In a nickel-cadmium cell the active material for the positive electrode consists of nickel hydroxide mixed with graphite for conductivity and additives such as barium or cobalt compounds for improved life and capacity. The active material for the negative electrode is prepared from cadmium hydroxide or cadmium oxide mixed with iron or iron compounds and sometimes with nickel. The iron and nickel materials are added to stabilize the cadmium, prevent crystal growth and agglomeration and improve conductivity. The electrolyte consists of potassium hydroxide with lithium hydroxide added for improved cycle life and high temperature performance. There are several physical designs for a cell of this chemistry including vented pocket, sintered plate, fiber and plastic bonded plate and others. The basic reversible chemical reaction remains the same for all of these designs.
2NiOOH + 2H2O + Cd = 2Ni(OH)2 + Cd(OH)2
On discharge, trivalent nickel oxy-hydroxide is reduced to divalent nickel hydroxide with consumption of water. Metallic cadmium is oxidized to form cadmium hydroxide. Upon charging the opposite reactions occurs. This type of cell produced a an open circuit voltage of 1.29 volts per cell.
Listed below are the typical operating characteristics of the various nickel cadmium cell designs.
Specific Energy Energy Density
Pocket plate single cell 20-27 Wh/kg 40-55Wh/L
Complete pocket plate battery 19-27 Wh/kg 32- 44Wh/L
Larger fiber plate batteries 40 Wh/kg 80 Wh/L
Plastic bonded plate batteries 56 Wh/kg 96Wh/L
Sintered plate designs 30-37 Wh/kg 58-96Wh/L
Occasional over-discharge or reversal of nickel-cadmium batteries is not detrimental, nor is complete freezing of the cells. After warming up they will function as normal again. They are a mechanically a very robust battery and can withstand severe mechanical abuse and rough handling. The electrolyte does not attack any of the cell components and therefore there is no risk of decreased strength during the lifetime of the battery. The thermal resistance of the cell is very good and can withstand temperatures up to 70 degrees C or greater without damage. Pocket plate nickel-cadmium batteries may be charged at constant current, constant voltage or modified constant voltage and as such are very forgiving to the charging process. Charging is normally carried out at the 5 hour rate and 7 hours for a fully discharged battery and can be conducted in the temperature range of -50 to 45 degrees C.
