[NSRCA-discussion] "Sick" Lipo Packs

[Ron Van Putte]

Yeah.  Now my head feels like my belly felt on Thanksgiving afternoon.

However, it begs the question of what over discharging or discharging  
too quickly actually is.  I can understand going past the mah rating  
as over discharging.  Can discharging to 75%, 85% or 95% of the mah  
rating be considered over discharging, if the rate of discharge is  
under the C rating?

Ron VP

On Dec 4, 2010, at 8:21 PM, John Fuqua wrote:

> 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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