NICKEL METAL HYDRIDE BATTERY USING MICROENCAPSULATED ALLOYS

Overcharging properties of nickel-metal hydride prismatic single cells (300 mA h) were examined in high pressure cells. The rise in pressure during overcharging increased with increasing charging rate and decreased with increasing content of copper or nickel in the surface coating on the alloy, i.e. in the following order: a mixed alloy containing 20 wt.% Cu or Ni; an alloy coated with 10 wt.% Cu or Ni; and an alloy coated with 20 wt.% Cu or Ni. The kind of coated metal used had no influence on the overcharging behavior. The gas in the cell using a 20wt.%Cu-coated alloy was mostly oxygen, while the cell using a copper-mixed alloy contained a large amount of hydrogen in addition to oxygen. The rapid pressure rise on overcharging was ascribed to insufficient charging of the metal hydride electrode. A sealed cell using a microencapsulated alloy showed a higher resistance to overcharging at higher charging rates.