Short answer: A measure of how fast a battery charges or discharges relative to its capacity, where 1C means fully charging or discharging in one hour.
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.
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
A 58 kWh Volkswagen ID.3 charging at its peak of 170 kW is experiencing about 2.9C. A 100 kWh Mercedes EQS charging at 200 kW is at only 2C, putting less relative stress on the cells.
Related terms
See a term you don't recognize? Scan it.
Point your phone at any charger screen. Coming soon.
Stuck at the charger? Open the app.
Step-by-step help for real charging problems. Log the experience. Free on iOS and Android.
Free to download · Available on iOS and Android