Efficiency of an Accumulator

Efficiency of an Accumulator Points : Efficiency of an Accumulator, E.M.F. of Accumulators Efficiency is defined as the ratio of output to input for any device. There are two efficiencies according to the units in which input and output are measured. If the measurements are made of the quantity of electricity, ie., ampere-hours, then the efficiency is called the quantity efficiency or ampere-hour efficiency; thus

(i) Quantity efficiency = amp. hours of discharge/amp. hours of charge

On the other hand, if output and input are measured in units of energy, i.e., kilowatt-hours or watt hours then the efficiency is called the energy efficiency or watt-hour efficiency, thus

(ii) Energy efficiency = watt - hours of discharge/ watt - hours of charge

As wattage is obtained by multiplying amperage by average voltage, the two kinds of efficiencies are inter-related as under:

Energy efficiency = average discharge voltage/average charge voltage x quantity efficiency

In determining the efficiencies experimentally, the state of the cell at the end of the discharge must be the same as its state at the commencement of charge.
E.M.F. of Accumulators The e.m.f. of a cell depends upon its charge or discharge. The terminal pd. Is considerably higher during charge than it is during discharge, because the internal voltage drop must be added to the e.m.f. during charge, while during discharge this drop must be deducted from the e.m.f. Thus

Terminal pd. during charge = e.m.f. + internal drop
Terminal pd. during discharge = em.f. - internal drop