Electronics: Testing Battery Capacity

Find out the true mAh of your 18650 or LiPo batteries using a load resistor and a multimeter.

1. The Circuit

Connect a fully charged battery (4.2V) in series with a known load resistor (e.g., 4 Ohm, 10W).

2. Measure Current

Measure the current flow: I = V / R. For a 4.2V battery and 4 Ohm resistor, I = 1.05 Amps.

3. Monitor Voltage

Note the time. Check the battery voltage every 15 minutes. Stop the test when the voltage reaches 3.0V.

4. Calculate mAh

Capacity = Current (mA) x Time (hours). If it took 2 hours at 1050mA, the capacity is 2100mAh.