Discharge Action
During discharge, he hydrogen ions (H+) remove oxygen from the lead peroxide (PbO2) of the positive plates and combine with it to form water (H2O). Loss of oxygen from the lead peroxide reduces it grey lead (Pb). The water formed by the action c the electrolyte so that as the cell discharges, the specific gravity (relative density) decreases Measurement of the specific gravity change with a hydrometer will show the state of charge of the cell.
At the negative side of the cell, sulphate ions (SO4ˉˉ) combine with the pure lead of the negative plates to form a layer of white lead sulphate (PbSO4) The lead sulphate layer increases during discharge and finally covers the active material of the plate so that further reaction is stifled. Some sulphate also forms on the positive plates but this is not a direct part of the discharge reaction.
A fully charged cell will be capable of producing 1.95 volts ad and the relative density of the .electrolyte will be at a maximum (say 1.280) After a period of discharge the electrolyte will be weak due to formation of water and the plates will be sulphated, with the result that the voltage on load will .drop .Recharging is required when voltage on load drops to say 1.8 volts per cell and the relative density is reduced to about 1.1 20.
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