Glossary

Depth of discharge

Unit % · Also DoD, Depth of Discharge

What is Depth of discharge?

The share of the rated capacity that may regularly be drawn without unduly shortening service life. Around 50 percent for lead-acid, considerably more for lithium.

What matters about Depth of discharge in practice?

The size of the battery bank follows from the daily balance and the depth of discharge: a requirement of 150 amp-hours at 50 percent permitted depth of discharge gives 300 amp-hours of rated capacity.

Depth of discharge is not a fixed value but a trade-off: regularly discharging a lead-acid battery to 80 percent gets you considerably fewer cycles. The data sheet usually shows this relationship as a curve.

What depth of discharge is usual for lead and lithium?

For lead-acid and AGM batteries you plan with around 50 percent, for LiFePO4 with about 80 percent of the rated capacity. The battery management system of a lithium battery disconnects at undervoltage, while a lead battery without protection simply keeps discharging.

What does that mean for battery size?

With 100 Ah daily consumption and two days without recharging, 200 Ah are drawn. With lead this needs 400 Ah rated capacity, with LiFePO4 about 250 Ah. That is why a lithium battery is usually much smaller and lighter for the same benefit.

How do you keep an eye on depth of discharge?

  • A battery monitor with shunt counts the amp-hours drawn and shows the state of charge in percent.
  • Load shedding switches loads off before the limit is reached.

The battery capacity calculator works out the right capacity.

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