The battery capacity is specified in ampere-hours (Ah).
Note : it indicates the amount of electricity which a battery can supply at the specified discharge rate, till its voltage falls to a specified value.
For a lead acid battery, the discharge rate is specified as 10 hours or 8 hours while the value of voltage to which it should fall is specified as 1.8 V.
Mathematically the product of discharge current in amperes and the time for discharge in hours till voltage falls to a specified value is the capacity of a battery.
Where ID = Discharge current in amperes
TD = Time of discharge in hours till voltage falls to a specified value.
Sometimes it is specified as watt-hours (Wh). It is the product of the average voltage during discharge and the ampere hour capacity of a battery.
The battery capacity depends on the following factors,
1. Discharge rate : As the rate of discharge increases, the battery capacity decreases.
2. Specific gravity of electrolyte : More the specific gravity of electrolyte, more is the battery capacity as it decides internal resistance of the battery.
3. Temperature : As temperature increases, the battery capacity increases. This is shown in the Fig. 1.
4. Size of the plates : This is related to the amount of active material present in the battery.