Depth of discharge (DoD) and battery lifetime

What depth of discharge is

The depth of discharge (DoD) says how much of the battery’s capacity you use in one cycle. If a 10 kWh battery delivers 8 kWh before being recharged, the discharge was 80%.

Why shallow discharges last longer

Each cycle wears the battery, and wears it more when it is deeper. The number of cycles a battery can take falls sharply with the depth of discharge, and the drop is steeper for lead-acid.

Calculate your battery’s life

Choose the type, the discharge of each cycle and the nominal capacity. The calculator interpolates the table and shows the cycles, the years (at one cycle per day) and the total energy the battery delivers over its life.

More cycles is not always more energy

Shallow discharges give more cycles, but each cycle delivers less energy. What matters is the total energy delivered over its life (capacity × DoD × cycles, an idealized estimate that ignores capacity loss over its life). For a 5 kWh lithium battery:

In the illustrative table used here, the range between 50% and 80% delivers the most accumulated energy. Raising the depth of discharge to 100% reduces total energy. In this example, operating at 30% DoD also reduces the calculated accumulated energy; the real choice also depends on the reserve you want and the operating conditions.

Using DoD in sizing

1. 1
   Set the energy per day

   The home needs 5 kWh from the battery per day.

2. 2
   Choose the design DoD

   With lithium at 80%, the nominal capacity is 5 ÷ 0.8 = 6.25 kWh. At 50% it would be 10 kWh.

3. 3
   Estimate the life

   At 80% that is 4,000 cycles: at one cycle per day, about 11 years (arithmetic equivalent at one cycle per day, ignoring calendar ageing) and 20,000 kWh delivered, as an idealized estimate.