I'm just finishing up a class demo unit that uses a 62 amp Motorcraft
alternator. I wasn't happy with the positive ground on that unit but was
cheap and had the external voltage regulator that I needed. 62 amps at
13 to 14 volts is around 850 watts. Calcs out to 1.15 HP theoretical.
Any of the 3 to 5 hp B&S, Tecumseh, import specials, etc should work
fine at full load.

To build what you are talking about, I'd get Delco alternator with the
internal voltage regulator. If you happen to have a used one, the
rebuild kits run about $15. Amperage varies considerably from a low of
about 60 amps to around 135 on the units used on police cruisers. You
probably don't care, you don't really want to charge the battery much
faster than 30 to 50 amps.

As for how fast to spin the alternator: Here is the measured performance
curve for the Motorcraft alternator: (rebuilt unit came with a printed
inspection tag)

rpm amps
1600 11
2000 24
2500 43
3000 53
4000 61
5000 65

I'd expect the Delco units to be similar so driving it 1:1 from a
3600rpm lawnmower engine would be fine. I've tried both belt drive and
direct drive using a Lovejoy connector. Using the Lovejoy required some
special machining to get it to connect up: the armature is axilially
posititioned in the bearing by the nut on the pulley. I think I'd stay
with a belt drive for your application.

On gotcha to keep in mind: the case on the alternator is one side of the
circuit. It's tough to make a mounting where the chassis of your
charging rig is not grounded to the case of the alternator. If all the
batteries and equipment you work with are negative ground, should not be
an issue. But if you have an old positive ground vehicle or one with
multiple batteries in series, you really have to watch where you set up
the charging rig.

And a last thought: there are some aftermarket add on boxes that use a
different voltage regulation curve to get 120 volt power out. Really
trashy frequency and voltage regulation but they will run a small
universal drill. These are pretty much gone now, the decent 12 volt
inverters are much more user friendly.

Have fun!

RogerN wrote:
I've been thinking about mounting an automotive alternator on a small
gasoline engine to make a portable battery charger with respectable output.
The purpose would be to charge auto batteries when there are no outlets
around. Of course you could do the same thing with a generator and battery
charger but I think you can get perhaps 70 amps or more from an alternator.
It would have been nice this winter when we had the ice & snow storm and the
battery was weak on the seldom driven 4WD diesel truck. Also could be
useful for camping and boating, could recharge the trolling motor battery on
the lake, etc...

RogerN