Mainly the battery efficiency is defined as the ratio of output during discharging to the input required during charging, to regain the original state of the battery.
It is defined in many ways as,
1. Ampere-hour efficiency or quantity efficiency
2. Watt-hour efficiency or energy efficiency
1. Ampere-hour Efficiency
It is defined as ratio of output in ampere-hours during discharging to the input in ampere-hours during charging. It is denoted as ηAh.
For lead acid battery, it ranges between 80 % to 90%.
2. Watt-hour Efficiency
It is defined as the ratio of output in watt-hours during discharging to the input in watt-hour during charging. It is denoted as ηWh.
Note : it can be seen that as average voltage during discharge is less than the average voltage during charge, the watt-hour efficiency is always less than the ampere-hour efficiency.
For lead acid batter, watt-hour efficiency ranges between 70 % to 80 %.