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Nickel-zinc battery
NiZN-AA.gif
PowerGenix NiZn AA cells with charger
Energy/weight 100 W·h/kg
Energy/size 280 W·h/L
Power/weight > 900 W/kg
Energy/consumer-price 2-3Wh/US$
Cycle durability 400-1000[1]
Nominal cell voltage 1.6 V

The nickel-zinc battery (sometimes abbreviated NiZn) is a type of rechargeable battery that may be used in cordless power tools, cordless telephone, digital cameras, battery operated lawn and garden tools, professional photography, electric bike and light electric vehicle sectors.

While nickel-zinc battery systems have been known for over 100 years, advancements have only recently made this technology viable and competitive with other commercially available rechargeable battery systems.

Contents

History

Thomas Edison was awarded a U.S. Patent for a rechargeable nickel-zinc battery system in 1901 (U.S. Patent 684,204).

The battery was later developed by an Irish chemist, Dr. James J. Drumm (1897-1974)[2] and installed in four 2-car Drumm Railcar sets between 1932 and 1948 for use on the Dublin-Bray line. Although successful they were then withdrawn when the batteries wore out. Early nickel-zinc batteries were plagued by limited number of discharge cycles.

Applications

Presently this battery technology has limited consumer availability with only AA cells offered for the digital camera market in some camera stores. Both D-cells and sub-C cells are currently used in commercial applications. Nickel-zinc batteries have a charge/discharge curve similar to 1.2V NiCd or NiMH cells - but with a higher 1.6V nominal voltage [3]

Nickel-zinc batteries perform well in high drain applications, and may have the potential to replace lead-acid batteries because of their higher energy to mass ratio and higher power to mass ratio (up to 75% lighter for the same power), and are cheap compared to nickel-cadmium batteries (expected to be priced somewhere in between NiCd and lead-acids). NiZn may be used as a substitute for nickel-cadmium. The European Parliament has supported bans on cadmium based batteries and nickel-zinc offers the European power tool industry a potential alternative.

Electrochemistry

Charge Reaction: 2Ni(OH)2(s) + Zn(OH)2(s) ↔ 2Ni(OH)3(s) + Zn(s)

Note that the stoichiometry above is different than below, but the reactions are identical. Technically, water is consumed and generated on the charge and discharge cycles.

Discharge Reaction: H2O + Zn + 2NiOOH ↔ ZnO +2Ni(OH)2

Electrochemical open circuit voltage potential: ~1.73V

Battery Life

The tendency of the zinc electrode to dissolve into solution and not fully migrate back to the cathode during charging has, in the past, presented challenges to the commercial viability of the NiZn battery. The zinc's reluctance to fully return to the same location of the solid electrode adversely manifests itself as shape change and dendrites or whiskers, which may reduce the cell discharging performance or eventually short out the cell, resulting in low cycle life.

Recent advancements have enabled manufacturers to prevent this problem. These advancements include improved electrode separator materials, zinc material stabilizers, and electrolyte improvements. One manufacturer (PowerGenix) claims battery cycle life comparable to NiCd or NiMH battery systems [4]

Battery cycle life is most commonly specified at a discharge rate of 80 percent of rated capacity and assuming a one hour discharge current rate. If the discharge current rate is reduced or if the depth of discharge is reduced then the number of charge/discharge cyles for a battery increases.

When comparing NiZn to other battery technologies it is important to note that cycle life specifications may vary with other battery technologies depending on the discharge rate and depth of discharge that were used.

Advantages

Nickel Zinc cells have an open circuit voltage of 1.8 volts when fully charged [5] and a nominal voltage of 1.6V. This makes NiZn an excellent replacement for electronic products that were designed to use alkaline primary cells (1.5V). NiCd and NiMH both have nominal cell voltages of 1.2v, which may cause some electronic equipment to shut off prior to a complete discharge of the battery because the minimal operating voltage is not provided. Li Ion cells have a much higher open circuit voltage (3.6-3.7v) and are not a suitable replacement for alkaline primary cells; however having resistors in the AA shell with Li-ion cells may approximate the correct cell voltage.

Due to their higher voltage, fewer cells are required (compared to NiCd and NiMH)to achieve a given battery pack voltage, reducing pack weight, size and improving pack reliability. They also have low internal impedance (typ. 5 milliohm) which allows for high battery discharge rates.

NiZn batteries use no heavy/toxic metals (mercury, lead or cadmium), or metal hydrides (rare earth metals) that are difficult to recycle. Both nickel and zinc are commonly occurring elements in nature and are not very hazardous. Zinc and nickel can be fully recycled.

NiZn cells use no flammable active material or organic electrolyte.

Properly designed NiZn cells can have very high power density and low temperature discharging performance.

Disadvantage

Currently, only Sub C and AA NiZN cells are available.

Charging

NiZn technology is well suited for fast recharge cycling as optimum charge rates of C or C/2 are preferred [6].

Known charging regimes include constant current of C or C/2 to cell voltage = 1.9V. Maximum charge time is 2 1/2 hours. Trickle charging is not recommended as recombination is not provided for and excess hydrogen will eventually vent adversely affecting battery cycle life. Charge is reinitiated after cell voltage has fallen below 1.6V.

References

External links

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