RE: AMA Lithium Safety Alert

[Rick Page]

This message from Rick Page <rick-page@shaw.ca> brought to you by EFLIGHT!

>The safest approach is to use only prepackaged battery packs assembled by
>reputable vendors with the appropriate protective devices installed,
I have never seen such a thing for hobby use. FMA and others have been
talking about "protective devices" but I don't think any exist for lithium
polymer packs yet.

Rick.


*** Any complaints or problems? Send an email to monitor@ezonemag.com
*** For help with list commands go to http://www.ezonemag.com/pages/mailhelp.htm
*** For the list rules go to http://www.ezonemag.com/pages/mailrule.htm

[Doug Ingraham]

This message from Doug Ingraham <dpi@rapidnet.com> brought to you by EFLIGHT!

On Fri, 23 May 2003, Rick Page wrote:

> This message from Rick Page <rick-page@shaw.ca> brought to you by EFLIGHT!
>
> >The safest approach is to use only prepackaged battery packs assembled by
> >reputable vendors with the appropriate protective devices installed,
> I have never seen such a thing for hobby use. FMA and others have been
> talking about "protective devices" but I don't think any exist for lithium
> polymer packs yet.
>
> Rick.

At the minimum it would be a really good idea to place a fuse in the
circuit. This would help prevent short circuit pack meltdowns. However,
protection for a pack during charge is an entirely different matter. A
reasonably simple solution might be to place a 4.3v zener diode across
each series cell. This would not make it impossible to melt down a pack
from overcharging but it would make it take a lot longer to get there.
It would also have the advantage of making out of balance packs more
tolerant of overcharge.

Doug Ingraham
Rapid City, SD USA



*** Any complaints or problems? Send an email to monitor@ezonemag.com
*** For help with list commands go to http://www.ezonemag.com/pages/mailhelp.htm
*** For the list rules go to http://www.ezonemag.com/pages/mailrule.htm

[Don Stackhouse @ DJ Aerotech]

This message from "Don Stackhouse @ DJ Aerotech" <djaerotech@erinet.com> brought to you by EFLIGHT!

Regarding the AMA Safety Committee's recent proclamation about the use of
Lithium batteries:

>The safest approach is to use only prepackaged battery packs assembled by
>reputable vendors with the appropriate protective devices installed, and use
>only chargers designed for this purpose. Buying individual cells which are
>modeler-assembled and charged with conventional chargers is discouraged.

As is often the case with "one size fits all" advice, there are some
problems with these recommendations if taken too literally and too widely
applied.

In the case of radio batteries and batteries used with a BEC (Battery
Eliminator Circuit) in the speed controller to power the radio from the
motor battery (as is typically used in the vast majority of park flyers,
and probably the majority of electric powered models in general), the LAST
thing you want is some protective circuit in the battery that will suddenly
and without warning kill the power to everything (including the radio!)
when the battery voltage gets too low.

Running a lithium battery too low in service may damage the battery's
ability to be recharged later, but at least in the batteries I'm familiar
with it is not going to cause the airplane to catch fire. However, suddenly
killing the power to the radio will in the vast majority of cases cause a
crash, which certainly could cause a shorted battery and a fire. The same
is true of nicad and nickel-metal hydride cells, and probably almost any
other type of battery. I have personally seen fires started by nicads that
shorted in a crash.

Having a protective circuit in the battery to shut it off at some preset
voltage during charging is also not necessarily a good idea. Such a device
introduces yet another possible failure mode. This protective circuit could
malfunction in flight, once again causing an in-flight radio failure, with
subsequent crash, possible fire, etc.. Something I learned when I was
involved in failure mode analysis in the aerospace industry was that when
you try to add safety in one area you usually end up taking away some
safety in another area, and that in general the safest system is usually
the simplest one, the one that has the fewest things that can fail.

The best place for these features is in the charger in the case of
overcharging, and in the electronic speed control in the case of low
voltage cutoff in flight. An ESC with the proper cutoff voltage setting
will shut down power to the motor while still maintaining power to the radio.

As far as the use of these cells for radios is concerned, once again the
use of an in-flight protection circuit to shut off the batteries is "penny
wise and pound foolish". Such a circuit would kill power to the radio,
saving the batteries (at least for the moment), but losing the entire
airplane! In addition, this circuit introduces its own failure modes, once
again increasing the probability of in-flight radio failure.

Lithium batteries can have problems re-charging if run too low, but other
than that they are no different than any other type of battery. With
lithiums, nicads or NiMh cells, if you stay up too long your radio will
stop working, and you will probably lose the plane (as well as possibly
causing all sorts of mayhem wherever what went up happens to come down).
The solution here is the same as for ANY type of battery. Know your
batteries, know how long they can fly the plane, do regular maintenance
checks to make sure they really do still perform as expected, keep track of
your flight time, and land the plane with a safe margin of power still left
in reserve in the batteries. This is how you avoid battery failures in
nickel-based and other types of batteries, and this is how you avoid the
problem with lithium cells. Radio failure due to battery shutdown is still
radio failure, regardless of whether it's because your lithium cell
protection circuit shut off or because your nicads simply ran out of
electrons. The end result is the same.

There are some in flight battery warning devices that will turn on a bright
LED or other warning device when the power starts to get low, which can be
helpful as long as you can see that warning device while the airplane is
in flight. In our experience, the batteries themselves decrease in voltage
as they discharge, and in an electric powered non-soaring model the
resulting power loss to the prop can force you to land while there is still
a safe minimum voltage left in the batteries.

However, ultimately we all need to recognize that first rule of all
aviation: that the "pilot in command" is responsible for the safety of the
aircraft. YOU must know the capabilities of your airplane and its systems,
and accept the responsibility to stay within them. No amount of add-on
devices of whatever sort will protect adequately against lack of attention,
recklessness or bad judgement.


Don Stackhouse @ DJ Aerotech
djaerotech@erinet.com
http://www.djaerotech.com


*** Any complaints or problems? Send an email to monitor@ezonemag.com
*** For help with list commands go to http://www.ezonemag.com/pages/mailhelp.htm
*** For the list rules go to http://www.ezonemag.com/pages/mailrule.htm