[NSRCA-discussion] Swelling Lipo
Bob Kane
getterflash at yahoo.com
Sun Dec 5 10:14:04 AKST 2010
Can I use this in a club newsletter ?
Bob Kane
getterflash at yahoo.com
--- On Sun, 12/5/10, Chris <cjm767driver at hotmail.com> wrote:
From: Chris <cjm767driver at hotmail.com>
Subject: [NSRCA-discussion] Swelling Lipo
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Date: Sunday, December 5, 2010, 9:30 AM
Here is the full article with much more detail.
Chris
What's Really Going On Inside A Dying Lithium Battery
Thu, 07/08/2010
Warning: Science ahead! Close your eyes and
turn away,
you've been warned!
Many radio-control enthusiasts experience disappointment with the
cycle life
of their Lithium-based batteries in electric aircraft. Often this
is because
they're not entirely sure what's going on inside the battery, and
choose a
capacity or voltage that's inappropriate for their application.
Ultimately,
this manifests itself in "swelling" or "ballooning" of a Lithium
battery. This editorial attempts to explain what's actually going
on when this
happens.
Chemically, there can be three causes for the swelling of a LiPo
battery,
and one exacerbating condition that makes it worse across the
board. These
occur in hard-shell Lithium Ion batteries, too, but the hard shell
can
withstand several atmospheres of pressure before expanding.
Note: This is MY understanding of the chemistry involved. I may
be off-base,
after all, I'm a college dropout. But I did love chemistry class!
Cause #1: WATER in the mix.
EDIT: Lithium manufacturers who's products are implicated in
this
assertion (read: Hextronik et al, circa 2006-2007, Thunder Power
circa 2008)
will dispute the assertion of contaminated Lithium. The most
common contaminant
is water, but there are many others that will cause lithium
oxidation in the
cell. Basically, any other substance containing oxygen that can
be freed by
electrolysis or heat will become a contaminant, and any
substance that isn't
the expected anode, cathode, or separator is a contaminant that
will reduce the
performance of the cell and cause swelling in other ways.
Manufacturers have a
fiduciary responsibility to claim that there was no product
defect, otherwise
they're responsible for a recall. I'll talk about the science
and let you draw
your own conclusions.
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.
Cause #2: Formula degradation from over-charge/over-discharge
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.
Cause #3: Poor separator construction
A number of cheap LiPos also use a bad separator formulation.
Ultimately, it
often boils down to using a dry separator with way too high of an
internal
resistance to hold up to manufacturer "C"-rating claims. The
internal
resistance grows over time because a higher and higher percentage
of the LiPo
is simple Lithium Oxide, and the balloon grows bigger as more
oxygen atoms are
freed.
I'd also lump "poor anode or cathode chemistry" into this
category, too. Ever get a bad battery out of a batch of good ones?
Often it's
because the mixture of chemicals was inconsistent, and you end up
with too much
or too little lithium on one side of the battery (well, in certain
plates, you
get my drift).
Exacerbating factor: HEAT.
A little heat makes everything work better for a Lipo. If you
could fly your
battery right at 140 Fahrenheit all the time, it would make
fantastic power and
be operating right in its happy zone. But it generates heat when
charging, and
when discharging. Hitting 150 results in significant metallic
lithium
generation, which as we can see from above is a major cause of
puffing and cell
destruction.
Similarly, the maximum 4.235v/cell limit is only at that mythical
140F. It
goes down steadily from there, to about 4.2v/cell at room
temperatures, and
around 4.0v/cell below 50F, beyond which the over-abundance of
electrons will
again break chemical bonds and free lithium to bond with oxygen
and create
lithium oxide... which is just a disaster waiting to bond with
humidity in the
air if the LiPo ruptures, to create Lithium Hydroxide.
Conclusion
Chemically, there are no LiPos that will not puff under certain
circumstances. But tightly-controlled humidity, a superb gel
separator,
nano-structured anode and cathode, and careful charging and
discharging within
manufacturer limits should also prevent puffing. Similarly,
putting a pack that
has been abused into a lower-discharge aircraft, even when puffed,
often serves
the purpose of stopping the puffing in its tracks because no more
metallic
lithium is being created in the cell by abuse.
And now you know the answer to today's geeky topic. Why lithium
polymer
batteries often puff up.
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