I just wired a new 7 pin on an old trailer I just bought. In no particular order this is what each pin is for:
I'll try to find the instructions that show which pin is which. But the way I see it, there are only 2 that could possibly charge the battery, the one for the brakes or the aux. Most likely its the aux. I would install the brake controller, get a test light and check for power on each one. I doubt the brake pin would have and power without pushing the brake. If the aux is the only thing with power when the switch is on, but all lights and brake off then it most likely to be the one.
Also, call the trailer dealer and ask them where the battery charges from to verify.
77 CJ-7- AMC 360, Edelbrock cam, MC2100, Fenderwells, CJ T18/D20, Chevy D44/EB 9" rear, Trutrac f/r, 4.11, SOA, 1.25 lift shackles, 35x12.50 BFG KM2s, Shackle Reversal