C-rate
The charge or discharge current of a battery relative to its capacity; 1C empties a full battery in one hour, 0.5C in two hours.
The C-rate normalises current to battery capacity so different sizes can be compared. A 100 Ah battery discharged at 100 A runs at 1C; at 50 A, 0.5C (C/2); at 200 A, 2C. For storage systems the same idea is applied to power and energy.
Duration (h) ≈ 1 / C-rate
Example: 100 MW / 400 MWh BESS = 0.25C, a 4-hour system
Higher C-rates increase resistive (I²R) heating and polarisation, reducing usable capacity and accelerating ageing. Datasheets therefore quote capacity and cycle life at a specified rate, commonly 0.5C or 1C. Typical ranges:
- Grid BESS for energy shifting: 0.25–0.5C (2–4 hours)
- Frequency regulation systems: 1–2C
- Home batteries: about 0.3–0.5C continuous
- EV fast charging: peaks of 2–4C on modern packs, tapering above about 60–80% SoC
Power-optimised cells use thinner electrodes and more conductive additives, sacrificing energy density; energy-optimised cells do the reverse. The C-rate is the battery counterpart of the power-to-energy ratio discussed under power vs energy.
Related terms
All terms →- Power vs energy (kW vs kWh)
Power (kW) is the rate of energy flow at an instant; energy (kWh) is power accumulated over time. Capacity is measured in power, output in energy.
- Cycle life
The number of full charge-discharge cycles a battery can deliver before its capacity falls to a defined end-of-life threshold, usually 70–80% of rated capacity.
- Depth of discharge (DoD)
The fraction of a battery's capacity that has been discharged, or the maximum fraction a manufacturer allows to be used per cycle.
- BESS (Battery Energy Storage System)
A complete grid- or site-connected battery installation, including cells, battery management, power conversion, thermal management and controls.
- DC fast charging
High-power EV charging in which an off-board charger delivers direct current straight to the battery, typically 50–400 kW and above.