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Depth of discharge is an alternative method to indicate a battery's state of charge. The depth of discharge is the complement of state of charge: as one increases, the other decreases. While the state of charge is usually expressed using percentage points (0 % = empty; 100 % = full), depth of discharge is usually expressed using units of A h (e.g, 0 is full and 50 A h is empty) or percentage points (100 % is empty and 0 % is full). The capacity of a battery may be higher than its nominal rating. Thus it is possible for the depth of discharge value to exceed the nominal value (e.g., 55 A h for a 50 A h battery, or 110 %).
At least in some battery technologies, such as lead-acid AGM batteries, there is a correlation between the depth of discharge and the cycle life of the battery. Depth of Discharge is defined as the capacity that is discharged from a fully charged battery, divided by battery nominal capacity. Depth of discharge is normally expressed as a percentage. For, example, if a 100 A h battery is discharged for 20 minutes at a current of 50 A, the depth of discharge is 50 * 20 / 60 / 100 = 16.7 %.
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