CELL FUNDAMENTALS 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.
Electrochemistry
The electrochemistry of the nickel-metal
hydride cell is generally represented by the following charge and discharge
reactions:
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.
At the positive electrode, the charge
reaction is based on the oxidation of nickel hydroxide just as it is in the nickel-cadmium
couple.
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. At the positive electrode, nickel
oxyhydroxide is reduced to its lower valence state, nickel hydroxide.
Nickel-metal
hydride cells, with the exception of the negative electrode, use the same general types of
components as the sealed nickel-cadmium cell.
The concept
of the nickel-metal hydride cell negative electrod followed 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 action occurs 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 appears to offer the best set of features for commercial nickel-metal hydride
cell applications. 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.
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. 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.
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.
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. The prismatic cells may fit more easily into
volume-critical applications. 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. 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. 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. 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 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 relieved.
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.
The principal
battery parameter 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. 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 faster 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. 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.
Surge and
Discharge Curve
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 mid-point voltage (MPV - the voltage
when 50 percent of the available capacity is discharged) provides a useful approximation
to average voltage throughout the discharge.
The environmental
influences on the location and shape of the voltage profile are the discharge temperature
and discharge rate. Small variations from room temperature (± 10oC) have 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.
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
duration.
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. 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.
The primary effects
of cell temperature on dischargeable capacity, assuming adequate charging, are at lower
temperatures ( < 0oC). Use of nickel metal hydride cells in cold
environments may force significant capacity derating from room-temperature values.
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.
Discharge
Application Considerations
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 nickel-metal 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.
State-of-Charge
Measurement
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 state-of-charge. 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.
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. 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, may
actually be less susceptible to long-term damage from cell reversal than the sealed
nickel-cadmium cell. 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 batteries.
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. However, 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 that this choice of end-of-discharge voltage (EODV) is dictated only by
considerations of preventing damage to the cell.
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 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: EODVMPV-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.
CHARGE CHARACTERISTICS
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.
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. 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 nickel-metal hydride
cells are exothermic. This difference is manifested in the interrelationships among
voltage, pressure, and temperature as discussed below.
Voltage, Pressure, Temperature
Interrelationships
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
slop 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.
Charge Acceptance at Temperature
The effect of temperature on charging
efficiency is one area of difference between nickel-metal hydride and nickel-cadmium
cells. Specifically charge acceptance in the nickel-metal hydride cell 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 ventilation.
Rate Effect on Charge Acceptance
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 or the peak in voltage. 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.ecommended
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. The 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 evolved: One-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 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 temperature-sensing 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 regime to ensure that the cell has received a full
charge. Three-step charging 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.
Temperature-Based Charge Control
Use of charge control based on the
temperature rise accompanying the transition of the cell to overcharge is 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 results in increasing temperature throughout charging.
This requires care in selection of setpoints to avoid premature charge termination. 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
Large 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.
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.
Despite the concerns voiced above system can provide an effective economical approach to
detecting early entry to overcharge. 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 overcharge.
Magnitude
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 charge-control strategy.
High Temperature
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:
a. The selection of setpoints, for both
temperature and voltage-sensing systems, can be affected if the cells are already at
elevated temperatures prior to starting charge.
b. Charge duration may have to be extended
due to the charge acceptance inefficiencies.
Low Temperature
Low temperature charge acceptance is better
for the nickel-metal hydride cell than for nickel-cadmium cells. 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.
Available Battery Charging Systems
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. 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.
The amount of
capacity available from nickel-metal hydride cells after standing for a given number of
days in four different thermal environments. The 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.
Recommended
Storage Conditions
Storage
recommendations for nickel-metal hydride cells parallel those for nickel-cadmium cells:
a. Store at the
lowest feasible temperatures (0 to 30°C being the generally recommended storage
temperatures).
b. Store cells/batteries open-circuit to eliminate loaded storage effects.
c. Storage 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
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 pre-storage capacities.
Cells and batteries
intended for storage for extended periods of time (pass 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 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. Nickel-metal hydride cell cycle life has received intensive development attention
with the result that operational life expectations are now competitive with those for
nickel-cadmium cells.
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:
a. 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.
b. 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. 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.
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.
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.
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.
Exposure to High Temperatures
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
high-temperature charging better than standard nickel-cadmium cells.
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
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.
DESIGNING FOR NICKEL-METAL HYDRIDE CELLS
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,
bothcharge-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. This guide allows estimation of
the run times available from specified cell sizes when exposed to a given constant
discharge rate. Included on the guide are nickel-metal hydride cell sizes available from
the manufacturer at the publication date. 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 scenarios.
Materials of Construction
The material for
the nickel-metal hydride cell external surfaces are, like the nickel-cadmium cell are
nickel-plated steel, and resistant to attack by most environmental agents.
Orientation
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. Like most other battery cells, nickel-metal hydride
cells are most comfortably applied in a near-room-temperature environment (-25°C);
they also can be successfully utilized when exposed to a much wider range of temperatures.
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.
Cells are best
stored in temperatures from 0 to 30°C although storage for limited periods of time at
higher temperatures is feasible.
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.
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. 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 ignition. 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.
Termination
The exterior of the nickel-metal hydride
cell is nearly identical to that of the nickel-cadmium 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 nickel-metal 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.
BATTERY DESIGN
Nickel-metal hydride
cells are versatile performers easily adapted to most application demands. Existing design
libraries for nickel-cadmium cells can usually be easily 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.
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. Injection molding of hard cases requires a substantial
investment for mold construction and is thus best suited for high volumes.
Lighter shrink-wrapped plastic packaging may be used when routine battery removal is not
expected. These packs 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 my 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.
Shape
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
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). 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. Materials for use in nickel-metal hydride
battery construction include those below.
Wires -- All wire insulation should be Teflon, Kapton â, 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.
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 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.
CARE AND HANDLING
Nickel-metal hydride cells should be handled in much the same manner as nickel-cadmium
cells. Major points are summarized below.
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.
Nickel metal hydride cells can generate high currents if shorted.
The active materials in the negative electrode can ignite on exposure to air.
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 cells:
Ship cells only in fully discharged state.
Provide appropiate packaging, considering the cells’ and batteries’ weight, to
avoid transit damage.
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.
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
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. .
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