Glossary

Rated capacity

Unit Ah

What is Rated capacity?

The capacity stated on the battery, measured under defined conditions — usually at a specific discharge time and temperature. It is not the usable capacity.

What matters about Rated capacity in practice?

Only the share released by the permitted depth of discharge is usable: about half for lead-acid, considerably more for lithium. A 200 Ah lead-acid battery therefore delivers around 100 usable amp-hours.

In addition, the figure applies to a specific discharge time — often twenty hours. Discharge faster and you get less out than the label says.

Under which conditions does rated capacity apply?

For lead batteries usually for a 20-hour discharge (C20) at around 25 °C, for lithium batteries often for a faster discharge. A 100 Ah lead battery therefore supplies 5 A for 20 hours. Discharged at 20 A, noticeably less comes out.

What reduces the available capacity?

  • Cold: lead batteries deliver noticeably less in frost.
  • High discharge currents, especially with lead.
  • Ageing: at the end of their service life often only 80 percent of rated capacity remains.

How do you plan with rated capacity?

From the rated capacity you calculate the usable capacity via the permitted depth of discharge and allow a reserve for ageing and cold. The battery capacity calculator works out the right size for your consumption.

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