motor bicycle magneto and battery

Discussion in 'The Projects Forum' started by dougan778, Jul 19, 2010.

  1. dougan778

    Thread Starter New Member

    Jul 19, 2010
    Hey all!

    I have a motorized bicycle. If you like projects, you should think about building one. They're fun. is one of many places you can get the kits.

    Anyway, the motor I have is equipped with a magneto/CDI setup. I want to have headlights on my "hog" and am pondering the ways. Wisconsin statutes mandate that the headlights on a moped or motorized bicycle have to be attached to a generator or magneto, possibly with a "wet battery" in-line.

    I checked my magneto with a multimeter and it runs 4.5 v to 9 v AC rms depending on the RPM. From what I've heard, you can really only put about a 3 watt draw on it before it starts to lose spark in the engine.

    So my intent is to get a small 6 v moped lead battery and mount it behind the bike seat. I want to create my own headlights with CREE LEDs (more fun that way) and run them off of the 6 v battery.

    Ideally, I would find a way to charge the battery with the magneto. I understand what a rectifier is and what it does, and I feel like I'm skilled enough to make a full wave rectifier to convert that AC magneto power to DC power. I think. :)

    I have a few questions off of the bat, though:

    1) Given rectified, DC current, what do I have to do to charge the battery? I wouldn't imagine I could simply hook up a + and - wire to the proper terminals on my battery. I need to keep the power drawn from the magneto under 3 watts (I don't care if it's enough to account for my headlight's draw. Law doesn't say anything about the charge being enough to keep the battery fully charged when in use. :) ). So far, in my head, the wiring diagram is the magneto, and then a full-wave rectifier like the one below, and then a question mark, and then the battery. So what do I have to do to have it charge the battery with a specific amount of power? It seems like this task is just barely out of my league, but I feel like I could get there with some guidance.

    2) Will the variance in voltage provided by the magneto (4.5-9) cause problems when charging the battery? I know cars vary a little bit and it's not a problem, but this is a bit wider. Also, will the up-and-down nature of rectified AC power be a problem? Again, comparing to a car, this is the case and doesn't cause problems...

    Thanks in ahead. I'm sure I'll have more questions, but at this point, I'm sort of just trying to identify the proper questions to ask. :)
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    If much of your riding is daytime, then 3W might give suficient charge to charge battery & operate a headlight. Connect bridge rectifier [+ ] thru 10Ω10W to battery, & try out- engine performance & battery condition. If all's well try 6Ω & repeat testing. Monitor battery V for low & high according to battery type.