Battery Internal Resistance Calculator
Internal resistance of a battery or cell from its voltage with and without a load, with the heat it loses and the current it could deliver into a short.
Results
Internal resistance
25.00
mΩ
Power lost inside the battery
10.00
W
Short-circuit current (theoretical)
504
A
Maximum power it could deliver
1588
W
What this tool does
Every battery behaves like a perfect voltage source behind a small resistor, which is why its voltage sags under load. Measure the voltage at rest, then under a known current, and the drop divided by the current is that resistance. It rises as a battery ages, gets cold or runs low, so it is one of the best indicators of health — a car battery whose resistance has doubled may still show a good voltage at rest. Measure quickly under load, before the voltage drifts with temperature and charge.
Formula
R_int = (V_open − V_load) ÷ I · I_sc = V_open ÷ R_int · P_max = V_open² ÷ (4 R_int)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
vo | Voltage with no load | V |
vl | Voltage under load | V |
i | Load current | A |
R | Internal resistance | mΩ |
PL | Power lost inside the battery | W |
SC | Short-circuit current (theoretical) | A |
PM | Maximum power it could deliver | W |
Worked example
- Voltage with no load12.6 V
- Voltage under load12.1 V
- Load current20 A
- Internal resistance25.00 mΩ
- Power lost inside the battery10.00 W
- Short-circuit current (theoretical)504 A
- Maximum power it could deliver1588 W
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.