how fast does an alternator spin?
then you need a little more rpm to get the alternator to charge above the battery voltage. About 13.5 volts. The average alternator is now between 1,300 to 2,100 rpm. I set up an old 35 amp alternator to run off a 1/3 hp 1,750 rpm washing machine motor at a 1:1 belt ratio. Using the stock regulator would stall the 1/3hp motor. I put a large adjustable resister on the field connection of the alternator.