a) E.M.F : The electromotive force of a battery is defined as the energy spent or the work done in taking a unit positive charge around the complete circuit of the battery i.e., in the circuit outside the cell as well as in the electrolyte inside the cell. When no current is drawn from a battery i.e. when the battery is in open-circuit condition, then potential difference between the terminals of the battery is its E.M.F.
b) Internal resistance: The resistance within the source that causes a drop in the source voltage when load current flows, is called internal resistance.
c) AH efficiency: Ampere-hour efficiency of a battery is defined as the ratio of the output of battery in amp-hr during discharging to the input amp-hr of battery during charging.
d) Watt – Hr efficiency: The ratio of the output of a battery, measured in watt-hours, to the input required to restore the initial state of charge, under specified conditions, is called Watt-hr efficiency.