What's Easiest Way to Charge Trailer Battery From Tow Vehicle

Published 12/22/2022

Question:

I want to connect the power/charge pin up to my vehicle so that I can charge/maintain the battery while towing. The vehicle currently has a 7 pin connected to a 4 way flat. Im not sure which would be better to use to prevent the trailer from draining the vehicle battery, either a solenoid or a relay switch as far as I can tell they do the same thing. Both will turn power on or off via a smaller control voltage, though neither controls the amount of amperage that will flow. I also am not sure where to connect the control circuit/voltage of if there is as specific device for making this connection. I am also unable to find how one would limit the current flow in order to prevent over heating the wire that needs to be run or 7 way plugs connectors. Is there a device for this or is it considered not needed because the trailer battery cannot draw that much amperage. What prevents the vehicle from drawing, or more importantly over drawing amperage from the trailer battery during engine start? Even a relay or solenoid once turned on by the control voltage will pass current in either direction. In order to prevent this control voltage would need to not be actuated until after the starter disengaged, or am I just over thinking things here? My thoughts were to use a 20 or 30 amp relay, with a 15 amp fusible link and 14GA wire to the power wire on the 7 pin connector. I know that the 15 amp fuse would blow if more than 15 amps were pulled, but would prevents something from pulling that much in the first place?

asked by: Eli

Helpful Expert Reply:

Typically people just run 10 gauge wire like part # 10-1-1 (sold by foot) from the positive post of the tow vehicle battery to the 12 volt accessory pin of the vehicle's 7-way and then run 10 gauge wire from the trailer's 7-way to it's battery that you want to charge and that does a decent enough job keeping the trailer battery topped off while not over taxing the charging system of the tow vehicle in any way. There would be no need to worry about the circuit creating issues when the vehicle is charging or overheating in any way. It's basically going to put a light enough charge on the circuit that anything like that wouldn't need to be worried about.

expert reply by: Jameson C