[NSRCA-discussion] "Sick" Lipo Packs

[Chris]

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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>
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