Originally Posted by Polwart
I think it depends - and unfortunately the OP is a little thin on details to be sure. My slightly newer Yam 20 has a fairly heavy duty cable for battery in/output but it is also electric start. IIRC some engines without electric start simply have a "lighting output". Because it is only for filament bulbs it doesn't need rectified - so may be a.c.
Zookeeper - do you have the model number for your engine? If not 2str or 4st? Electric start or manual only?
I think the lighting output may still be DC. beacuse with no regulator you need a battery connected to act as a "shunt regulator" and reservoir.
otherwise at tickover any lamps connected will be dim and then as you rev the engine up the filaments will burn out.
my old honda had a lighting output, the voltage varied between 8 at tickover and about 30v at 5000 rpm. with a battery connected the voltage stays around 12-14 but the rpm varies charge current from 0-5 amps.
with this system too small a battery would boil dry fairly quickly from overcharging, a battery that will handle a few amps trickle charge continuously is needed. usually capacity of 10 times the charge current.
so if your charge coil is say 60 watts. that 5 amps. probably nearer 2-3 amps at normal rpm so go for a battery around 25- 35aH
maybe nissan micra size or a big bike battery.