Enter the rated energy (Ah) and the current (amps) of charge or discharge into the calculator to determine the C rate and time to charge. This calculator can also determine the rated energy or current if the other variables are known.

C Rate Formula

The following formula is used to calculate the C rate of a battery.

Cr = I / E 

Cr = I / E

  • Where Cr is the C rate
  • I is the current of charge/discharge (Amps)
  • E is the rated energy of the battery (Ah) (amp hours)

To calculate a c rate, divide the current of charge or discharge by the rated battery energy in amp hours.

C Rate Definition

C-Rate is defined as the inverse of the time it takes, in hours, to charge or discharge a battery. For example, a battery that takes 2 hours to charge has a C Rate of 1/2 =.5.

Example Problem

How to calculate the C Rate of a battery?

For the first example, we will use the current and rated energy to calculate the C rate.

First, determine the rated energy storage of the battery. In this problem the rated energy is found to be 200 Ah.

Next, determine the current of charge or discharge. This is found to be 50 amps.

Finally, calculate the C rate using the formula:

Cr = I / E

= 50/ 200

= .25

In this next example, we will calculate the C Rate using the time it takes to charge.

After taking a battery from full charge to 0 the time to discharge is measured to be 2 hours.

Using the formula Cr = 1/t, the C rate is found to be 1/2 = .5.