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ITFrequently Asked Questions · Batteries
What does depth of discharge (DOD) mean for batteries?
Depth of discharge shows how much energy has been drawn from the battery relative to its total capacity.
Depth of discharge is expressed as a percentage. If a 100 Ah battery delivers 30 Ah, the depth of discharge is 30%, while the remaining state of charge (SOC) is 70%. The two figures are complementary and always add up to 100%.
DOD matters because it directly affects service life. The deeper a lead-acid battery is discharged in each cycle, the fewer cycles it will withstand in total. This is why manufacturers always state the number of cycles against DOD: the same battery lasts many more cycles at 30% than at 100%.
In a UPS, the batteries usually stay fully charged and discharge only during outages, so the DOD each time depends on the length of the outage and the load. In solar systems or applications with daily use, DOD becomes a key design criterion.
Practical tip: when calculating backup time, do not plan for the battery to be drained to 100% every time. A larger capacity that keeps daily discharge lower extends battery life considerably.