State of charge (SOC) quantifies the remaining capacity available in a battery at a given time and in relation to a given state of ageing. It is usually expressed as percentage (0% = empty; 100% = full). An alternative form of the same measure is the depth of discharge (DOD), calculated as 1 − SOC (100% = empty; 0% = full). It refers to the amount of charge that may be used up if the cell is fully discharged. State of charge is normally used when discussing the present state of a battery in use, while depth of discharge is most often used to discuss a constant variation of state of charge during repeated cycles.