[NSRCA-discussion] "Sick" Lipo Packs

Sun Dec 5 04:59:47 AKST 2010

Chris ,

Great article!
Were can we find the rest of it.

On Sat, Dec 4, 2010 at 8:36 PM, Chris <cjm767driver at hotmail.com> wrote:

>  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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