On our boat, Batt1 and Batt2 are connected to their respective terminals on the battery connector switch. Batt3 is connected to Batt1 through a high current
continuous relay which is activated by the ignition switch. When the engine
is on, Batt3 is paralleled with Batt1, and when the Ign switch is off, Batt3 is isolated. Actually, not quite this simple, since Volvo
, in their infinite wisdom, dump all other loads when the key is turned to the START position.
A diode between the Start terminal and Aux terminal solved