This is a quote found in myt "How to Build Trailers" book.
"Lights will be needed of course and you should install an outlet plug to carry a lead to your car battery. Or you may pick up and install a battery in the trailer itself, belting an old auto generator to one of the trailer wheels for chargning purposes"
Interesting idea huh? Of course now you could just spend 75 bucks and get a one wire alternator. Mount a pulley to the axle and a swing arm mount for the alternator and pop on a belt!