Charging power and C-rate

C-rate expresses charging or discharging current relative to battery capacity. It is a better measure of cell stress than charger power alone, because the same number of kilowatts means something very different for a small and a large battery.

What C-rate means

A C-rate of 1C corresponds to a current that would fully charge or discharge the battery in about one hour; 2C would take about half an hour and 0.5C about two hours. To estimate it for a car, divide the charging power in kW by the battery capacity in kWh. As an illustration, 11 kW into a 55 kWh pack is about 0.2C, while 110 kW into the same pack is about 2C.

This is why the same charger affects different cars differently. A large pack absorbs a given power at a lower C-rate than a small pack, so each cell carries less current.

Why higher C-rates add stress

At higher currents, lithium has to enter the anode faster than it can diffuse into the graphite particles. This raises the risk of lithium plating, especially at low temperature and high SOC, and generates more heat. Cycle-life studies on LFP and comparative tests across chemistries found that higher C-rates generally accelerate capacity loss, with the size of the effect depending on chemistry and temperature.

Research on the trade-off between fast charging, energy density and cycle life identifies plating as the central limiting mechanism. Cells designed for high energy density tend to be more limited in how fast they can be charged without additional ageing.

The charger is not the C-rate

A charger's nameplate power is the maximum the station can deliver, not what your battery receives. The vehicle's BMS sets the actual current based on its charging curve, the pack temperature and the current SOC. Many cars charge at high power only in a limited SOC window and taper as the battery fills.

Home AC charging typically results in low C-rates and is generally considered gentle in terms of current. That does not make AC charging free of ageing: calendar effects such as time at high SOC still apply.

How Voltgevity handles C-rate

Voltgevity estimates the effective C-rate of your charging sessions from the charging power you use, your vehicle's battery size and a typical charging curve, rather than from the station's nameplate rating. The result feeds into the cycle ageing estimate together with temperature and SOC window.

Key takeaways

C-rate is charging power relative to battery capacity, a better stress indicator than kilowatts alone.
Higher C-rates raise lithium plating risk and heat, especially when cold or at high SOC.
The vehicle's charging curve, pack temperature and SOC determine the actual current, not the charger's nameplate.
Low-power AC charging keeps C-rates low, but calendar ageing still applies.