LiPo Batteries in Gliders

On Feb 24, 8:50*pm, 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'

Most of the dire "explosion-fire" warning here are several years out
of date. Chemistry's like LiFePo4 have no such dangers. Most 5AH and
larger packs have internal PCB's for safety.

Check out US manufacturers like Tenergy which offers a number of
"heavy duty" Li-Po and LiFePo4 packs with internal PCB's which limit
charge and discharge voltages and currents to safe levels.

One problem not mentioned is that 4-cell Li-Po can deliver as much as
17V when fully charged which is more than some avionics allow. I
would use a DC-DC converter to deliver a tightly regulated 13.8 V to
the avionics bus. DC-DC converters allow a wider choice of battery
voltages.

This thread reminds me of the old saying: pioneers are the ones with
the arrows in their backs. SLA batteries are cheap, and their risks
are known. Why mess around and take a chance on an in-flight fire?

-John

On Feb 25, 10:12*am, bildan wrote:
On Feb 24, 8:50*pm, 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'

Most of the dire "explosion-fire" warning here are several years out
of date. *Chemistry's like LiFePo4 have no such dangers. *Most 5AH and
larger packs have internal PCB's for safety.

Check out US manufacturers like Tenergy which offers a number of
"heavy duty" Li-Po and LiFePo4 packs with internal PCB's which limit
charge and discharge voltages and currents to safe levels.

One problem not mentioned is that 4-cell Li-Po can deliver as much as
17V when fully charged which is more than some avionics allow. *I
would use a DC-DC converter to deliver a tightly regulated 13.8 V to
the avionics bus. *DC-DC converters allow a wider choice of battery
voltages.

Second the max voltage warning. ILEC has already seen some fried
avionics from customers experimenting with these things...
Be careful out there (check max voltage for pack and everything
in your panel FIRST),
Best Regards, Dave "YO electric"

From: "Dave Nadler"

Second the max voltage warning. ILEC has already seen some fried
avionics from customers experimenting with these things...

This is just one more reason to use LiFe instead of LiPo or LiIo. A
fully charged A123 LiFe pack delivers 14,4V (3,6V x 4), same as SLA.
20 A123 cells in 4s/5p configuration have almost the same dimensions
as a 12V/7Ah SLA but offer slightly more voltage, 11,5Ah and power to
start a car at 70% of the SLAs weight. And you can fast charge the
pack in 15 minutes if your charger can deliver 50 amps :-)

Michael

On Feb 26, 1:56*am, "Michael Huber" wrote:
From: "Dave Nadler"

Second the max voltage warning. ILEC has already seen some fried
avionics from customers experimenting with these things...

This is just one more reason to use LiFe instead of LiPo or LiIo. A
fully charged A123 LiFe pack delivers 14,4V (3,6V x 4), same as SLA.
20 A123 cells in 4s/5p configuration have almost the same dimensions
as a 12V/7Ah SLA *but offer slightly more voltage, 11,5Ah and power to
start a car at 70% of the SLAs weight. And you can fast charge the
pack in 15 minutes if your charger can deliver 50 amps :-)

Michael

Ahh.. 14.4V is the NOMINAL voltage for a 4-cell LiFePo4. However, put
a good DVM on a 4-cell pack after a full charge and you can see
anything up to 17V. It will drop back to 14.4V pretty quick but,
'apparently', can do some avionics damage before it does. (My
Microair manual lists 16V as the maximum.) An alternative to a DC-DC
regulating converter is to just discharge a fresh pack a bit until the
slight overcharge is dissipated.

Ahh.. 14.4V is the NOMINAL voltage for a 4-cell LiFePo4.

No, 14.4V (3.6V x 4) is the recommended charger cut off voltage for 4s LiFe,
nominal voltage is 13.2V (3.3V x 4). If a good DVM reads 17V on a 4s LiFe
Pack you are overcharging the cells, probably using LiIo or LiPo settings on
your charger. Go to http://www.a123systems.com/a123/products , choose
"download specs" and check yourself in the data sheet.

If you are really overcharging a 4s LiFe to 17V at least you demonstrated
why it is MUCH better to use LiFe than LiPo ;-)

Michael