Real batteries have internal resistance, so their terminal voltage drops when they deliver current. A weak or cold battery has a higher internal resistance – which is why a car may not start on a frosty morning.
How to use the Internal Resistance Calculator
- Electromotive force ε
- Load resistance R
- Current I
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the EMF (open-circuit voltage), the load resistance and the measured current.
Inputs explained
| Field | What to enter |
|---|---|
| Electromotive force ε | Units: V, mV, kV, µV, MV |
| Load resistance R | Units: Ω, kΩ, MΩ, mΩ |
| Current I | Units: A, mA, µA, kA |
What each result means
- Internal resistance r = [Electromotive force ε]÷[Current I] – [Load resistance R]
- Terminal voltage = [Current I]×[Load resistance R]
- Power lost inside = [Current I]^2×([Electromotive force ε]÷[Current I] – [Load resistance R])
- Efficiency = [Load resistance R]÷([Electromotive force ε]÷[Current I])×100
Example values
How the main result changes when Electromotive force ε changes and everything else stays at the example values above:
| Electromotive force ε (V) | Internal resistance r (Ω) |
|---|---|
| 6.3 | -0.95 |
| 9.45 | -0.425 |
| 12.6 | 0.1 |
| 15.75 | 0.625 |
| 18.9 | 1.15 |
| 25.2 | 2.2 |
Formula
r = ε/I − R, V_terminal = I·R = ε − I·r
Worked example
Inputs — Electromotive force ε: 12.6 V; Load resistance R: 2 Ω; Current I: 6 A.
Results — Internal resistance r = 0.1 Ω; Terminal voltage = 12 V; Power lost inside = 3.6 W; Efficiency = 95.2381 %.
Key terms
- EMF
- Electromotive force, the open-circuit voltage.
Frequently asked questions
How do I measure EMF?
Measure the voltage with no load connected.
Why does terminal voltage drop under load?
Current through the internal resistance causes a voltage drop.
What is maximum power transfer?
When the load equals the internal resistance.
Does internal resistance change?
It rises with age and at low temperatures.