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Depth of Discharge

What is Depth of Discharge and why is it so important?

A battery’s depth of discharge (DoD) indicates the percentage of the battery that has been discharged relative to the overall capacity 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 %.

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 Ah (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 %).