How To Wire and Install On-Board Auxiliary Trailer Battery to Power Lights

Expert Reply:

In order to charge an on-board trailer battery you'd need to have a seven way connection on both the vehicle and trailer, this would contain the necessary circuits to pass a charge from the vehicle battery to the trailer battery. If you don't already have one on the trailer you can use the etrailer Trailer Wiring 7-Way Upgrade Kit part # e99011. If you don't already have one on the vehicle we'd need the year, make, and model to inform of options. The seven way would then contain a circuit called the "battery charge line" or twelve volt power" which would be used to provide a small trickle charge to an on-board trailer auxiliary or breakaway battery.

In all honesty, if you were simply using the on-board battery for some lights inside the trailer you probably wouldn't even need to worry about draining the vehicle's battery assuming there wasn't any sort of backfeed protection built into the vehicle's trailer wiring circuits. That said, I understand wanting to be safe and not sorry so in which case I recommend the Redarc In-Vehicle BCDC Battery Charger part # RED96FR which does a few different things. It will not only make sure there is no backfeed but it will also ensure your trailer battery is fully topped off at all times. It will only pull power from the vehicle battery when it's receiving a charge from the alternator. The "battery charge line" or twelve volt power" circuits from the vehicle only provide a trickle charge whereas this unit will provide a full charge. I like this option for you as it opens the door for you to power other accessories from the battery without having to worry about a dead battery.

I have attached a wiring diagram below.

How To Wire and Install On-Board Auxiliary Trailer Battery to Power Lights
(click to enlarge)

expert reply by: Conner L

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