What is a Battery C-Rate? Definition and Calculations

C-rate refers to the rate at which a battery charges or discharges relative to its maximum capacity. In other words, the speed at which delithiation and lithiation occurs in a lithium-ion battery. The higher the C-rate the faster charging or discharging occurs.

How is the C-rating of a Battery Represented?

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The C-rating of a battery is given as a number followed by C (eg. 1C) or C divided by a number (eg C/10). A 1C battery c-rate means that it takes one hour for the battery to charge (or discharge) to capacity at a given current.

• A high C-rate results in a battery charging/discharging at higher power for a shorter period of time.
• Example: 10C means it will take 6 mins (1/10 hr) to reach capacity or for a fully charged 100 mAh battery to provide 100 mA current.
• A low C-rate results in a battery charging/discharging at low power for a longer period of time.
• Example: C/100 or 0.01C means it will take 100 hrs to reach capacity or for a fully charged 100 mAh battery to provide 1 mA current.

Here are a few examples of different C-rates, the time associated with that rate as well as a potential application for a battery with that C-rate:

C-rate	Time	Application
5C	12 mins	Power Tool
2C	30 mins	Drone
0.5C or C/2	2 hr	EV Battery
0.1C or C/10	10 hr	Grid-level Storage
0.05C or C/20	20 hr	Laptop Battery

The rate at which a battery charges or discharges is hugely influential over which applications it is suitable for. Battery designers have to balance C-rates with battery stability and lifetime. High-capacity materials (e.g., NMC with high nickel content) tend to degrade faster, while lower-capacity, more stable materials (e.g., LiFePO₄) last longer.

How to calculate the C-rate (charge or discharge rate)

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When calculating the C-rate of a given battery there are two crucial parameters:

• Battery Capacity
• Current

Battery capacity for a given lithium-ion battery is dependent on the cathode material as it directly dictates how many lithium ions can be stored and released.

Equation

C-rate (h^-1) = Current (mA) / Battery Capacity (mAh)

Battery Capacity (mAh) = Capacity of Cathode Material (mAh/g) x Mass Cathode Materials (g)

Make sure all your units match up. If the capacity of the cathode material is in Ah/g then the battery capacity will be in Ah. The same goes for the unit of mass.

To work out charging or discharging time:

Time (h) = Battery Capacity (mAh) / Current (mA)

Charge Rate Calculations

To calculate the charge rate for a battery you must consider the applied current and battery capacity as mentioned above.

For a battery receiving an applied current of 100 mA that has a battery capacity of 500 mAh then the C-rate will be:

C-rate (h^-1) = 100 mA / 500 mAh = 0.2C

If you want to charge a battery for a certain amount of time to capacity then you must alter the applied current. If the battery capacity is 520 mAh and has a 2C C-rate then the calculation is:

Current (mA) = Battery Capacity (mAh) x C-rate (h^-1)

Eg. Current (mA) = 520 mAh x 2C (0.5 h^-1)

Current (mA) = 1040 mA = 1.04 A

Discharge Rate Calculations

To calculate the rate at which a battery discharges then you must consider the battery capacity and c-rate.

If you have a 10 Ah lithium battery:

C-rating	1C	5C	0.1C
Calculation	1C = 10 Ah x 1C = 10 A discharge current available 1C = 10 Ah/10 A = 1 hour discharge time	5C = 10 Ah x 5C = 50 A discharge current available 5C = 10 Ah / 50 A = 0.2 hours (12 mins) discharge time	0.1C = 10 Ah x 0.1C = 1 A discharge current available 0.1C = 10 Ah / 1 A = 10 hours discharge time
Answer	The battery can be used for 1 hour with a current load of 10 A	The battery can be used for 12 mins with a current load of 50 A	The battery can be used for 10 hours with a current load of 1 A

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