Calculator KB-26
Battery internal resistance calculator
You can measure the main indicator of battery health with a simple multimeter and a load.
Result
In brief: Measure a battery's internal resistance from the voltage drop under load: resistance of the battery and cell, losses, short-circuit current, and an estimate of wear. Example: Internal resistance: 100 mΩ.
Calculation example
| No load | 4.15 V |
|---|---|
| Under load | 3.95 V |
| Load current | 2 A |
| Cells in series | 1 pcs |
| Internal resistance | 100 mΩ |
|---|---|
| Per cell | 100 mΩ |
| Voltage sag | 200 mV |
| Heat inside | 400 mW |
| Short-circuit current | 41.5 A |
How it is calculated
P = I² · R
Isc = Uoc / R
A battery can be modeled as an ideal source with a resistor inside. Under load, some of the voltage drops across this resistor. Measure the voltage immediately after connecting the load, before the battery has cooled down or discharged.
What to look for
- Measure directly at the cell terminals: wires and contacts add their own resistance. A four-wire setup is best.
- Resistance increases with age and in the cold. It’s useful for comparing cells in the same pack.
Frequently asked questions
How do I measure a battery’s internal resistance with a multimeter?
Measure the voltage with no load and under a known current, then divide the difference by the current. A drop from 4.15 to 3.95 V at 2 A gives 100 mΩ.
What internal resistance is normal for an 18650?
Healthy cells are typically around 20–80 mΩ, depending on the model and measurement method. An increase to 1.5–2 times the new-cell value indicates wear.
Why does the result differ from a 1 kHz tester?
A 1 kHz tester measures AC-IR and doesn't capture slow chemical processes, while the load method gives a higher DC-IR value. You can only compare measurements made using the same method.
