It's the concerns Alex lists that turned me away from Li cells for
glider use. Their main advantage appears to be light weight, and the few
pounds saving for an instrument battery is insignificant compared to the
problems.

It appears NiMh batteries are much more practical, for several reasons:

* similar voltage to SLA (sealed lead-acid) and Ni-Cd cells (1.2 volts -
10 cells makes a 12 volt battery)
* self-balancing of cells with charge rates similar to the SLA batteries
we use
* more capacity per cubic inch than SLA, NiCd, and Li (important if you
are trying to stuff more amphours into your battery box)
* charging at the 10 hour rate seems relatively safe, as long as it's
timer controlled.

THe problem I'm having is getting good data sheets on the 8 to 12
amphour cells. The sheets from major manufacturers like Sanyo and
Panasonic are very limited in their data, and the sheets from the
"off-brand" units, which are much cheaper, are even worse, or not
available at all.

Another problem is there are no 8+ amphour batteries available, so you'd
have make your own pack, or have one custom built. The units with
interesting capacities are not being pushed to consumers, but more to
OEMs. Plenty of AAA, AA, C and D cells and chargers available!

The cost is high, too, with a battery the physical size of a 12 volt, 7
amphour SLA (about $35) costing ~$150 just for the cells. Right now, it
seems using NiMh cells to extend your glider's capacity is for the
electrically knowledgeable person that really doesn't want to put in
another battery box.

I'm in contact with one pilot that is routinely using them, however. If
there are others, I'd like to hear from them, especially about replacing
12 volt, 7 amphour SLA batteries.


Alex wrote:

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




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Eric Greenwell
Washington State
USA