Abstract: A battery protection circuit for an engine ignition system comprises first and second terminals for connection in series with a discharge circuit of the battery and normally ON switching means connected between said terminals. If the battery voltage falls below a predetermined level, for example, as a result of leaving the headlights on in a parked motor vehicle, incorporating the circuit in its ignition system, the switching means turns OFF and isolates the battery, so as to prevent further discharge of the battery. The switching means can be reset by the user, for example, by means of a signal from a handheld I.R. transmitter on returning to the vehicle, enabling the ignition coil to be energized from the battery and the engine to be started. Thus the circuit can be used to prevent a vehicle battery from going flat when the vehicle is parked.