LiPo Batteries in Gliders

GM wrote:
Hi,

it's still winter with lousy weather in the northern hemisphere, so it
is time to warm up an old discussion. I searched the forum for threads
reg. Lithium-Polymer Batteries and there have been none for a while.
Here are the questions:
- Does anyone have real life experience with LiPo's in a glider? (real
gliders, that is - not models)
- Can I combine two 7.4V packs in series to get a 14.8V unit or is
there a risk with that?
- How would one charge a combo like this? Each unit by itself and then
hook them together or with one charger while hooked together?

The prices have dropped to a very affordable level; so have the
chargers/balancers. The weight savings are considerable. My 12V/7.5Ah
standard sealed gel-cell weighs 2.35kg (5.17lbs) and it lasts a few
days. I found two 7.4V/5.0Ah units on e-bay that weigh in at 0.3kg
each (0.6kg or 1.32 lbs in total)

Any input is welcome.

Uli Neumann
'GM'



Hello Uli,

Glider applications are a fairly deep cycle requirement (more than 30%
down from fully charged). Lipo batteries are excellent but have some
serious problems in our deep cycle application.

Lipo batteries are 3.7v when fully charged. To get higher voltages you
just put more cells in series (7.4v, 11.1v, 14.8v, etc). They discharge
at a very flat output voltage until they suddenly drop in voltage. The
problem is that if you discharge a lipo beyond the safe threshold (3v
per cell) you destroy the battery. If over discharged the battery will
overheat during recharge and burst. The material inside the batteries
plastic wrap ignites upon contact with oxygen and sticks to everything
like napalm. You will burn your house down.

Also, in an accident if the battery case if punctured then the battery
will burn.

The cell phones and model builders use electronics that turn off the
units when the voltage takes that critical drop in order to save the
battery. Someone would have to build a similar unit for sailplane use
before they could be safely used (and more importantly re-used).

I own many, many lipo batteries and sealed lead acid is what I use in my
glider.

Cheers,
Pat Russette
ASW20B-Hobbs CQ