-temperature hydrogen (>200°C). Although these gases will
quickly disperse and cool, all materials used in cell construction must be capable of
withstanding elevated temperatures while remaining inert in a hydrogen environment.
Recommended materials for use in nickel-metal hydride battery construction include
those below. Consult with the cell manufacturer regarding specific material specification
Wires: All wire insulation should be Teflon® , Kapton® , or other material with a
minimum temperature rating of 200°C.
Sleeving: All shrink sleeving should be able to withstand 200°C. PVC sleeving is not
generally recommended. Kraft paper or fishpaper sleeving should be approximately
0.007 inches thick.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 33 of 36
Insulation: All cell insulation should be able to withstand 105°C for 24 hours. Vent
shields must be constructed of Nomex® or other insulating material capable of
withstanding 210°C.
Case Material: Plastic cases must meet UL 9V40. Case materials without a rating of
210°C DTUL (Deflection Temperature Under Load) must be provided with vent shields
over the positive ends of the cells.
Interconnections and Terminations
Cell interconnections typically consist of nickel (Ni200) strip spot-welded from one cell
terminal to the adjacent cell’s case. Nickel bus strips offer good conductivity, ease of
welding, and resistance to corrosion. Minimum recommended nickel strip size is 0.187
inches wide by 0.005 inches thick. Wire interconnections are rarely used because of the
difficulty in attachment since soldering directly to cells is forbidden.
Battery terminations come in a variety of configurations ranging from simple flying leads
(wires soldered to weld lugs which are then welded to the cells) in permanent
installations to much more elaborate contact or connector systems on removable
battery packs. Removable battery packs should be designed with a connection system
that produces a minimum of 2 pounds of force while incorporating a wiping action on
insertion to cut through oxide layers on the connection surfaces
Other Components
Nickel-metal hydride batteries typically require more components than nickel-cadmium
batteries because of the emphasis on careful, redundant charge control including
adequate fail-safe charge termination in case of excessive temperatures. These
components include the follows:
PTC Resistor: Positive temperature coefficient resistors such as Raychem’s
PolySwitch® circuit protector provide a latching, but resettable device for protection
against short-circuit conditions.
Thermostat: Thermostats or other resettable thermal control devices are typically used
for backup to the primary charge control system to guard against extended overcharge
and the resulting elevated temperatures.
Thermal Fuse: Thermal fuses that open at a suitably elevated temperature (nominally
90°C) are often used as a third tier of thermal protection (after the normal charge control
system and thermostat). They are a fail-safe measure since the battery charging system
will become inoperative.
Thermistor: Thermistors are normally used for the temperature sensing necessary for
recommended charge control schemes.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 34 of 36
Standard Configurations
A wide variety of standard battery configurations have been developed by cell
manufacturers encompassing permutations of cell size/capacity, voltage, terminations,
and charge control and termination sensors.
As a minimum, Energizer Power Systems recommends that the following be included in
any standard battery design:
Primary Charge Control System—The standard temperature or time-based charge
control system to switch to maintenance charging.
Backup Resettable Thermal Protection—Terminates charging if the primary control
system should fail to switch prior to extended overcharge. Normally set to 70°C.
Fail-Safe Thermal Fuse—Permanently opens charge circuit if battery temperature
exceeds acceptable limits. Normally set to 90°C.
Short-Circuit Protection—Provides protection in cases of excess discharge current.
Vents and Vent Shielding—Gas management system to diffuse and cool a vented stream of
While battery location is generally influenced by product design constraints such as
available space, influence on center of gravity, and ease of access, battery locations
should also provide adequate ventilation, isolation from ignition sources and separation
from major heat generators.
Nickel-metal hydride cells should be handled in much the same manner as nickelcadmium cells. Major points are summarized below. Contact the cell manufacturer for
additional information pertinent to specific applications.
General Safety Precautions
Nickel-metal hydride cells are generally well behaved; however, like any rechargeable
cell, they should be treated with care. Issues in dealing with nickel-metal hydride cells
include the following:
Nickel-metal hydride cells operate on an exothermic, hydrogen-based charging and
oxygen recombination process. Precautions should be taken to avoid venting. Should
venting occur, the vent gases must be properly managed.
Nickel metal hydride cells can generate high currents if shorted. These currents are
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 35 of 36
sufficient to cause burns or ignition of flammable materials.
The active materials in the negative electrode can ignite on exposure to air. They electrolyte is
also corrosive and capable of causing burns. For these reasons, the cell should be maintained
intact and sealed.
Shipping and Handling
Shipping and handling of nickel-metal hydride cells is straightforward. The following
suggestions ensure maximum performance, reliability, and safety in working with the
Ship cells only in fully discharged state.
Provided proper packaging, considering the cells’ and batteries’ weight, to avoid
transit damage, either to cells or adjacent items.
Do not store cells or batteries in loaded or shorted condition.
Use product on a first-in, first-out inventory management policy.
Avoid keeping excessive product in inventory.
Avoid excessive handling of charged cells and batteries outside the end-use product.
Although disposal procedures for nickel-metal hydride cells are still evolving, as a
minimum, observe the following precautions:
Discharge fully prior to disposal.
Do not incinerate.
Do not open or puncture cells.
Observe all national, state, and local rules and regulations for disposal of rechargeable cells.
Incoming Inspection
Normal incoming inspection techniques consist of physical examination of the cells for
any dents, bulges, or leakage and selection of a representative sample for capacity
testing. In general 100 percent capacity testing is discouraged because of the
cost/schedule impact. Specialized incoming test procedures are normally developed for
each application by consultation between the product designer and the cell
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Nickel-Metal Hydride
Page 36 of 36
This reference manual contains general information on all
Energizer/Eveready batteries within the Nickel Metal Hydride chemical
system in production at the time of preparation of the manual. Since the
characteristics of individual batteries are sometimes modified, persons and
businesses that are considering the use of a particular battery should
contact the nearest Energizer Sales Office for current information. None of
the information in the manual constitutes a representation or warranty by
Eveready Battery Company, Inc. concerning the specific performance or
characteristics of any of the batteries or devices.
Copyright ©2001 Eveready Battery Co. Inc. - All Rights Reserved
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF