Glossary

C-rate

Unit C

What is C-rate?

Charge or discharge current relative to the rated capacity. 0.5 C means half the numerical value of the capacity as current — for 200 Ah, that is 100 amps.

Current in A = C-rate × capacity in Ah

What matters about C-rate in practice?

Data sheets usually state capacity for a specific discharge, such as C20 — that is, over twenty hours. Discharged faster, the same lead-acid battery delivers considerably less. This effect is called the Peukert effect.

On the charging side, the permitted C-rate limits the sensible charging current. For lead-acid it lies between 0.25 and 0.4 C depending on the type; for TPPL and lithium types it is considerably higher.

How do you calculate with the C-rate?

Current = C-rate × rated capacity. For a 200 Ah battery, 0.5 C is therefore 100 A and 1 C is 200 A. The other way round: an inverter drawing about 190 A at 12 V loads a 200 Ah battery at almost 1 C.

Which C-rates are usual?

  • Lead batteries: charging currents mostly between 0.1 and 0.3 C, depending on design.
  • LiFePO4: charging and discharging often up to 0.5 C or more; the data sheet states the limits.
  • Short peaks, for example at start-up, may be higher than the continuous current.

Why does the C-rate matter in planning?

It shows whether a battery suits large loads such as inverter, anchor windlass or bow thruster, and which charging current charger and DC-DC charger may deliver. A small battery with a large inverter is quickly overloaded. Batteries with charge and discharge current data are listed under LiFePO4 battery for boats.

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