Apparatus for emergency power supply

An apparatus for emergency power supply to the starter motor of a motor vehicle comprising a switch (10) having two pairs of terminals which are connected by electrical leads (5-8) to contact terminals (1,3;2,4) for coupling to the battery of the motor vehicle and to an external battery. The switch (10) can be switched to an open state or to one of two following closed states, direct or crosswise, by an electronic circuit (13) which identifies the polarities of the terminals and sets the switch to one or other closed state in order to connect the terminals having the same polarity to each other. In order to avoid overloads, the circuit (13) detects a current which is greater than a predetermined value and, by means of a timer (13-4) and a control circuit (13-3), switches the switch (10) to the open state when the current which is greater than the predetermined value lasts longer than a predetermined period of time. The circuit (13) can also prevent the switch from being closed for a predetermined period of time after a signal for opening the switch.