Keeping your trailer batteries charged with your tow vehicle, shore power, and other options

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

NOTE: AS OF JULY, 2022, ALL TRAILERS ARE SHIPPED WITH THE 7-PIN TO BATTERY WIRING SYSTEM DESCRIBED IN THIS ARTICLE CONNECTED AT THE FACTORY.  It is still worthwhile to review this document as it details how this charging system works.    Your connection will be fused, the fuse is located between the 7 pin box and the house battery bank.

Shore Power: when not in use, it is recommended best practice to ALWAYS keep your shore power plug connected to 110V service for both restorative and float/trickle charging Year Round.  Refer to the owners manual for additional details.


Tow Vehicle Trickle

When plugged in, your tow vehicle's 12v electrical system will be connected to the trailer house batteries via the auxiliary 7 pin lead.   Most vehicles will only supply power to the auxiliary lead when the key is on - ie: the vehicle is in accessory mode or the engine is running.    To check if your vehicle is charging the house batteries, you can look at their voltage on the trailer's usb voltage display, or with a volt meter.   When charging, the vehicle's battery will usually be more than 13 volts.    Your house battery voltage should increase from 12.5, 12.6 volts (if fully charged,) to above this value when you connect the 7 pin.

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. 

Instructions for hooking up this wire in early trailers:

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 holder

Important!  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 to early trailers 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: