One of the 7 pins in a 7 way plug is already designated as trailer charging. Not sure on the bricks and bulls, but on the OBS it’s fed by a relay so that it only feeds the trailer with the key in the run position. This keeps it from drawing down the batteries in the truck when stopped. All that needs to be done is connect the trailer battery to that pin in the plug.
PLEASE DO NOT wire a standardized connector incorrectly. It can only cause problems and confusion for the next guy. What if you borrow someone else’s trailer, or you sell the truck or trailer.
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TNBrett,
I think we are backing into this from different ends to get a solution. I think we both make good points, our fellow members will have to decide what is best for them.
My concern with using the standard power pin you suggested, that is used to charge the brake away battery and I believe used by some electric over hydraulic pump's as I recall. (Which I have.) So not sure I would want to be goofing with brakes to charge a trailer winch battery that is dead and drawing away the power. Or worse yet blows a fuse while underway. To your point maybe a charged onboard battery in parallel saves the day!
I have yet to see any trailer that had back up lights, so this terminal is not generally used unless you have surge brakes and at that a back up valve. (Trigged by power from the back up lights.) I have also seen what I have described on two or three different wiring diagrams and spec'd for travel trailers so guessing it's not an uncommon way to wire for charging the house batteries.
Yes to your point, sell your truck and don't mention. Next guy to own has a snowmobile trailer with surge brakes and back up valve, he will find out quickly when he backs up he has an issue. (Turns out I was that guy!) I have a trailer with surge brakes and the back up valve so I am wired this way now. But when I bought the truck he had house power via a isolator and then going to the reverse pin. I figured it out in under 10 minutes. Then decided to rewire as mechanic before did a half baked job (Splices, taps, loose wires, etc.) wiring for a trailer socket and a 5th wheel socket in the bed. Now its all ship shape and working great.