What does C-Rate mean?

Explanation

C-rate expresses charging or discharging speed as a multiple of the battery's capacity. A 1C rate means the battery charges (or discharges) its full capacity in one hour. A 2C rate does it in 30 minutes. A 0.5C rate takes two hours. For a 75 kWh battery, 1C equals 75 kW of power.

C-rate is useful for comparing charging speeds across cars with different battery sizes. A 50 kWh battery charging at 100 kW is charging at 2C. A 100 kWh battery charging at 100 kW is only at 1C. The smaller battery is being stressed more relative to its size, even though both receive the same power.

Higher C-rates generate more heat and create more stress on the battery cells. Most EV batteries are designed to handle peak charging C-rates between 1C and 3C for short periods. Sustained high C-rates accelerate degradation. This is another reason why larger batteries tend to maintain their health better over time: the same fast charger represents a lower C-rate for a bigger battery.

For current specifications, see National Renewable Energy Laboratory.

Where you'll see this

• In vehicle specifications

Common confusion

C-rate is not a fixed speed. It is relative to battery size. A 100 kW charger represents a very different C-rate depending on whether the battery is 40 kWh (2.5C) or 100 kWh (1C).

Example

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