Depth of discharge

Depth of discharge (DoD) is the fraction or percentage of the capacity which has been removed from the fully charged battery. It is an alternative method to indicate a battery's state of charge (SoC). The depth of discharge is the complement of state of charge: as one increases, the other decreases.

Definition

For almost all known rechargeable battery technologies, such as lead-acid batteries of all kinds like AGM, there is a correlation between the depth of discharge and the cycle life of the battery. Depth of discharge is defined as the total amount of energy that is discharged from a battery, divided by the battery nominal capacity. Depth of discharge is normally expressed as a percentage.

While the state of charge is usually expressed using percentage points (0% = empty; 100% = full), depth of discharge is either expressed using units of Ah (e.g. for a 50 Ah battery, 0 Ah is full and 50 Ah 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 Ah for a 50 Ah battery, or 110%).

Sample calculation

If a 90 Ah battery is discharged for 20 minutes at a constant current of 50 A, the depth of discharge is:

${\frac {50{\text{ A}}\cdot {\frac {20{\text{ mins}}}{60{\text{ mins}}}}{\text{ hours}}}{90{\text{ Ah}}}}\cdot 100\%=18.522\%$ 