How this calculator works
A simple real-cell model places an ideal emf in series with an internal resistor. The same discharge current flows through the internal resistor and the load. Terminal voltage is below emf by I times the internal resistance. This is a local steady-state approximation; battery resistance varies with temperature, state of charge, age, current and measurement time.
Formula
I = emf/(Rload + r). r = emf/I - Rload. Vterminal = I Rload = emf - I r.
Worked example
A 12 V emf delivering 2 A through a 5 ohm load has internal resistance 1 ohm and terminal voltage 10 V. A 9 V emf with 8 ohm load and 1 ohm internal resistance delivers 1 A at 8 V.
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Frequently asked questions
Can internal resistance be zero?
Yes. That represents an ideal voltage source, provided the total series resistance stays positive.
Why reject negative resistance?
It is inconsistent with the passive cell model. Check the measured current, load and open-circuit voltage.
Does this model charging?
The current sign and battery dynamics during charging require a different convention. This page models nonnegative discharge.