[Voneville]

The little bit I know about electrical may not be any help, but I'll try. A capaciter stores charge. Your bsa should run fine without a battery, but a capaciter will help when you kickstart because there is enough electricity stored up to give it the boost it needs. When you add lights to the equation you need a battery or some sort of battery eliminator (like a magneto or box). A capaciter alone isn't enough on the british bikes I don't think. Again I'm no expert. One trick is to buy the smallest battery you can find. Honda Scooters from the 80's have tiny little batteries and they actually take a beating well. I know a couple of guys that run them in their cafe racers with a lot of success.

If I'm wrong about any of this stuff please chime in and correct me, I'm not an electrician.