Internal Resistance Calculator

Internal Resistance of a Power Source

Every real battery or power source has some internal resistance that causes its terminal voltage to drop below its ideal EMF (electromotive force) when current flows. It’s calculated as r = (EMF − V_terminal) / I, where I is the load current drawn from the source.

A fresh, healthy battery typically has low internal resistance, while a depleted or degraded battery has higher internal resistance, causing its voltage to sag more under load. This measurement is commonly used to assess battery health and predict how much current a source can deliver before its voltage drops too far to be useful.