It didn't keep his batteries charged because, as I said, his bike was not putting out on average the same amount of power that his lights were using, thereby causing the lights to rely on the battery as a supplement.
When you're running a 35 watt 12 volt bulb on a moped, it's probably very rarely reaching it's full abilities, where when you connect it to a battery, it's going to draw the same amount of current the whole time; a full 2.9 amps.
I also would suspect that some mopeds would fry a battery if too much time was spent at full speed. It may be a good idea to put a 13.7 volt regulator in there.