It permanently compares the connection between the starter battery, the body battery and their connected loads. It switches the charging power to the body battery as soon as the voltage of the starter battery rises (charging by alternator). If the voltage of the starter battery drops below 12.8 volts, it disconnects the on-board battery from the charging process. A D+ signal is not necessary and the wiring is reduced to a minimum. When the on-board battery reaches 10.5 volts, it switches off all consumers to protect the battery from deep discharge.