Originally Posted by Earthman
Short answer - no, the white wire will not work with a 20 Watt light. The magneto was designed to only make several Watts of power at 6 volts +/- of AC current, which is enough to run the engine and a low-power (3 Watts max) head & tail light system. You can run low-power LED lighting systems off the white wire, but the light output is relatively low and/or focused into a tight beam. You can run a 20 Watt incandescent lamp (like your photo) if you run it off a battery. You can trickle charge the battery from the white wire. Since the magneto makes alternating current, you will need at least one diode to rectify the current, but a full-wave bridge rectifier (basically 4 diodes) would be better. If you want to charge the battery properly, you should have a voltage regulator as well, but the extra cost may not be worth the trouble - try a single diode or a full-wave bridge rectifier (both available from RadioShack) with a small 6 volt lead-acid battery and see how well the simple system works for you....
All good info but I would like to add the following:
Even though it is possible to get more charging power with a full wave bridge, there are problems. A full wave uses both half cycles to charge the battery, including the half cycle that generates the spark. Draw too much power and you kill the engine. The single diode half-wave rectifier will not kill the spark since it can be setup to draw power from the unused half cycle. Also, if you use a bridge, you cannot use a chassis ground. All of your components will have to be electrically isolated from ground. All of my components ground to the frame and use a single power wire.
I have found that you do not need a regulator when using a single diode and an 6v SLA battery. I have run it with and without a regulator. The battery never overcharges or gases.
The white wire can charge your battery. But if you have large lights and run them often enough, you may still need an auxiliary charger. The white wire charger works best with LED lights and maybe occasional use of a larger incandescent headlight.