Linden D., Reddy T.B. (eds.) Handbook of batteries

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31.20 CHAPTER THIRTY-ONE

FIGURE 31.11 Nickel-zinc speciﬁc energy as a function of discharge rate. Batteries were discharged

at room temperature. 2C: battery was discharged to 1.05 V cutoff; others: battery was discharged to

1.2 V. (Courtesy of Evercel Corp.)

Temperature Dependence of Performance. Temperature has a strong inﬂuence on battery

performance. As a general rule of thumb, nickel-based batteries achieve optimal performance

when charged cold and discharged warm. Unfortunately, in many applications, temperature

is ambient and uncontrolled. Figure 31.12 shows the effect of temperature on the discharge

capacity of the nickel-zinc battery at four different rates. At lower rates, the discharge ca-

pacity is a linear function of temperature. At the 6C rate, the relationship starts to become

nonlinear, primarily due to the increased conductivity of the electrolyte above 30

⬚C.

Cell Balance on Discharge. Cell balance becomes an important issue in the discharge

performance of multicell batteries. The capacity of the battery will be reduced by weaker

(lower capacity) cells. Newly manufactured cells typically fall within a narrow range of

capacity (for example, plus or minus two or three percent over a manufacturing lot). As a

battery is cycled, the individual cells may diverge in performance. In practical terms this

means that the cells in the battery exist at different states-of-charge. This affects battery

performance because the discharge capacity of the battery is dominated by the weaker cells,

which tend to depress the overall battery voltage. If the cells in a battery become extremely

unbalanced, battery performance and cycle life may be adversely affected. This is strongly

inﬂuenced by system level design, particularly thermal design. Temperature differentials in

the battery will cause cells to diverge in performance and thereby reduce the overall per-

formance of the battery. It is therefore recommended that systems be designed such that a

minimum temperature difference (

⌬T) exists among the cells in the battery.

Reconditioning. Many alkaline rechargeable nickel-based batteries, such as nickel-

cadmium, nickel-hydrogen and nickel-metal hydride, are capable of being reconditioned.

Typically this means that the battery is taken to a very low state-of-charge and then recharged

at a moderate rate. Sometimes the reconditioning effect is only seen after several such cycles.

NICKEL-ZINC BATTERIES 31.21

FIGURE 31.12 Discharge capacity of a nominal 30 Ah nickel-zinc battery as a function of temperature at different

discharge rate. C / 20, C/ 5, C rate: battery was discharged to 1.2 V, 5C, 6C: battery was discharged to 0.95 V. (Courtesy of

Evercel Corp.)

Reconditioning is not currently recommended for nickel-zinc batteries. Should more data

become available in the future, reconditioning may become an option for the nickel-zinc

system. In the meantime, the nickel-zinc battery should not be treated as a generic nickel-

based rechargeable battery with respect to reconditioning.

31.5.2 Charge Retention

Fully charged nickel-zinc batteries lose only about 20% of their original capacity within a

month on open-circuit stand at 25

⬚C. The self-discharge rate of nickel-zinc, like that of most

batteries, increases with temperature.

Typical charge retention data for the nickel-zinc battery is shown in Fig. 31.13. As with

most batteries, the voltage decays exponentially the ﬁrst few days and then levels out to a

very slow rate of decline. Capacity decay during open-circuit stand amounts to less than 1%

per day after the initial higher loss rate that occurs during the ﬁrst 4 or 5 days. If the battery

is allowed to stand for very long periods of time, the battery voltage may reach zero volts.

If this happens the battery usually requires 3 or 4 cycles to fully recover capacity when the

battery is brought into service again.

31.22 CHAPTER THIRTY-ONE

FIGURE 31.13 Self-discharge of a nickel-zinc battery at room temperature. (Courtesy of Evercel

Corp.)

31.5.3 Cycle Life

The nickel-zinc battery is capable of delivering more than 500 cycles at 100% depth-of-

discharge. Cycle life is largely a function of the application, including factors such as duty

cycle, depth-of-discharge, the charging regime used, the cumulative amount of overcharge,

the level of abuse the battery receives and the thermal and mechanical environment. As the

battery is cycled, the capacity gradually declines due to physical changes and degradation

processes in the battery. This gradual decline in capacity is both normal and predictable for

a given battery design. Fig. 31.14 shows the discharge capacity for a 12 VDC nickel-zinc

battery (30 Ah nominal capacity) as a function of cycling. The battery under test is a standard

design built by Evercel Corporation using a plastic-bonded graphite composite nickel elec-

trode and a plastic-bonded calcium zincate zinc electrode. The battery achieved more than

600 cycles at 80% DOD while retaining in excess of 80% of its designed capacity. During

each cycle, the battery was discharged to 80% of its rated capacity at the C/ 5 rate (6

Amperes) and was charged using a two-step CC /CV method, described in Sec. 29.6. The

discharge capacity of the battery was checked at 25 cycle intervals, which adds approximately

24 additional cycles performed at 100% DOD. The data shows a gradual and predictable

decline in performance which is important from a system level. This allows the system

designer to account for battery aging in the overall product speciﬁcation and design. At

100% DOD, the battery achieved 550 cycles before the capacity dropped to 80% of its rated

value. The rate of decline of capacity tends to increase near the end-of-life which provides

an early indication that failure is imminent.

Figure 31.15 shows voltage performance as a function of cycle life for a single-cell nickel-

zinc battery. The chart plots the mid-point discharge voltage (MPDV) as a function of the

number of cycles. The MPDV is deﬁned as the loaded cell voltage at the mid-point of

discharge as calculated based on the capacity removed from the cell (i.e. half of the actual

capacity of the cell). The battery was cycled at 100% depth-of-discharge, both charging and

discharging at the C /2 rate (15 Amperes). This represents an accelerated test in which

slightly more than 3 cycles per day can be accumulated, whereas in most applications only

a single cycle per day is performed. The loaded discharge voltage of the battery gradually

NICKEL-ZINC BATTERIES 31.23

FIGURE 31.14 Cycle life of 12 V, 30 Ah nickel-zinc battery. 80% DOD testing performed by JBI. (Courtesy

of JBI and Evercel Corp.)

FIGURE 31.15 Nickel-zinc battery MPDV, measured at the C / 2 rate, as a

function of cycle number. (Courtesy of Evercel Corp.)

31.24 CHAPTER THIRTY-ONE

FIGURE 31.16 Nickel-zinc cycle life as a function of DOD. (Courtesy of Evercel Corp.)

decreases with cycling. The effect is similar to that observed with the gradual decline in

capacity. The voltage declines primarily due to the gradually increasing impedance of the

battery as the electrodes slowly degrade as it gradually dries-out. The decay in voltage is

linear and predictable until a point is reached at which the battery can no longer support the

imposed discharge current. When this happens the battery has essentially ‘‘failed’’ from the

standpoint that the required load voltage can no longer be supplied. If the discharge current

is reduced, it will continue to function and will still deliver greater than 80% of its rated

capacity. The battery shown in Fig. 31.15 delivered nearly 600 cycles under these test con-

ditions.

In each set of data, the batteries were cycled at 100% depth-of discharge (DOD).

Depth of Discharge. Cycle life is a direct function of depth-of-discharge, as well as being

a function of many other factors discussed elsewhere in this chapter. Generally, cycling the

battery at deeper depths-of-discharge results in reduced cycle life. The general relationship

between cycle life and depth-of-discharge for the nickel-zinc battery is shown in Fig. 31.16.

This empirical data can be used as a guide in system level design to achieve the required

cycle life by limiting the depth-of-discharge of the battery. Battery life is reduced when

cycled at deeper depths-of-discharge because of the higher stress levels induced in the elec-

trodes. Mechanical expansion and contraction, zinc electrode solubility issues and electro-

chemical issues are all involved in the process.

Temperature Dependence. Temperature is also an important factor in relation to cycle life.

Temperature affects all aspects of battery performance. In general, nickel-based alkaline

batteries perform best at moderate temperatures in the range of 10

⬚Cto30⬚C. Outside of

this temperature range, performance and cycle life may be less than optimum. If the system

design and the application environment are able to maintain the battery within the optimum

range, improved battery performance and cycle life will result.

Failure Mechanisms. Failure mechanisms for earlier nickel-zinc batteries include zinc mi-

gration, shape change, dendritic shorting and hydrolysis of the cellulose-based separator.

These have been substantially eliminated in modern nickel-zinc battery technology. Dendritic

shorting and shape change have been virtually eliminated through the use of reduced solu-

NICKEL-ZINC BATTERIES 31.25

bility calcium zincate electrode technology. Zinc migration has been substantially reduced.

Separator systems have also improved substantially. Stable polymeric zinc migration barrier

materials are used in place of cellulose-based separators. There remain two primary failure

mechanisms in sealed nickel-zinc batteries, failure of the zinc electrode and cell dry-out.

Even with the use of reduced solubility calcium-zincate technology, the zinc electrode

still has some ﬁnite solubility in the alkaline electrolyte. Zinc can form complex zincate

anions in the electrolyte and diffuse throughout the battery. Some of this zincate is deposited

within the pores of the nickel electrode. This may adversely affect the performance of the

nickel electrode and thus the performance of the battery. It is possible that the gradual

decrease in capacity observed is partially for this reason.

In plastic bonded electrodes, the ﬁbrillated Teﬂon

䉸

structure minimizes mechanical fatigue

in the electrode and provides dimensionally stable long-term performance by allowing the

electrode active materials to expand and contract during charge and discharge. Conventional

sintered or pasted electrodes provide no mechanism for this expansion and contraction. In

addition, a ﬁbrillated Teﬂon

䉸

structure prevents zinc migration and shape change by locking

the active material in place within a stable three-dimensional structure. This effect also

reduces the extrusion of active material from the nickel electrode into the separator.

31.5.4 Memory Effect

Nickel-zinc batteries may exhibit only a very mild memory effect that is associated with the

nickel electrode. Nickel-cadmium batteries commonly exhibit what is termed ‘‘memory ef-

fect’’ or ‘‘fading.’’ This is a reversible phenomenon usually caused by repetitive cycling

at less than full depth-of-discharge. The observed effect is a depression in the discharge

voltage (as much as 120 millivolts) when the battery is discharged below the depth at which

it was previously cycled. Nickel-zinc batteries are only slightly affected by a similar

phenomenon.

31.6 CHARGING CHARACTERISTICS

Proper charging of the nickel-zinc battery is a critical factor in achieving maximum per-

formance and cycle life. The goal of recharging any battery is to input the correct quantity

of charge to deliver the optimum discharge capacity. Charging beyond this point is not

productive and in many cases may cause degradation in battery performance and cycle life.

This is particularly true in the nickel-zinc battery because the zinc electrode is susceptible

to increased zinc dissolution/ migration during extreme overcharge. The critical factor in

charging system design is how to detect when the battery has achieved a full state-of-charge.

Several methods can be used for charge termination including temperature compensated

voltage, the rate of change of voltage with respect to time, increased battery temperature or

a variety of other commonly used techniques.

Development of the commercial nickel-zinc battery necessitated the development of

charging methods and algorithms in order to supply charging systems for commercial ap-

plications. Extensive testing has been performed to fully characterize the nickel-zinc system

as a function of both charge rate and temperature. Several charging algorithms and methods

of charge termination have been evaluated. The deﬁning characteristic for a charging system

is cost, as the cost of the charger must be proportional to the cost of the battery and the

system in which the battery is used.

It should be stressed that the manufacturer’s recommendations should be strictly adhered

to in charging any battery. Excessive overcharge, current which is too high or too low, or

the use of an inappropriate charging algorithm may result in reduced performance, reduced

cycle life and potential safety hazards. Use only a charger speciﬁcally designed for the nickel-

zinc battery.

31.26 CHAPTER THIRTY-ONE

31.6.1 Charging Regimes

The most common charging method is a two-stage constant current/constant voltage (CC/

CV) regime, with the CC phase cutting off at a temperature-compensated termination voltage.

The CV phase can be terminated when the current falls below a predetermined value or after

a speciﬁed time. Other charging schemes can also be used with the nickel-zinc system, but

it is important to follow the manufacturer’s recommendations. The primary charging criterion

is the required time for recharging the battery. This affects battery and system design and

also determines the cost of the charger.

31.6.2 Fast Charging

For applications where frequent, repeated use is anticipated, a high-rate fast charging system

can be used, with more sophisticated charging algorithms and charge termination methods.

Fast charge systems are capable of achieving a fully charged battery in as little as 2.5 h.

This method works particularly well for smaller systems such as bicycles and small battery

applications where extremely high currents are not required. At higher charging currents,

charge termination becomes much more critical as Ampere-hours accumulate much faster.

Higher currents also may induce higher temperatures in the battery which may reduce its

overall charge efﬁciency. A balanced approach to charging at high rates is required in order

to achieve maximum performance and cycle life.

In a typical high-rate charging cycle, the battery is charged at the C /2 rate up to nearly

95% state-of-charge. A ﬁve-minute rest is included at the end of the higher rate C/2 portion

of the charge to allow depolarization of the battery and voltage relaxation. When the current

is reapplied, the battery is clamped at a constant voltage and the current is allowed to taper,

topping off the battery to full capacity. The electrochemistry of the battery is such that large

quantities of charge can be accepted at a lower state-of-charge. As the state-of-charge in-

creases, parasitic gas evolution reactions become signiﬁcant and charge efﬁciency decreases.

This two-step method greatly reduces oxygen evolution at the nickel electrode during over-

charge and improves overall charge efﬁciency. Fig. 31.17 shows a standard two-step charging

method used for nickel-zinc batteries. The voltage rise near the end of charge signals that

the battery is approaching full charge. The voltage rise occurs because the electrochemistry

of the cell is transitioning from the normal charge reactions to oxygen evolution, which

occurs at a different characteristic voltage. This phenomenon provides an easily implemented

method of charge termination. Once this point is reached, the charge current is reduced to

compensate for the reduced charge efﬁciency of the battery and the lower current thereby

reduces the gas evolution rate. Typically only about 10% of the rated capacity of the battery

is input during the second step. A back-up charge termination time limit should also be

provided to prevent thermal runaway.

Charge termination voltage is a function of both temperature and charge rate. At higher

charge rates, the end-of-charge voltage increases as a function of cell impedance and polar-

ization. The charge termination voltage also increases at colder temperatures, primarily due

to the decreased conductivity of the electrolyte. Because of this strong temperature depend-

ence, the battery charger must be temperature compensated, requiring the use of a thermistor

to detect battery temperature. This slightly increases the cost of the charging system but

provides much more efﬁcient charging of the battery.

NICKEL-ZINC BATTERIES 31.27

FIGURE 31.17 Seven-cell, 30 Ah nickel-zinc battery charge proﬁle at room temperature. C/ 1.75 charge to

14.7 V then 14.35 V charge to I ⬍ C/4. (Courtesy of Evercel Corp.)

31.6.3 Slow Charging

A slow charge method can be used for applications where rapid recharging is not required

or where the cost of the charger must be reduced. This might include commercial applications

where only one battery discharge is required per day. Slow charging in these types of ap-

plications is typically done overnight. Charge termination is less critical when charging at

low rate. After charge termination, the battery can be left on a low rate trickle charge,

however this is typically not necessary as the self-discharge rate for the battery is low. Trickle

charging is generally not recommended for the nickel-zinc system.

31.6.4 Charge Termination

Charge termination is the critical issue for all types of charging methods, either fast or slow

charging. It is necessary for the charger to detect when the battery has achieved a full state-

of-charge and either stop charging or reduce the charging current. There are a number of

charge termination methods which are used in nickel-based alkaline rechargeable batteries

such as nickel-cadmium and nickel-metal hydride. These include voltage,

⫺dV/ dt (the neg-

ative derivative of voltage with respect to time), temperature rise and a variety of other

methods based on the current /voltage proﬁle of the battery as it nears a full state-of-charge

or other properties of the battery. Similar techniques can be used with the nickel-zinc system.

As seen in Fig. 31.17, the reproducible rise in voltage at the end of charge can be used

as a charge termination method. This voltage is a function of several factors including cur-

rent, temperature and the speciﬁc battery design. Fig. 31.18 shows the relationship between

the end-of-charge (EOC) termination voltage and charging current at three different temper-

atures which bracket many normal operating conditions. The charge termination voltage is

nearly a linear function of charging current at each temperature. It is apparent that the

temperature has a very large effect over the range of 0

⬚Cto40⬚C. Figure 31.19 shows the

31.28 CHAPTER THIRTY-ONE

FIGURE 31.18 EOC voltage as a function of charging current and temperature for a 30 Ah single-cell

nickel-zinc battery. (Courtesy of Evercel Corp.)

FIGURE 31.19 EOC voltage as a function of temperature and charge rate. (Courtesy of Evercel Corp.)

NICKEL-ZINC BATTERIES 31.29

relationship between charge termination voltage and temperature graphically as a function

of charging current. The data show that the charge termination voltage is linear as a function

of temperature (at rates between C/10 and C/ 2) up to about 25

⬚C. Above this temperature,

the slope of the end-of-charge voltage versus temperature relationship declines signiﬁcantly.

At temperatures above 25

⬚C, the EOC termination voltage varies very little with increasing

temperature.

31.6.5 Overcharging

Overcharging is detrimental to most batteries, including nickel-zinc. Overcharge is based on

the nominal (or actual) capacity of the battery being deﬁned as 100% state-of-charge. Charge

input above the 100% state-of-charge level is deﬁned as overcharge. Overcharge simply

means that more Ampere-hours of charge were put into the battery than was removed from

the battery on the previous discharge. It is typically expressed as a percentage of the nominal

(or actual) capacity. Batteries are more tolerant to overcharge at lower charge rates. But even

low current can damage a battery if enough total Ampere-hours are input. As a battery

approaches full state-of-charge, parasitic gas evolution reactions occur which generate heat

and can cause a battery to vent. Excessive overcharge may also degrade the electrodes and

reduce battery performance and cycle life. Therefore, it is highly recommended to prevent

severe overcharging of the battery.

31.6.6 Cell Balance During Charging

Cell balance also becomes an issue in multicell batteries from the standpoint of charging.

As a battery is cycled, the individual cells may diverge in performance. Many of the concepts

from the discussion of the effect of cell balance in discharge performance also apply here.

Cell balance affects charging because cells may receive differing amounts of overcharge

depending on the state-of-charge of the individual cell. Cell balance may also affect the

efﬁciency of various charge termination methods, making charging the battery less uniform

and reliable. If the cells in a battery become extremely unbalanced, battery performance and

cycle life may become adversely affected. Thermal imbalance can cause electrical unbalance

in the battery. It is therefore recommended that systems be designed such that a minimum

temperature difference (

⌬T) exists among the cells in the battery.

31.7 APPLICATIONS

Nickel-zinc provides the lowest-cost option for a long-cycle-life alkaline-rechargeable

system. The nickel-zinc system is suited for mobile applications such as electric bicycles,

electric scooters and electric and hybrid vehicles or other deep cycle applications. Nickel-

zinc may also replace other nickel based batteries with a less expensive system.