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?

  • 1
    \$\begingroup\$ You’ll want some fuses to protect the cables in there as well. \$\endgroup\$
    – Kartman
    Commented Sep 14, 2021 at 6:29
  • 1
    \$\begingroup\$ On my boat I have basically the same setup as you plan to build. I’ve “solved” the emergency starting issue by having some heavy jump start cables that I can connect between the engine and house battery banks when needed. It’s more versatile and cheaper and helps keep complexity down. It’s only rarely needed anyway. \$\endgroup\$
    – StarCat
    Commented Sep 14, 2021 at 7:30
  • \$\begingroup\$ I should make it clear my setup uses just a “smart” VSR. I also have heavy duty switches between each battery bank and their loads. \$\endgroup\$
    – StarCat
    Commented Sep 14, 2021 at 7:42
  • \$\begingroup\$ I have fuses to use in the circuits. I'm pretty confident what I've done will work, just not sure about the inner workings of the VSR but I presume I can use it the way I've designed. Thanks for the responses. \$\endgroup\$
    – HobbyCoder
    Commented Sep 15, 2021 at 9:35

1 Answer 1


Below is a simple simulation of using ideal diode rectifier that has 30mV voltage drop. When the aux bank is drained, it will take all the current.

Connecting together aux. bank with a start battery could boil your start battery - only the voltage difference and internal resistance would limit the current.


simulate this circuit – Schematic created using CircuitLab


simulate this circuit

For a such large battery bank, I think it would be better to install yet one separate alternator.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.