"Eric Greenwell" wrote

I don't know the Grob 103 details, but most self-launchers do have an
alternator. If not, finding a way to mount an alternator in what is
usually a very cramped location, and making it sturdy enough to keep the
two-stroke engine from vibrating it to pieces, and integrating it into the
electrical system would be a major project.

The recharge current is limited by the battery size, and the 28 Ah battery
means a limit around 5 or 6 amps. Since the current drops off as the
battery charges, replacing 6 Ah means an engine run of at least 1.5 hours.
Nobody wants to put an hour and a half on their engine to top off a
battery - the noise alone is enough to seek other solutions!

I see your points, but still am not sure that some increased charging system
could not provide a good answer.

Most engines like that have a flywheel magnet and stator system, and are
pretty anemic in the amount of current they put out. One of the small
alternators from something like a small tractor could be mounted, and a
larger battery installed, probably a wet cell, or other type that could
handle more charging current. If not able to fill the larger battery in a
20 minute run to altitude, it would be darn close, and the larger capacity
would have more than enough to run your electronics all day, even at only
1/2 or 2/3rds capacity. I agree about not running an engine a long time,
just to charge, but if you ran the battery down one day on a marathon
flight, a little run would keep your electrons zinging around the
microcircuits!
--
Jim in NC