A decent modern car alternator should be rated at 60 to 80 Amps, which at 14 Volts is 840 to 1120 Watts (1.1 to 1.5 hp). Including the losses in the belt drive, they are not a lot better than 60 percent efficient, so you need an engine that can deliver 1.8 to 2.5 hp continuously. A 3.5 hp Briggs can't do that - it is rated based on gross power, not nett power - but a 5 hp Briggs, Honda or chonda can do it. Of course an alternator with built-in voltage regulator is best, then the only wiring you need is two heavy wires: one from battery positive to alternator output and a second from battery negative to alternator ground, plus a very light wire with a switch in it from battery positive to the alternator's field terminal. I doubt a car alternator would object to running for the few hours a battery charger operates with its rotor vertical, so you can use either a vertical or horizontal crankshaft engine - whichever falls to hand. The whole charging device should be carryable with one hand for short distances. In practice of course you would only use that large an alternator on a full sized car or truck battery, or you'd overheat the battery and ruin it.
