Why internal resistance matters
Every battery behaves like an ideal voltage source in series with a small internal resistance. When current flows, part of the voltage is lost inside the battery and turns into heat. A battery with high internal resistance cannot deliver high currents: the voltage sags, devices brown out and a car may not start, even if the battery still holds plenty of charge.
Measuring it with a voltmeter
- Measure the open-circuit voltage with nothing connected.
- Apply a known load and measure the voltage under load and the current.
- R = (V_open − V_load) ÷ I
Take the loaded reading after a second or two, once the voltage has settled, and keep the test short so the battery does not discharge or warm up noticeably.
Example
A 12 V car battery reads 12.6 V at rest and 12.1 V while supplying 25 A, so R = 0.5 ÷ 25 = 20 mΩ. At a 150 A cranking current it would sag to 12.6 − 150 × 0.020 = 9.6 V, which is the usual pass/fail threshold of a battery load test. During the 25 A test, 12.5 W of heat is produced inside the battery.
What else the calculator shows
- Efficiency: loaded voltage ÷ open-circuit voltage, the share of energy that reaches the load.
- Short-circuit current: V ÷ R, the theoretical maximum. Never try it: real batteries can deliver enormous currents and cause burns, fire or explosion.
- Maximum power: V² ÷ 4R, delivered when the load resistance equals the internal resistance, at only 50% efficiency.
Typical values
| Battery | Typical internal resistance |
|---|---|
| New 12 V car battery | 4 to 8 mΩ |
| Worn car battery | 15 mΩ and more |
| 18650 lithium-ion cell | 15 to 50 mΩ |
| AA NiMH | 20 to 50 mΩ |
| AA alkaline | 150 to 300 mΩ |
Interpreting results
- Internal resistance rises with age, cold and low state of charge, so compare readings at similar temperatures and charge levels.
- Dedicated testers often measure at 1 kHz (AC resistance), which gives lower values than this DC method.
- In a battery pack, a cell with much higher resistance than its neighbors will heat up and age faster; it is a good candidate for replacement.