12-Volt Accessory Circuit from Trailer Connector Not Keeping Trailer Battery Charged

Question:

I have a 7 point adaptor to my 4 point plug. I cant get it to charge my trailer battery as im driving down the road. When the trailer is plugged into the car my lights work on the inside of the trailer. All my outside lights work properly, just my inside lights dont work unless the car is plugged in to the trailer. Leaving me to believe my car plug isnt charging my battery on the trailer. That being said, I charged the batteryon a charger, put it back on the trailer and the lights work on the inside. Therefore its not the battery. ??

asked by: Richard D

Expert Reply:

For starters, be aware that the limited wire gauge feeding this 12-volt circuit from the 7-way plus the distance the two batteries are away from each other means that although the tow vehicle battery can provide a maintenance charge to the trailer battery, it won't have sufficient power to charge it once it's discharged or nearly so. A good battery charger like the NOCO # 329-GENIUS5 should be used to fully charge the battery when shore power is available (which it sounds like you're doing already).

You'll want to begin by using a circuit tester like # PTW2993 on the pin in the 1:00 position on the 7-way at the rear of your Subaru. With the ignition on, you should be seeing 12-volts at that pin. On the trailer, there should be a wire from the trailer connector that runs to the positive battery terminal. Disconnect this wire, and with the vehicle ignition on and the trailer plugged in, check that wire for 12-volts.

If you're not getting power at the 7-way on the vehicle, you'll need to review how it was installed. Typically the black wire on the # 37185 adapter is ran to the vehicle's positive battery terminal via a 40 amp breaker. Make sure the breaker hasn't blown. Make sure the connector on the trailer isn't corroded and that the trailer battery terminals aren't corroded.

Using a DC to DC battery charger like the # RED46FR in combination with a battery charger would be a good solution for keeping your battery charged up. This charger only should be used with the vehicle running, and can be used in conjunction with a solar panel if you wish. I'll link you to a help article that will help outline the options depending on how much power you wish to provide to the batteries.