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Construction
Rechargeable Ni-Cd batteries are comprised of a positive electrode plate which uses nickel hydroxide as its main active material, a negative electrode plate which uses a cadmium compound as its main active material, a separator made of a thin non-woven fabric, an alkaline electrolyte, a metal case, a sealing plate provided with a self-sealing safety valve, and other components. The positive and negative electrode plates, isolated from each other by the separator, are rolled in a spiral shape inside the case and sealed by the sealing plate by means of an insulation gasket.
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Battery Reactions
Generally, in rechargeable cells there are three different electro-chemical reactions: the discharge reaction which supplies electrical power to the load of the battery, the charge reaction which restores that electrical power, and the oxygen gas generation reaction resulting from the electrolysis of water on the positive electrode which occurs after the completion of charge, or, in other words, during overcharge. For rechargeable Ni-Cd batteries, the charge and discharge reactions are illustrated by the formula shown below.
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In positive:
Charge: Ni(OH)2 +OH→NiOOH+H2O+e
Over charge:4OH-→2H2O + O2 + 4e
Discharge:NiOOH+H2O +e→Ni(OH)2+OH
Over discharge:2 H2O+2e→H2+2OH
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In negative:
Cd(OH)2+2e→Cd+2OH
2Cd+O2+2H2O+4e→2Cd(OH)2
Cd+2OH→Cd(OH)2+2e
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The overall:
2Ni ( OH )2 + Cd ( OH )2 < -------> 2 NiOOH + Cd + 2H2O ± e |
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