Internal Resistance Calculator

Find a battery's internal resistance from EMF and terminal voltage.

Internal resistance (Ω) 0.4

Formula: r = (EMF − V) ÷ I

Step-by-step with your numbers:

1. Values used:

2. EMF (open circuit) = 1.5 V

3. Terminal voltage = 1.3 V

4. Current = 0.5 A

6. Internal resistance = 0.4Ω

Did we solve your problem today?

Every real battery has internal resistance that drops its voltage under load.

The math behind it

r = (EMF − V_terminal) ÷ I. The bigger the current, the more the terminal voltage sags.

1.5 V EMF dropping to 1.3 V at 0.5 A → 0.4 Ω.

Why does a weak battery dim under load?

High internal resistance causes a large voltage drop when current flows.