Question

A source of what potential difference is needed to charge a battery of 20V e.m.f and internal resistance of 0.1 ohm at arate of 70A.

Answer 1

Source potential will need to exceed the 20 V of the battery as well as overcome the drop across the internal resistance. Twenty seven volts would do that, but that may produce excessive charging current. If the specifications for the battery state a charging current of 2 amps then the potential of the charging source would be approximately 21 volts.