This diagram is the plan for my van to have a secondary bank of 12 V batteries in the back to run an inverter to power some equipment.
Originally my only extra requirement was to make sure the original starting battery in the van was not drained by the battery bank running the equipment in the back. My initial thought was to install a simple Voltage Sensing Relay battery isolator, an off-the-shelf item that monitors the voltage such that when the alternator starts to charge at 13.8 V the VSR closes a circuit to the secondary battery bank and allows charging, when the vehicle is switched off voltage drops below 13.8 V and the VSR disconnects the circuit and isolates the starting battery from the secondary bank.
Then I decided I wanted to include the ability to use the secondary battery bank to start the vehicle in a situation where the starting battery has failed. I found a VSR that has a simple override switch that will manually connect the circuit and allow me to use the secondary battery bank to start the vehicle except that the VSR is only 140 amp rated and this diesel starter motor might pull a up to 500 amps.
My solution is the include a 500 amp solenoid and 500 amp rated wiring between the secondary battery bank and the starting battery. When the VSR relay is manually (or automatically with > 13.8 V) energized it completes it's own circuit to the secondary battery bank but also energizes the solenoid that completes the 500 amp circuit back to the starter providing a high current path back to the starter and not destroying the VSR ........
What do you think?