TABLE OF CONTENTS The nickel-metal hydride battery has replaced nickel-cadmium in a number of electric vehicle applications in recent years, since it offers a better performance without the toxicity problems of cadmium.
The GM-Ovonic Battery Company has been the major supplier of nickel-metal hydride battery technology and has developed particular expertise in its application to electric vehicles as a result of being awarded a major contract by the US Advanced Battery Consortium in 1992. The battery has a negative electrode of complex metal hydride alloy compacted onto a conducting substrate, a potassium hydroxide electrolyte, and a positive electrode whose active material is nickel hydroxide. During charging, hydrogen generated by reaction with the cell electrolyte is stored in the alloy as a hydride phase, while nickel hydroxide is oxidised to nickel oxyhydroxide [7]. The cell construction is shown in Figure 5.3. The cell can tolerate both overcharge and overdischarge because the gas recombination reactions which take place prevent the build-up of excessive pressure inside the cell. This tolerance also simplifies battery management during charge and discharge when many cells are connected in series in an electric vehicle application.
An energy density of greater than 70 Wh/kg is reported combined with a high power density of 200 W/kg. The battery can accept over 600 full charge/discharge cycles to 80 per cent depth of discharge and can be fast recharged to 80 per cent of full capacity in 35 min. Cooling by air or liquid means...
