[NSRCA-discussion] "Sick" Lipo Packs

[ehaury]

Chris

Thanks for the info - pretty much answer's the puffing question.

Let me add my thoughts / observations as to why things can not be all good with charging / balancing related to pattern packs and the common chargers we use.

So what about "charge abuse"? My observations suggest that balancing equipment doesn't always do what is expected. Many balancers function by drawing charge current away from the high cell(s) which allows the low cells to "catch up". Part of the problem lies with the balancers capacity to divert that charge current - usually 300 - 500mAh - which isn't sufficient when charging at 5A. If the charger primarily terminates on total pack voltage, the high cell(s) are pushed too high until the average reaches the target pack voltage. This will often leave the weak cell low and some too high and won't be noted unless the individual cell voltages are observed. (The way to charge packs packs with large imbalance is at a rate that doesn't exceed the diversion capacity of the balancer - takes time.) On a good note, most systems will terminate charge (with notification) if the high cells are pushed too far - still not good for them (especially without consideration for temperature) but an important safety feature that should prevent charging eruptions as well as providing notification that something isn't right. Then there are chargers that balance by either charging each cell independently or by adding current to the low cells to help them "catch up". This is a good strategy and that appears ideal except for issues with connectors / leads.

A common problem is that both balance methods can be compromised by the balance leads / connectors themselves which can result in the balancer actually misbalancing the pack. The connectors are small and of materials that will oxidize over time. They also tend to lose tension with use. Both the looseness and oxidation results in a poor connection. A marginal connection will prevent the balancer from "seeing" the correct voltage and will become worse as balance current flows. (Ever have a "balance lead not connected" or "incorrect cell count" report - these are indications that the connectors are not doing their job. This all is 10X true if charging at 5A is attempted through these faulty connections! Additionally, the balance leads themselves are small gauge wire of low strand count (not very flexible) which often results in several strands breaking from the connector with use. Eventually the wire might break off - in the meantime it's a resistor in the balancing circuit! I've also seen extensions (have all the same problems) where only a couple of strands were captured in crimping to the connector! The good news is that much of this can be avoided with maintenance and observation. Contact cleaners will help keep the connections clean, the some female connectors can be removed from the housing (one at a time if on the batt) and "bent" back closed when they loosen. Check connections on new leads / extensions and always fix the problem if some lead "wiggling" is needed to make the charger happy.


Earl



---- Original Message ----- 
  From: Scott Pavlock 
  To: General pattern discussion 
  Sent: Sunday, December 05, 2010 7:59 AM
  Subject: Re: [NSRCA-discussion] "Sick" Lipo Packs


  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.

      a.. Cathode: LiCoO2 or LiMn2O4 
      b.. Separator: Conducting polymer electrolyte 
      c.. 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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