Abstract: A battery has an exposed negative terminal (2), but the positive terminal (10) is hidden within the casing of the battery and instead only a dummy terminal (3) is visible. When the vehicle is parked, power is fed from the positive terminal (10) through an isolator relay (12) to an INHIBIT circuit (C). When power is made available through the ignition switch to a terminal (6), power is fed via a socket (4) to a keypad. A predetermined code has to be set into the keypad in order to pass an operating signal to an electronic lock (13), which is effective to switch the relay (12) to connect power to an ENABLE circuit (B) and to disconnect the supply from the INHIBIT circuit (C). This causes a solenoid (9) to operate for a time determined by a timer (14) so that the positive terminal (10) is connected to the dummy terminal (3) through a plunger (9A). When the ignition key is moved to the start position power is supplied through the starter lead connected from the terminal (3) to a starter motor.