OOB thinking - boosting voltage to charge trailer bank from towing vehicle

rickst29 over on the DIY Solar Forum has a neat solution to passing power from the towing vehicle to the trailer’s bank. He feeds ~440w to the trailer bank with what he calls a “virtual panel”.

He overcomes the current limits in the 7-pin wiring by ~tripling the voltage, thereby reducing current by 2/3rds. Power (~13A) is carried on the cable’s black trailer battery charge wire.

simplfied version

The original post contains a lot of detail and automation. Here is a simplified version:

On the truck: isolator -> DC-DC upconverter (36v) -> Bargman 7-pin trailer connector

On the trailer: 7-pin -> MPPT controller input

At the MPPT input: heavy switch to choose between real panels and “virtual” panels (tow vehicle charging)

Rick automates the controller input switching (and running the DC upconverter, as far as I can tell) with relays. When I need to switch inputs on my little 10A mppt (portable panels vs. the DC power supply (DIY converter) I use a manual battery switch. Rick says the most complex part of the project was the automation, so if you want a cheap/simple version a manual battery switch might suffice.


I think Rick is using Bargman because of the heavier wiring in the 7-pin; the barg looks like it is a gauge heavier than typical OEM cablles on each pin. According to Bargman their version is:

  • 14ga - Yellow: center auxiliary
  • 14ga - Green: tail and license lights
  • 14ga - Brown: right turn and stop lights
  • 14ga - Red: left turn and stop lights
  • 12ga - Blue: electric brake
  • 10ga - Black: battery charge
  • 10ga - White: ground (return)


twitter comment thread for this post