[NSRCA-discussion] "Sick" Lipo Packs
Bill Glaze
billglaze at bellsouth.net
Mon Dec 6 10:43:15 AKST 2010
O.K. Ron, now get a clean sheet of paper and two #2 pencils; in ten minutes
there's going to be a Zip quiz worth 50% of your final
grade.............................. Bill
----- Original Message -----
From: "Ron Van Putte" <vanputte at cox.net>
To: "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
Sent: Sunday, December 05, 2010 12:00 AM
Subject: Re: [NSRCA-discussion] "Sick" Lipo Packs
> 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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