I have a late 70's moped (not scooter... it's a puch) in which I have installed an aftermarket lighting coil. At idle, it puts out less than 10 volts, but at wide open throttle, it can put our around 20. I would like to install a small SLA (sealed lead acid) battery off of which to run lights and would like to keep the battery charged with the power coming off the engine.
So, the question is, what do I need to do in order to keep the battery charged properly? It seems to me that if I were to simply wire the output from the coil to the positive node on the battery (and ground the ground to the frame) that when the engine is off, the lighting coil will then essentially become a short in the battery and either burn out the battery or fry the lighting coil.
as you correctly state the coil would be a dead short.
the coil is a rudimentary alternator in that it will be producing AC voltage..Though the PUCH was originally six volts, I believe the later TOMOS were 12 volts. so these might be a source of parts.
1. you need a rectifier to change the AC alternating current to DC. A simple half wave rectifier will suffice as you only have one output as the other is grounded to the frame.
2 you will need a solid state voltage regulator. Maplin and Radio shack make a suitable one but you can also get one from ebay that is variable.
3 you will need a ZENNER diode to act as an avalanche diode to relieve the rectifier and the voltage regulator when the alternator it producing too high a current/ voltage to prevent the circuits from being over loaded and therefore burnt out.
4 Fit a suitable fused circuit to control the output of the battery and to prevent the battery from being damaged by a high discharge rate..
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