The invention provides a system for jump starting vehicle batteries in which the service end of each of two jumper cables has a self-adjusting locking clamp, and each jumper cable is segmented into two parts that are joined by a plug and socket configured such that the cable parts cannot be incorrectly joined or will be relatively harmless if incorrectly joined. Auxiliary wiring is provided for recharging free-standing source batteries that are used for jump-starting. The system enhances user safety, simplifies making and breaking of electrical connections, and facilitates the use of back-up batteries.