Calculator KB-24
Battery C-rate calculator
At 1C, a battery discharges in one hour; at 2C, in half an hour. See what that means for your cells.
Result
In brief: C-rate calculator: battery charge and discharge current by capacity, time, power, voltage sag, and heating from internal resistance. Example: Current: 6 A.
Calculation example
| Capacity | 3 Ah |
|---|---|
| C-rate | 2 C |
| Voltage | 3.6 V |
| Internal resistance | 30 mΩ |
| Current | 6 A |
|---|---|
| Full discharge time | 30 min |
| Power | 21.6 W |
| Voltage sag | 180 mV |
| Heat inside the cell | 1.08 W |
How it is calculated
t = 1 / C
Pheat = I² · R
C-rate shows current relative to capacity. The time shown here is theoretical: at high currents, a battery delivers less capacity, while heat increases with the square of the current, so twice the current means four times the heating.
What to look for
- Check the continuous current rating in the datasheet, not the seller's claim on the label.
- A lithium cell should not heat above 60 °C during discharge.
Frequently asked questions
What does 1C mean for a battery?
It’s a current numerically equal to the battery’s capacity: for a 3 Ah battery, 1C is 3 A and 2C is 6 A. In theory, 1C discharges a battery in an hour, and 2C in 30 minutes.
What C-rate is acceptable for an 18650?
Check the datasheet for the specific model: high-capacity cells are usually rated for up to 1–2C, while high-current cells can handle up to 10C. Ratings on cheap cells are often exaggerated.
How does C-rate affect battery heating?
Heat increases with the square of the current: P = I² · R. At 6 A and 30 mΩ, that’s 1.08 W per cell; at 12 A, it’s already 4.3 W.
