Efficiencies of the Cell
The efficiencies of a cell can be considered in two ways :
1. The quantity or ampere-hour (Ah) efficiency
2. The energy or watt-hour (Wh) efficiency
The Ah efficiency does not take into account the varying voltages of charge and discharge. The Wh efficiency does so and is always less than Ah efficiency because average p.d. during discharging is less than that during charging. Usually, during discharge the e.m.f. falls from about 2.1 V to 1.8 V whereas during charge it rises from 1.8 volt to about 2.5 V.
The Ah efficiency of a lead-acid cell is normally between 90 to 95%, meaning that about 100 Ah must be put back into the cell for every 90-95 Ah taken out of it. Because of gassing which takes place during the charge, the Ah available for delivery from the battery decreases. It also decreases
(i) Due to self-discharge of the plates caused due to local reactions and
(ii) Due to leakage of current
because of faulty insulation between the cells of the battery.
The Wh efficiency varies between 72-80%. If Ah efficiency is given, Wh efficiency can be found from the following relation :
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From the above, it is clear that anything that increases the charge volts or reduces the discharge volts will decrease Wh efficiency. Because high charge and discharge rates will do this, hence it is advisable to avoid these
Read article – Back E.M.F.
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