Flyfisherman128 wrote: Most altenators put out between 60 and 120 amps most chargers do not.
Most batteries - even sizable banks - won't accept 120 amps for very long. They will quickly taper down and the charge time won't be all that much better than a converter.
Remember, the faster you charge a battery, the less it will be charged when you reach absorption charge voltage.
I could see the advantage if you could find a engine that idles real quiet. For example, my rototiller with a B&S OHV engine is very quiet at idle, and wouldn't be annoying. (it's sure noisy under load though!)