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

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31.30 CHAPTER THIRTY-ONE

31.7.1 Temperature and Environmental Considerations

All batteries are limited in the operational and non-operational environments to which they

can be exposed. Alkaline batteries are limited in the low temperature extreme primarily by

the freezing point of the electrolyte. This can vary from about

⫺25⬚C for 20% KOH to below

⫺60⬚C for 31% potassium hydroxide. Twenty-ﬁve percent potassium hydroxide freezes at

about

⫺38⬚C. Nickel-zinc battery electrolyte is typically in the range of 20% to 25% potas-

sium hydroxide, while most nickel-based alkaline rechargeable batteries use 31% potassium-

hydroxide or even higher concentrations in some cases. Dissolved zinc anions in the elec-

trolyte will also effectively lower the normal freezing point of the pure electrolyte. The lower

concentration of electrolyte is used in order to reduce zinc solubility and extend cycle life.

It is preferable to maintain the battery above the freezing point of the electrolyte or permanent

damage to the battery may occur. In some cases, nickel-zinc batteries can be optimized for

extreme cold weather applications by the use of electrolyte additives which enhance con-

ductivity at cold temperature and depress the freezing point.

Temperature Extremes. The operating temperature range for most nickel-zinc batteries is

typically speciﬁed as

⫺10⬚Cto⫹50⬚C. Batteries which have been optimized for colder

temperatures can operate down to

⫺30⬚C or even slightly colder. At warmer temperatures,

the problem becomes one of charge efﬁciency. The charge efﬁciency of the battery drops

sharply at temperatures above

⫹40⬚C, resulting in reduced battery capacity. Battery perform-

ance is affected by operating temperature as discussed in the performance section of this

chapter. The non-operating temperature range for the nickel-zinc battery is typically speciﬁed

⫺30⬚Cto⫹50⬚. Long-term storage above ⫹50⬚C is not recommended or reduced battery

performance may result.

31.7.2 Electric Bikes and Scooters

Bicycles and scooters are the primary method of personal transportation in many countries,

including China, Taiwan and many other parts of Asia. Scooters are also very popular in

parts of Europe, such as Italy, and the Mediterranean countries. When a bicycle is used for

transportation and for carrying personal goods, human power becomes very limiting in terms

of range and convenience, particularly when traveling uphill. For this reason electric power-

assist bicycles are being marketed in many countries. A typical electric bicycle is shown in

Fig. 31.20.

Similarly, scooters are widely used as a primary source of transportation in many coun-

tries. Highly polluting small gasoline-powered scooters are endemic in Asian cities and clean

electric scooters are beginning to replace noisy gas-powered scooters. Under strict test con-

ditions at the Energy and Resources Lab of Taiwan Industrial Technology Research Institute,

the nickel-zinc battery for electric scooters demonstrated a cycle life for 14,168 km of travel.

As a result, scooter manufacturers who use this battery would receive a government incentive

of US $750 per scooter. Moreover, the Taiwanese government has stipulated that at least 2%

of every custom scooter manufacturer’s output must be electrically powered. The ﬁrst com-

mercial production scooter to use the new deep cycle nickel-zinc technology is the EVT,

shown in Fig. 31.21. This scooter is manufactured in China and is currently also being

distributed in the United States.

Most standard operating voltages for electric bicycle and scooter batteries are 24 V and

48 V, respectively. Capacity ranges from 10 Ah up to 40 Ah. An electric bike equipped with

a nickel-zinc battery has a minimum range of 15 km before recharging, not including ad-

ditional range supplied by human pedaling. Motors powered by nickel-zinc batteries allow

electric bikes to achieve a maximum speed up to 20 miles per hour. An electric scooter

equipped with a nickel-zinc battery has a range of 80 km before recharging and can achieve

a maximum speed up to 50 miles per hour.

NICKEL-ZINC BATTERIES 31.31

FIGURE 31.20 Nickel-zinc powered ZAP electric bicycle. (Courtesy of Evercel Corp.)

FIGURE 31.21 Nickel-zinc powered EVT scooter. (Courtesy of Evercel Corp.)

31.32 CHAPTER THIRTY-ONE

FIGURE 31.22 Nickel-zinc powered electric lawnmower. (Courtesy of Evercel Corp.)

31.7.3 Deep-Cycle Applications

Deep-cycle applications for nickel-zinc batteries include trolling motors, electric bicycles and

scooters, wheelchairs, golf carts, electric lawnmowers, electric vehicles and similar uses. In

general, nickel-zinc batteries have good deep-cycle capability. Cycle life and performance

issues at high depths-of-discharge are discussed elsewhere in this chapter. In general, ad-

vanced nickel-zinc batteries are capable of exceeding 500 cycles at 100% depth-of-discharge

when operated within the speciﬁed temperature limits and charged according to the manu-

facturer’s speciﬁcations. It is important to follow the manufacturer’s recommendations for

charging when operating the battery at high depth-of-discharge because cell imbalance be-

comes an even more critical issue. Cells tend to diverge in performance more rapidly as the

depth-of-discharge is increased. Lawn and garden applications are becoming increasingly

important as the disadvantages of the noise and pollution of small gasoline-powered engines

becomes more apparent in both cities and suburban neighborhoods. An example of a nickel-

zinc powered electric lawnmower is shown in Fig. 31.22.

Deep cycle marine applications, such as trolling motors, are an ideal use for nickel-zinc

batteries. Nickel-zinc batteries have the required discharge rate capability, high cycle life at

deep depths-of-discharge, lightweight rugged construction and are capable of being stored

over the winter months without any maintenance charging. Nickel-zinc batteries are capable

of storage for several months in a fully discharged state.

NICKEL-ZINC BATTERIES 31.33

FIGURE 31.23 Nickel-zinc powered PIVCO Citi Bee. (Courtesy of Evercel Corp.)

31.7.4 Hybrid and Electric Vehicles

As the automotive industry moves forward with the development of electric and hybrid

vehicles and other alternatives to the conventional gasoline-powered automobile, existing

battery technologies are being stretched to their limits in terms of weight, size, cell balance

issues, environmental concerns and cost reduction. Nickel-zinc may be applicable to hybrid

and electric vehicle applications because of its light weight, high power capability and deep

cycle ability. Nickel-zinc is also lower in cost than other nickel-based rechargeable batteries.

Several demonstration electric vehicles have been powered with prototype nickel-zinc bat-

teries. A full-size 11.2 kWh nickel-zinc electric vehicle battery was built by Evercel Cor-

poration. This battery was tested in a PIVCO Citi Bee (Ford purchased PIVCO in 1998 and

the vehicle is now sold under the brand name TH!NK). The vehicle achieved ranges of 78.8

to 131.5 km at average cruising speeds of 30 to 50 km /h. This vehicle, along with the

nickel-zinc battery which powers it, is shown in Fig. 31.23.

An EV made by Trapos was tested at 40 km /h with both nickel-zinc and lead-acid

batteries. A 205 kg (12 kWh) nickel-zinc battery provided a range of 172 km, whereas a

280 kg (7.0 kWh) lead-acid provided a range of only 69 km. The nickel-zinc battery provided

a speciﬁc energy of 58 Watt-hours per kilogram while the lead-acid battery only pro-

vided 25 Wh /kg. The nickel-zinc battery weighed 25% less than the lead-acid battery but

provided 70% more energy storage capacity with more than double the range of the lead-

acid battery. Reduced battery weight is an important advantage for large batteries such as

those used in electric and hybrid vehicles. Eagle-Picher manufactured an 18 kWh, 200 Am-

pere-hour monobloc nickel-zinc electric vehicle battery. The battery was tested in a Solectria

䉸

converted Geo Metro

䉸

vehicle. Another program at Eagle-Picher, sponsored by the Air Force

31.34 CHAPTER THIRTY-ONE

Wright Aeronautical Laboratories, developed a nickel-zinc battery for a remotely piloted

vehicle (RPV).

31.7.5 Standby/Float Charge

Nickel-zinc batteries may be used in ﬂoat charge and standby applications such as emergency

lighting, emergency power back-up systems and uninterruptible power supplies (UPSs). If

ﬂoat charging is performed, the ﬂoat charge rate should balance and offset the self-discharge

rate so that the battery receives no net overcharge. This ﬂoat charge rate may vary slightly

depending on the battery design and the environmental conditions of the application, but it

is typically in the range of C/ 40 to C /70. The battery manufacturer’s recommendations

should be followed to obtain maximum performance and life.

31.7.6 Military Applications

Nickel-zinc batteries may be useful for military application requiring deep cycle capability.

Nickel-zinc batteries may also be employed in torpedoes, swimmer delivery vehicles, and

other submersibles. Nickel-zinc has long cycle life and low cost and meets the deep cycle

requirements of torpedoes and submersibles.

31.7.7 Special Applications

Microscopic nickel-zinc batteries are being developed for use in micro-electro-mechanical

systems (MEMS), remote autonomous sensors, and other microelectronics.

Photolithogra-

phy and patterned electrodeposition are used to fabricate 1000 cells at a time on a silicon

chip. Polypropylene laminate is glued to a silicon wafer and etched with oxygen plasma into

cell cavities and cell walls. NiOOH is deposited into the cell cavities, while Zn is deposited

onto ledges surrounding the NiOOH. The cells are then ﬁlled with aqueous 25% KOH and

sealed with polypropylene. The speciﬁc capacity achieved by these cells is 2 to 4 C /cm

and is limited by the positive electrode. Each cell occupies only 10

⫺

to 10

⫺

. These

batteries can sustain a current density of 100 mA /cm

for 2 seconds (MEMS require only

10 to 15 ms of high power discharge). Higher current densities can last for a few millisec-

onds. The low internal surface area of today’s nickel-oxyhydroxide microelectrodes results

in high polarization at high current densities, rendering thin ﬁlm nickel-zinc cells incapable

of sustaining current densities greater than 1 A /cm

. An SEM photomicrograph of a micro-

scopic nickel-zinc battery is shown in Fig. 31.24.

FIGURE 31.24 SEM-micrograph of a Ni / Zn cell.

(Courtesy of Bipolar Technologies Corp.)

NICKEL-ZINC BATTERIES 31.35

31.8 HANDLING AND STORAGE

The nickel-zinc battery does not impose any special handling or storage requirements beyond

those of any other commercially available battery. However, it must be realized that any

battery does impose some safety and handling issues such as electrical hazards or corrosive

electrolyte spillage. A material safety data sheet (MSDS) and operating/handling instructions

should be obtained from the battery manufacturer.

31.8.1 Cell Venting Hazards

Nickel-zinc, like many other commercially available batteries, incorporates a safety vent to

prevent an overpressure condition in the battery which might result in the rupture of the

battery case. During normal operation, the battery operates in a sealed manner. The vent is

present as a safety feature only. The nickel-zinc battery typically uses a resealable valve so

that venting does not necessarily lead to immediate battery failure. This low-pressure valve

also serves to regulate internal pressure, which assists in gas recombination within the battery.

Since it is possible for the valve not to open, some potential hazard does exist. The primary

concern is the ﬁre and explosion hazard due to potentially ﬂammable gasses being vented.

It is also possible that some small quantity of corrosive electrolyte may be entrained within

the gas. These potential hazards to personnel and equipment must be recognized and taken

into consideration during operation of the battery. Venting is most likely during severe over-

charge or overdischarge conditions, but might possibly occur during normal battery operation

as well.

31.8.2 Overcharge and Overdischarge Protection

The battery should be protected from excessive overcharge and overdischarge. Extreme abuse

may reduce battery performance and cycle life or cause a potential safety hazard. The system

in which the battery is installed should provide protection for the battery under adverse

conditions. Voltage exceeding the normal charging voltage should not be applied to the

battery under any circumstances.

31.8.3 Fire and Explosion Hazard

Batteries may produce potentially explosive gases, particularly near the end of charge or

when over-discharged. Any battery which has a safety relief vent has the potential for venting

hydrogen or other explosive gases. The nickel-zinc battery presents no special hazard in this

respect compared to other batteries which are capable of venting.

31.8.4 Storage

Nickel-zinc batteries are similar to other nickel-based alkaline rechargeable batteries. In

general, nickel-zinc batteries are tolerant of storage at low states-of-charge. Because of self-

discharge, most storage occurs at low state-of-charge. It is generally advisable to store bat-

teries in a controlled manner when possible or at least within the non-operating environ-

mental conditions speciﬁed by the manufacturer. It is preferred to store the batteries in a

cool and dry place. Long-term storage at temperature extremes may adversely affect battery

cycle life. Long-term storage above

⫹50⬚C is not recommended or reduced battery perform-

ance may result. Smoking should not be permitted in the storage area. If batteries are being

placed into storage prior to use, they should be withdrawn from storage on a ﬁrst in /ﬁrst

31.36 CHAPTER THIRTY-ONE

out basis (FIFO). Nickel-zinc, like most other battery chemistries, should be stored on open-

circuit. If the battery is stored with a load across the terminals, cycle life may be reduced.

The non-operating (storage) temperature range for the nickel-zinc battery is typically spec-

iﬁed as

⫺20 to 50⬚C. The battery manufacturer’s recommendations should be followed.

Short-term Storage. There are no issues regarding short-term storage of the nickel-zinc

battery for periods of less than 6 months. If the battery has been unused for some period of

time, a few ‘‘wake-up’’ cycles may be required before the battery recovers full capacity. This

effect is typically not noticeable in most applications. It is possible for the battery to exhibit

a lower than normal charge voltage which would preclude normal end-of-charge voltage

termination. The charger should have a back-up charge termination mode to counter this

possibility, including a time limit charge termination as a minimum.

Long-term Storage. Long-term storage is typically considered to be a period exceeding 6

months in which the battery is stored at essentially zero state-of-charge (due to self-

discharge). Nickel-zinc batteries can be stored for up to 3 years. Long-term storage should

be done in a controlled environment to minimize adverse effects on the battery of temperature

extremes. If stored for periods of more than 3 years, some degradation in performance may

be observed. A more aggressive charge may be required to fully recover capacity after long-

term storage greater than 6 months. If possible a ‘‘pre-conditioning’’ charge can be performed

which terminates at a slightly higher voltage than the normal charge termination voltage.

This serves to overcome passivating oxide layers that may build up on the electrodes during

long periods of non-use. Pre-conditioning allows 60% capacity recovery on the ﬁrst use after

storage and 90% recovery on the second use. If a preconditioning charge is not possible, the

battery may require several cycles to achieve full capacity using the normal charging method.

As mentioned above, long-term storage above

⫹50⬚C is not recommended or reduced battery

performance may result.

Acknowledgment

The authors would like to acknowledge the signiﬁcant contributions of Jonathan O’Neill,

Frank Cao and Kendra Pruneau, Evercel Corporation, in researching the scientiﬁc literature,

compiling background information and assisting in the preparation and editing of this chapter.

REFERENCES

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1048.

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(1998).

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(USSR) 35 (1962), 1246.

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NICKEL-ZINC BATTERIES 31.37

12. U.S. Patent 5,460,899.

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14. U.S. Patent 5,863,676 (1999), Charkey and Coates.

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32.1

CHAPTER 32

NICKEL-HYDROGEN BATTERIES

James D. Dunlop, Jack N. Brill, and Rex Erisman

32.1 GENERAL CHARACTERISTICS

A sealed nickel-hydrogen (Ni-H

) secondary battery is a hybrid combining battery and fuel-

cell technologies.

The nickel oxide positive electrode comes from the nickel-cadmium cell,

and the hydrogen negative electrode from the hydrogen-oxygen fuel cell. Major advantages

and disadvantages are listed in Table 32.1.

Salient features of this hybrid Ni-H

battery are a long cycle life that exceeds any other

maintenance-free secondary battery system; high speciﬁc energy (gravimetric energy density)

compared to other aqueous batteries; high power density (pulse or peak power capability);

and a tolerance to overcharge and reversal. It is these features that make the Ni-H

battery

system the energy storage subsystem currently employed in many aerospace applications,

such as geosynchronous earth-orbit (GEO) commercial communications satellites, and low

earth-orbit (LEO) satellites, such as the Hubble space telescope.

Application of the Ni-H

battery has mainly been directed toward the aerospace ﬁeld.

Recently, however, programs have been started for terrestrial applications, such as long-life

stand-alone photovoltaic systems.

TABLE 32.1 Major Advantages and Disadvantages of the Nickel-Hydrogen Battery

Advantages Disadvantages

High speciﬁc energy (60 Wh/ kg)

Long cycle life, 40,000 cycles at 40% DOD

for LEO applications

Long lifetime in orbit, over 15 years for GEO

applications

Cell can tolerate overcharge and reversal

pressure gives an indication of state of

charge

High initial cost

Self-discharge proportional to H

pressure

Low volumetric energy density:

50–90 Wh /L (IPV cell)

20–40 Wh /L (battery)

32.2 CHAPTER THIRTY-TWO

32.2 CHEMISTRY

The electrochemical reactions of the Ni-H

cell for normal operation, overcharge, and reversal

are

Normal operation:

discharge

⫺

—— —



NiOOH ⫹ HO⫹ e Ni(OH) ⫹ OHNickel electrode

———



charge

discharge

⫺

—— —



–

H ⫹ OH H O ⫹ eHydrogen electrode

———



charge

discharge

—— —



–

H ⫹ NiOOH Ni(OH)Net reaction

———



charge

Overcharge:

⫺

–

2OH → 2e ⫹ O ⫹ HONickel electrode

22 2

⫺

–

O ⫹ HO⫹ 2e → 2OHHydrogen electrode

22 2

Reversal:

⫺

–

HO⫹ e → OH ⫹ HNickel electrode

222

⫺

–

H ⫹ OH → HO⫹ eHydrogen electrode

22 2

32.2.1 Normal Operation

Electrochemically, the half-cell reactions at the positive nickel oxide electrode are similar to

those occurring in the nickel-cadmium system. At the negative electrode, hydrogen gas is

oxidized to water during discharge and is reformed, during charge, from the water by elec-

trolysis. The net reaction shows hydrogen reduction of nickel oxyhydroxide to nickelous

hydroxide on discharge with no net change in KOH concentration or in the amount of water

within the cell.

32.2.2 Overcharge

During overcharge, oxygen is generated at the positive electrode. An equivalent amount of

oxygen is recombined electrochemically at the catalytic platinum electrode. Again, there is

no change in KOH concentration or the amount of water in the cell with continuous over-

charge. The oxygen recombination rate at the negative platinum electrode is very rapid,

sustaining very high rates of continuous overcharge, provided that there is adequate heat

transfer away from the cell to avoid thermal runaway. This is one of the operational advan-

tages of the Ni-H

cell.

32.2.3 Reversal

During cell reversal, hydrogen is generated at the positive electrode and consumed at the

negative electrode at the same rate. Therefore the cell can be operated continuously in