Batteries for Electric Vehicles: Nickel Cadmium

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

Chemistry

The negative electrode consists of cadmium hydroxide on a cadmium conductor, and the positive electrode consists of nickel hydroxide on a nickel conductor. The electrolyte is alkaline, normally potassium hydroxide with about 20% lithium hydroxide in aqueous solution, although sodium hydroxide has been used for high temperature cells.

On discharge, the following reactions occur:-

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

Negative: Cd + 2OH- -> 2e- + Cd(OH)2

Mass of reagents to produce 1 mole of electrons:- (Ni=58.7; Cd=112.4; O=16.0; H=1.0) 165.9g.

Voltage: 1.2v nominal; 1.0v discharged; 1.55v charged

Maximum discharge: ~5C

Cycle Life: 800-1000

Typical failure mode: dendritic formation from the cadmium to the nickel electrodes. This dendritic formation is caused by the high electric field around a point, and results in threads of cadmium shorting out the cell. Cells that are shorted by dendritic formation can often be recovered by applying a fat current. Dendritic formation occurs on charged cells, so these cells are best left flat.

Nickel Cadmium Technology

Charge and Discharge Rates

Nickel cadmium cells may be rapidly discharged and charged. They have a very large cycle life for the flooded varieties, although the sealed cells tend to suffer from dendritic formation. For charging the sealed cells, it is normal to charge at a constant current until the temperature rises. For the flooded varieties, normal CC-CV or CC-ZDV charging techniques are suitable.

Fuel Metering

The current integration method is perfectly acceptible for this technology. 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.

It is important to factor in self-discharge rates.

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 Cadmium 1.2 26.8Ah
32.2Wh
0.166kg - 161W 32W - 800-1000 - 194.0 - 1 mole of electrons
Aerobatt D500J Nickel Cadmuim 1.2 5Ah
6Wh
0.155kg D cell 18W 6W $4 800 - 38.7 $0.83 Small Cell

Electric Vehicle Suitability

The problem with NiCd cells is the cadmium, which is banned in many countries, because it is so poisonous that recycling it is a problem. Nevertheless, nickel cadmium batteries have been used in a number of successful electric vehicle projects, including the Renault Kangoo Electrique, and the Dodge TeVan.

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.