An interface circuit charges a rechargeable battery from a power source. A normally-closed relay with normally-closed switch contacts and a coil having a first end and a second end has its normally-closed switch contacts and the coil coupled in series between first and second input terminals. An EMF conduction diode is coupled between a fourth output terminal and a junction between the normally-closed switch contacts and the first end of the coil. The second end of the coil is coupled to a third output terminal. Current supplied from the power source energizes the coil and causes the normally-closed switch contacts to oscillate between an open condition and a closed condition. Energy transferred from the power source to an electromagnetic field of the coil when the normally-closed switch contacts are in the closed condition is transferred by discharging the electromagnetic field in response to current through the EMF conduction diode when the normally-closed switch contacts are in the open condition.