My EE friend put on the hitch before we swapped cars. He used the X5 occasionally (usually a Ram pickup, or the Subaru project) to tow his dirt-bike to local sites. He had a 4.5'x12' trailer w/o brakes, so he rigged a 4-way for the lights.

I now own that small trailer (we traded trailers, too), and I have two pickups modded for towing anything, that I can use for my braked trailers, so I probably will take my time installing a proper 7-way (with 12v feed wire) on the X5. Otherwise, your idea of using that to charge the battery would be perfect.