I've dealt quite a bit with lithium-ion and lithium-polymer batteries,
and I have to say that they are *extremely* sensitive to how they are
charged, discharged, and what temperature they operate at. If you're
planning on putting a Li-ion pack in your glider, make sure you do your
research and be very careful.

As far as voltage regulation goes, a switched DC-DC converter is the
way to go in terms of light weight and high electrical efficiency.
This is the least of your problems. The biggest problems with Li-ions
is looking after them during charging and discharging. Most Li-ion
cells operate at a nominal voltage of 3.7V. The voltage of the cells
*must* remain between 3.0V and 4.2V, and also within the stated
operating temperature range otherwise the cells are very likely to
out-gas, explode, catch fire, or all three.

Charging them is not a problem since, presumably, you would use an
off-the-shelf charger intended for Li-ion batteries. Discharging,
however, takes places in the glider -- in the air -- and will require
some kind of protection circuitry.

I have experience with these batteries through the solar car racing
community (a "sport" not too dissimilar to gliding!), and here is the
information on Li-ion batteries that is sent to teams:

http://www.americansolarchallenge.or...01-01-0959.pdf
http://www.americansolarchallenge.or...iumcontrol.pdf

As stated in the above documents there are also further issues with
cell equalisation in battery packs consisting of more than one cell in
parallel. There's a lot to consider, and the consequences could be
fatal in a glider. You can always jump straight out of a solar car
without much damage to the car, but getting out of a glider is another
issue altogether...

Personally, I don't see the advantage of using Li-ion cells in gliders.
Model gliders/airplanes gain an advantage through longer run-time for
the same amount of weight. Full-size gliders don't really have an
issue with a few more pounds of weight that an extra Pb-acid battery
adds.

_Alex