Batteries for Electric Vehicles: Nickel Zinc

Dangers include (but are not limited to): very high discharge currents; mildly corrosive electrolyte; mildly poisonous and environmentally damaging

Chemistry

The negative electrode is made of zinc, and the positive electrode consists of nickel hydroxide on a nickel conductor. The electrolyte is alkaline, normally potassium hydroxide with zinc oxide dissolved to form a potassim zincate solution.

On discharge, the following reactions occur:-

Positive: e- + NiO2H + H2O -> Ni(OH)2 + OH-

Negative: Zn + 4OH- -> 2e- + ZnO22- + 2H2O

Mass of reagents to produce 1 mole of electrons:- (Ni=58.7; Zn=65.4; O=16.0; H=1.0) 143.4g.

Voltage: 1.65v nominal; 1.0v discharged; 2.1v charged

Maximum discharge: ~3C

Cycle Life: 300-500

Typical failure mode: redeposition of the zinc occurs in different places to the original solution, so the electrode becomes more and more misshapen.

Nickel Zinc Technology

Dangers include but not limited to: massive energy can start fires, cause burns or cause the car to become uncontrollable; potassium or lithium hydroxide electrolyte in the battery is very corrosive, as well as poisonous, and will eat your face off if it spills on you. These batteries may spill if they are broken open, and so may well spill in an accident. Also the batteries are heavy, so you can injure yourself handling them, or make your car dangerous by overloading it.

This battery is similar to NiMH, but cheaper.

Charge and Discharge Rates

These batteries are subject to heating if they are rapidly discharged, as they have a comparatively high internal resistance (typically 3.3mOhm for the 30Ah cell). This means that if we drew say 200A from this sort of battery, each cell would experience about 150w of internal heating. That in turn means that discharge will have to involve careful observation of cell temperature.

Discharge depth is a big determinant for cycle life. Here is our cycle life table:-

Discharge depth Number of cycles discharge storage product
10%110001100C
20%4000800C
30%2700800C
40%2000800C
50%1200600C
60%1000600C
70%800560C
80%700560C
100%300300C

This table is a little unusual as it reduces with fewer cycles. So the best size of battery in terms of efficiency probably depends on the shelf life - if the battery lasts 3 years and is charged once per day, it is not worth designing for more than 1100 cycles. But this implies that the ideal discharge rate is probably 40% or less.

Fuel Metering

The current integration method is perfectly acceptible for this technology - as Peukert's Number is about 1.04, it must be used to factor the higher currents. Determining the capacity using off-load voltage is harder as the off-load voltage curve is very flat. It may be possible to use this to estimate discharge states only when very deep discharges have occurred. It is possible to determine battery state quite well from the voltage when discharging into a small load: so for instance a current of, say, C/20 might be used to determine fuel level, by applying the load and waiting for a minute to see what the output voltage becomes.

Here is a table of discharge voltage against capacity for a C/20 load. Even under load, the discharge line is very flat, with a difference of 1mV resulting in a change of 5% of capacity at near 50%. This will still make it hard to determine battery wear unless the batteries are deeply discharged.

Fortunately voltage at low currents is virtually unchanged from 0C to 40C, so providing the temperature is within these limits some accuracy can be retained.

Voltage at C/20 SOC (%) DOD (%)
1.9 100 0
1.76 83 17
1.72 67 33
1.69 50 50
1.67 33 67
1.64 17 83
1.61 0 100

Charging

A bulk charge rate of 0.5C should be used until a voltage of 2.1v per cell is obtained. Then the cells should be rested for five minutes before a finishing charge is applied, consisting of a constant voltage of 2.05v per cell until the current drops to less than C/4. Then the cells should be disconnected.

Bulk charging should not be attempted at temperatures over 25C: at these temperatures C/10 should be applied until 1.9v/cell is obtained. This may not provide full charge capacity.

Regenerative charging should be attempted at the same conditions as bulk charging: which is to say at a current of 0.5C.

Data on Nickel Zinc batteries is available from Evercel's website.

Practical Cells

Manufacturer Part Chemistry Voltage Capacity Weight(kg) Dimensions(mm) Peak Power Continuous Power Cost Cycles Peukert Number Energy Weight
Wh/kg
Wear Cost
(per kWh per cycle)
Notes
Theoretical Reagents Only Nickel Zinc 1.7 26.8Ah
45.6Wh
0.143kg - 161W 32W - 800-1000 - 318.6 - 1 mole of electrons
Evercel MB-100 Nickel-Zinc 13.2 85Ah
1.122kWh
22kg 298x171x225 6.6kW 3.3kW $350 300
2000@40%
1.04 51 $1.04
$0.39
Capacity at 3C rate
Low toxicity
may be discontinued
Drumm Cell ??? Nickel Zinc 1.7 600Ah
1020Wh
50kg ??? 1700W 1530W ??? ??? - 20.4 ??? 1932 design

Electric Vehicle Suitability

Nickel Zinc cells are hampered by their short cycle life, but are otherwise a good choice for electric vehicle, as they have a good performance, low Peukert number and are mostly benign environmentally.

This technology is being advanced in various ways, most of them concerned with making good electrical contact with the nickel oxide in the negative electrode. Various approaches, including nickel wire wool and nickel-plating the grains of oxide have demonstrated very improved performance. Some researchers have claimed 95% plus utilisation of the nickel hydroxide electrode, which would lead to excellent battery performance.


This page is part of an Open Source Electric Car Project, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.