> But sitting there at camp hearing the engine run, it's not clear to me
> how fast the engine needs to run, nor how long it needs to run, to bring
> the aux battery back to fully-charged status.

I had similar questions, since I had to run my engine to
recharge my aux battery while camping 3 days and running
my computer via an inverter.  Idling for 30+ minutes
appeared to charge it back up--this by watching the
voltage level in the system.  (Of course, max voltage
depends on the temp of the battery.)  I hadn't run my
aux down very much--the inverter was very sensitive to
the voltage and quit when it started to drop a little.

A while back, someone stated that the engine needed high rpm
to charge.  I doubted that and spent some time searching
the web.  Result: at idle, the output of the alt (amps,
not volts) is maybe 70% of max, so you don't need to sit
there rev-ing it.  Of course, this doesn't answer your
question of "how long...."

Here are the refs I found:
Chart of RPM vs amps:
http://www.fordmuscle.com/archives/2004/07/6GAlternator/index.shtml
Looks like a pretty flat "curve" on this.

Balmar (marine alternators) with very good tables:
http://www.balmar.net/PDF/Alternator-Regulator%20Guide.pdf

                        Richard A Jones
                        Boulder, Colorado