A battery charger supplies a current of 10 A to charge a storage battery which has an open circuit voltage of 5.6 V. If the voltmeter connected across the charger reads 6.8 V, what is the internal resistance of the battery?

0.12 Ω

Let r be internal resistance of the battery. Since the battery is charging,

∴  V = ε + Ir

Here, I = 10 A, ε = 5.6 V  and  V = 6.8 V

∴  6.8 = 5.6 + 10r      or      r = $\frac{1.2}{10}$ = 0.12 Ω