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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.<br>
<br>
Chris<br>
<br>
Part of a good article:<br>
<p><i>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.</i></p>
<p><i>Here's the science. You have three ingredients that are
functional in a LiPo
battery. The rest is wrapping and wiring attachments.</i></p>
<ul>
<li><i>Cathode: LiCoO2 or LiMn2O4</i></li>
<li><i>Separator: Conducting polymer electrolyte</i></li>
<li><i>Anode: Li or carbon-Li intercalation compound</i></li>
</ul>
<p><i>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.</i></p>
<p><i>Remember your chemistry class? Note the absolute <strong>lack
of any
hydrogen atoms in the reaction</strong>. None, zero, zip,
nada. If you have
<strong>water inside your battery</strong> -- 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.</i></p>
<p><i>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.</i></p>
<p><i>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.</i></p>
<p><i>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.</i></p>
<p><i>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.</i><br>
</p>
<i>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.</i>
<p><i>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...</i></p>
<p><i>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.</i></p>
<p><i>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.</i></p>
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<p><i>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.</i></p>
<br>
<br>
On 12/4/2010 8:23 PM, Stuart Chale wrote:
<blockquote cite="mid:4CFAE99E.9030508@optonline.net" type="cite">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 :)
<br>
<br>
On 12/4/2010 6:45 PM, Ron Van Putte wrote:
<br>
<blockquote type="cite">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.
<br>
<br>
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.
<br>
<br>
The three 5S packs I have left are "rock solid". Experienced
electric-pilots will know just what I mean.
<br>
<br>
I have flown these packs and they seem to perform just as they
did in their "youth".
<br>
<br>
My questions are: Why do lithium polymer cells "puff"? What is
the likely future of my recovered 5S packs?
<br>
<br>
Ron VP
<br>
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