Batteries
Battery Internal Resistance Calculator
This calculator evaluates internal resistance for a single batteries scenario. It uses open-circuit voltage, loaded voltage, and test current and shows the equation behind the displayed value.
Run the Battery Internal Resistance scenario
Do not mix values recorded under different conditions.
Measurements and entries
Separate nameplate capacity from measured usable capacity. Write down whether each entry is measured, rated, assumed, or calculated.
A denominator entry cannot be zero. Confirm decimal placement whenever a source uses a different unit scale. For internal resistance, use Source Internal Resistance Calculator.
- Open-circuit voltage
- Default example: 12.8 V. Enter open-circuit voltage in V.
- Loaded voltage
- Default example: 12.2 V. Enter loaded voltage in V.
- Test current
- Default example: 20 A. Enter test current in A.
From inputs to result
The arithmetic is based on r = (Voc − Vload) ÷ Itest. Only Open-circuit voltage, Loaded voltage, and Test current enter the numerical result.
For the example shown, internal resistance equals 0.0300 Ω. Treat that value as a demonstration, not a recommendation.
Calculate battery internal resistance from a documented load test. If you also need loaded voltage, continue with Battery Voltage Sag Calculator.
Changing Open-circuit voltage
Change Open-circuit voltage from 12.8 V to 15.36 V and preserve the rest of the case. This moves the answer from 0.0300 Ω to 0.1580 Ω.
Keep both cases when the changed input represents genuine uncertainty.
What the number means
Read Internal resistance as the outcome of this equation, not as automatic equipment approval. Compare it with manufacturer discharge data at the intended rate.
Save the raw entries before testing another scenario.
Checks before using the answer
Measure both voltages at the same state of charge and temperature.
The arithmetic intentionally leaves out chemistry-specific limits and manufacturer test conditions. Test an additional scenario when an omitted effect has a plausible range.
Separate nameplate capacity from measured usable capacity.
Questions about inputs and assumptions
When should I round internal resistance?
Retain the raw result for comparison and create a separately rounded reporting value if needed.
Which input can create an undefined result?
Test current is used in a denominator and must be greater than zero.
How should I prepare the Battery Internal Resistance inputs?
Use open-circuit voltage, loaded voltage, and test current from one operating condition. Separate nameplate capacity from measured usable capacity.
What assumptions affect Internal resistance?
Measure both voltages at the same state of charge and temperature. A broader review should include temperature, cell imbalance, BMS cutoffs, aging, and rate dependence.