Keeping your trailer batteries charged with your tow vehicle

This article details connecting the tow vehicle charging system via the 7 pin harness, and also has information on a DC/DC charger.


When plugged in, your tow vehicles 12v electrical system will be connected to the trailer house batteries via the auxiliary 7 pin lead.  


7 Pin Junction Box

Inside of the 7 pin junction box, located between the fridge and the batteries, are wires that can be connected to the tow vehicle's charging system.    On the far right, the black and white leads, both 10 AWG.

7 pin box only

The auxiliary power pin, black wire is already connected to charge the battery in the emergency break away box for the electric brake system. 


Charging the trailer batteries via this auxiliary pin is up to the customer.    If you find a wire inside the 7 pin junction box and see that it routes to the trailer battery system via a continuity check, you can use it to connect the systems.   OGT's recommendation is that a fuse is added to suit the wire gauge.   If you are using a 16 gauge wire, a 20 amp fuse can be used.  If you are using 10 gauge wire, a 30 amp fuse can be used.  This is assuming short wire runs of about 3 feet or less.    The best way to install a fuse is with an inline fuse holder either crimped or soldered into the lead, as close to the end connected inside the 7 pin box as possible:

inline fuse holderImportant!  When using the auxiliary pin in the 7 pin box, It is not recommended to connect a vehicle with charged batteries to a trailer with discharged batteries (below 12.5 volts.)  The fuse will protect the circuit, but current flow can be very high.  Charge the trailer batteries first, and monitor their state of charge.


In order for the auxiliary connection to work well, grounding is also important.   The connection from the ground pin in the 7 pin box to the trailer battery bank should be checked and improved if necessary.  With the only connection between the vehicle and the trailer being the 7 pin connector (IE: with the trailer hich disconnected and with the chains not in contact,) continuity should be confirmed between the ground pin in the 7 pin box and the trailer's batteries.   If there is a high resistance here, it is recommended that a minimum 10 gauge ground wire is run from the ground pin in the 7 pin box to the trailer frame, and to the battery bank.


All of these modifications must be vetted by the customer or their chosen installer and are not the responsibility of OGT!   



DC/DC Charger

The premium, high performance option for vehicle charging is a DC/DC charger, installed according to the manufacturer's recommendations.

We've had good luck with the redarc unit: