Calculate battery C-rate, current, or required capacity in Ah or mAh, with ideal runtime, stored energy, load power, and charge/discharge planning.

Optional voltage

Blank omits nominal energy and power estimates.

C Rate Formula

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

Cr = I / E
  • Where Cr is the C rate
  • I is the current of charge/discharge (amps)
  • E is the rated capacity of the battery in amp-hours (Ah)

To calculate a C-rate, divide the current of charge or discharge by the rated battery capacity in amp-hours.

C Rate Definition

C-rate is the current in amperes divided by the rated capacity in amp-hours. Under an ideal constant-current, full-capacity model, it is the inverse of the time in hours: an ideal 2-hour interval corresponds to 0.5C. Actual charging and discharge times depend on the battery and operating conditions.

Example Problem

How to calculate the C Rate of a battery?

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

First, determine the rated capacity of the battery. In this problem the rated capacity is 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 idealized constant-current example, we will estimate the C-rate from the full-capacity discharge interval.

Assume the full rated capacity is delivered at constant current over 2 hours. This is an idealized model, not an instruction to discharge a real battery to zero.

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