On Sun, 8 Feb 2009 01:24:35 -0800 (PST), wrote:

A good idea, BUT....work out how much motor power you would need to
get the alternator up to 70 amps.

That's easy. 70A at 14.4V is about 1kW, or 1.4 HP. An alternator has
pretty good efficiency, so a lawnmower engine should be just fine.

..not a small motor.

My model helicopter engine weighs 406 grams, and produces 1.9 HP. I'm
not saying it would be suitable for this application, I'm just saying
the engine shouldn't need to be all that huge.

...and coupling
it to the motor is no trivial matter either - but thats OK,

In the car, the alternator is driven by a simple belt. If you can find
or make a suitable pulley for the engine, it seems quite doable.

no way
could you charge a battery at 70 amps anyway.

In the car, the battery IS charged by the alternator. You'd just have
to be careful if you use the alternator from a large car to charge a
small battery. Back off the throttle some, and you should be fine.

btw, you
would need to work out pulley ratios to get the alternator into its
power band without revving the motor to blazes -

The alternator does have a maximum limit. It also has a lower limit
where it will not produce any power at all. The lower the speed, the
less power it will deliver.

and some sort of
governor..(only if your obsessive...)

In the car, the alternator works over a wide rev range. As long as you
stay below the maximum rated speed, you should be quite safe. If you
fit an amperemeter, you will be able to see what's going on.
--
RoRo