[NSRCA-discussion] "Sick" Lipo Packs

Sat Dec 4 17:21:36 AKST 2010

First good explanation of why Lipos swell that I have seen.  Thanks for the
insight.

From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Chris
Sent: Saturday, December 04, 2010 7:37 PM
To: General pattern discussion
Subject: Re: [NSRCA-discussion] "Sick" Lipo Packs

Stu is right, all cells in the pack were subjected to the same overcharge or
over discharge and will also fail as the first cell did.  One event will
probably not show up as a swelled cell but it is the overcharge / over
discharge over many cycles that will result in swelling. Notice the article
says that max voltage is temp related, and most chargers don't make this
adjustment so even with a "good" balance charger, you can still overcharge.

Chris

Part of a good article:

This was the common problem with many cheap Chinese LiPos of around
2005-2008. Most are better now, but it's the #1 cause of premature LiPo
failure: water contamination in the plant. Many of China's LiPo factories
are on the coast, where the altitude is very low and the humidity is high.
You can't run the humidity too low on the assembly floor, because you're
working with volatile chemicals that could explode in the presence of a
spark, and you can't run it too high because then you end up with a
worthless LiPo that swells on first use.

Here's the science. You have three ingredients that are functional in a LiPo
battery. The rest is wrapping and wiring attachments.

*	Cathode: LiCoO2 or LiMn2O4
*	Separator: Conducting polymer electrolyte
*	Anode: Li or carbon-Li intercalation compound

I'm going to be a little vague in my language here. The chemicals involved
vary according to manufacturers, so I don't want to make any assumptions.

Remember your chemistry class? Note the absolute lack of any hydrogen atoms
in the reaction. None, zero, zip, nada. If you have water inside your
battery -- and virtually all batteries have a little bit -- you've got
problems. When the chemical bond of H20 is broken by electrolysis and heat,
you end up with free oxygen. You also have free-roaming hydrogen that
typically ends up bound to your anode or cathode, whichever side of the
reaction it's on and depending on the state of charge of your battery.

Now, this is a pretty unstable situation that's exacerbated by any
over-discharge or over-charge condition creating metallic lithium in your
cell. The end result is Lithium Hydroxide: 1 atom of lithium, one atom of
hydrogen, and one atom of oxygen.

But you still have a free oxygen atom floating around inside the battery
casing, that typically combines with one other oxygen atom -- O2, or what we
sometimes think of as "air" -- or two other oxygen atoms, to form a
characteristic tangy, metallic-smelling substance called "ozone", or O3.
Gases expand with heat and contract with cold. Chuck a swollen battery in
the freezer and it might come out rock-hard again... until it heats up. It's
not frozen, it just got cold enough that the gases inside didn't take up
much space at all.

And that free O2 or ozone is just waiting to pounce and oxidize some lithium
on the slightest miscalculation on your part. The modest over-discharge
during a punch-out, or running the battery a little too low or letting it
get a little too hot, or running the voltage up to 4.235v/cell on a cold day
when the actual voltage limit per cell is more like 4.1v. All of these
create the perfect storm for a puffy battery to quickly turn itself into a
ruined battery or an in-flight fire.

Understanding the role of free oxygen in your battery, from water and other
causes, is CRUCIAL to understanding why batteries fail, and why sometimes
you can get by with flying a puffy battery, and sometimes you can't.

If a Lithium battery is overcharged or charged too quickly, you end up with
LOTS of excess free lithium on the anode (metallic lithium plating), and
free oxygen on the cathode. A free oxygen atom is small enough to freely
traverse the separator without carrying an electric charge, resulting in
lithium OXIDE on the anode. Lithium "rust", in reality. Useless to us at
this point, just dead weight being carted around inside your battery's
wrapper. 

But lithium oxide uses fewer oxygen atoms than existed in the ionized state,
so you end up with, again, FREE OXYGEN. And people wonder why if you
over-charge a LiPo underwater, it still ignites despite the lack of open
air...

If it's over-discharged or discharged too quickly, the reverse is true, but
you end up with Lithium Oxide on the cathode, but at a lower rate because
there's simply less there. Basically, an abused battery quickly develops
corrosion on both poles of the battery inside the wrapper. And the more it's
abused, the worse it gets as the resistance goes up and it still gets driven
hard.

This, by the way, is the most common cause of swelling today for our
aircraft when flown with a high-quality pack (not knock-off eBay leftovers
from expensive Chinese mistakes of 2004-2009). The reality is, these kinds
of cells, regardless of their 'C' rating, are built for use where they last
for several hours... not several minutes. While the chemistry if used as
designed is good for thousands of cycles, we're driving them so far out of
spec that we're lucky to get hundreds of cycles out of them.

In most cases, too, our batteries are under-specced. If slow-charged and
slow-discharged, many of these packs would often hold considerably more mAh
than we think they do. That's one of the reasons we get the performance we
do from them. Higher-C-rated packs also often introduce gelled electrolyte
into the separator, and carbon or phosphorous nano-structures on the anode
and cathode mixtures rather than the "pound it out thin and hope it's mixed
right" approach used with sheets of anodes & cathodes today.

On 12/4/2010 8:23 PM, Stuart Chale wrote: 

Been there done that but my experience is that before long additional cells
will fail and the cycle will continue.   With the cost of the lower priced
packs, ie: Zippy's I would no longer bother :) 

On 12/4/2010 6:45 PM, Ron Van Putte wrote: 

Those of you who use lithium polymer battery packs to power their
competition airplanes are familiar with "puffed" packs.  I recently had four
elderly 5S packs "puff".  We all know that's not good, but what I'd like to
know is what's actually happening. 

I know it's probably not wise for consumers to take lithium polymer packs
apart, but that's exactly what I did with four packs.  I discovered that in
three of the "puffed" packs, only a single cell was "puffed".  In the last
pack, there were two "puffed cells.  I did a little arithmetic and quickly
discovered that I could make three "unpuffed" packs from the good cells I
had.  So, I unsoldered the "puffed" cells from the four packs and
cannibalized one pack to make three 5S packs from what I had left.  This
process is obviously for the timid or the careless.  I was careful and had
no mishaps.  However, I would suggest that anyone who says "Oops" a lot
should not attempt doing this. 

The three 5S packs I have left are "rock solid".  Experienced
electric-pilots will know just what I mean. 

I have flown these packs and they seem to perform just as they did in their
"youth". 

My questions are:  Why do lithium polymer cells "puff"?  What is the likely
future of my recovered 5S packs? 

Ron VP 
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