[jakobnev]

I built one of these. It's 14.8V 13Ah, and I connected it to the car through both filaments (in parallel) of an H4 headlight bulb.

I doesn't provide as much current as the alternator of course, but when you EOC >85% of the distance it certainly helps. Before it, I had problems keeping the battery charged in the winter, when I always needed the head lights. (Still dark when driving to work, and already dark when coming home)