This is something I played around with a few years ago hoping to be able to power a few lights and a laptop with pedal power

I used a Honda Civic alternator with pushbike parts to gear up the speed.
I dont 100% remember all the specs now but I think I could put out 20 to 30
amps for maybee 30 seconds maybee 300 watts.

very hard to get working right on 12 volts due to the current needed to excite the coils and the pedal power required

Resistance in the charge circuit (long length of small diameter wire)droped the amps and made it easier to pedal

I finally used it to charge into 6 volts at about 5 to 10 amps 30 to 60 watts
which was comforatable for extended periods of time.

I used switching relays to reconect the 6 volt batteries back to series
when I was not using charger

I did no modifications to the alternator and I think a good permanent magnet generator would be a better
choice but did not know of a good cheap supply at the time.

I realised also perhaps the biggest design flaw was no flywheel. On heavy load this was especially