i'm interested in why your moped wouldn't put out enough power to charge a battery. my readings indicate between 20 and 30 volts at full rpm. probably close to 20-30 watts with the right loading.
also curious to know how much power is getting slammed into the battery at full throttle. I've got a couple of these little sealed gel-cell batteries laying around that i'd like to do something like this with, but they can only take like .3 amps max at 12v to charge, from what i've measured the moped alternator would smash that.