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2 pack faling behind with charging and get unbalanced (update 9/07/2020)
#31
(08-12-2020, 02:33 PM)gauss163 Wrote: Recall that the entire cell can is negative, and neighboring (touching) cans in (laptop) packs can have different potential (as newbie pack harvesters often learn the hard way by accidentally shorting them during disassembly). If the wrapper had a low melting point then it would not work well as an electrical insulator (between neighbors) during thermal events. So wrappers with higher melting points are preferred. That's why it is not unusual to see the wrapper survive seemingly intact after localized thermal events such as in the OP (though all bets are off with lower-tier cells whose wrappers may melt at very low temps - Ultrafire's are well-named!)

Yes, we know that the whole can is Negative. However, in a pack of parallel cells, it doesn't matter if the can touches another as they are all at the same potential. Another thing to note is that the OP had the packs built inside of a constructed housing that kept space between the cells of one pack from those of another.

However, in this instance, you can see that that particular pack was up against something "other" than another pack. So we know that it wasn't a voltage difference between packs. Buuuuut, what was that pack up against? Only the OP can inform us.

This is just a theory: Perhaps the unit that these packs were stored in was metal. Perhaps also the Negative side of the string was grounded to the storage rack somehow, perhaps the inverter/charger was bare metal to bare metal. This would make the rack essentially 0V potential. This particular pack, being in the middle some where, would have a different potential relative to the metal rack, perhaps even only 4V at full charge if this was the 2nd pack in the string. Then the OP pushed the pack up against the back of the metal rack. At the point of contact, by pure coincidence, there was a metal bur on the metal rack that slightly pierced the shrink wrap.
This metal bur could possibly even be on the end of a bolt that held the rack together. Then over time, that spot slowly heated up until it finally burned the wrapping and created a small hole. Basically it was a slow spot welding. Until the metal can burned enough metal away to disconnect from the bur.

I ask barry to check the back of the rack he installed the packs on and check for any damage to the rack itself.

Of course, this theory is all moot if he built a wooden rack to install these on. In which case, it would appear something else pierced the wrapping.
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#32
(08-12-2020, 02:47 PM)Korishan Wrote: Yes, we know that the whole can is Negative. However, in a pack of parallel cells, it doesn't matter if the can touches another as they are all at the same potential.

It appears you missed the word "series" in "two side-by-side series cells". These (and other mixed potential neighbors) occur frequently in commercial packs (e.g. laptops, powerbanks, etc), esp. those having odd shape. Of course the manufacturers must spec the wrappers to handle these cases, not only parallel neighboring cases as in DIY powerwalls.

As for "we know....", my experience is that almost all beginners have no idea that the entire can is negative, since that is contrary to most cells at consumer level.  One of the primary purposes of safety warnings is to help protect beginners - where most (but not all) accidents occur (e.g. vaper pants-on-fire accidents due to carrying naked cells in pockets with keys, coins, etc). And of course every active site has many beginners at any given time (often hidden due to lurking). As such, it is helpful to emphasize (in passing) these common oversights, since osmosis often works wonders for learning.
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#33
(08-12-2020, 03:02 PM)gauss163 Wrote:
(08-12-2020, 02:47 PM)Korishan Wrote: Yes, we know that the whole can is Negative. However, in a pack of parallel cells, it doesn't matter if the can touches another as they are all at the same potential.

It appears you missed the word "series" in "two side-by-side series cells". These (and other mixed potential neighbors) occur frequently in commercial packs (e.g. laptops, powerbanks, etc), esp. those having odd shape. Of course the manufacturers must spec the wrappers to handle these cases, not only parallel neighboring cases as in DIY powerwalls.

I didn't miss the word "series".

(08-12-2020, 03:02 PM)gauss163 Wrote: As for "we know....", my experience is that almost all beginners have no idea that the entire can is negative, since that is contrary to most cells at consumer level.  One of the primary purposes of safety warnings is to help protect beginners - where most (but not all) accidents occur (e.g. vaper pants-on-fire accidents due to carrying naked cells in pockets with keys, coins, etc). And of course every active site has many beginners at any given time (often hidden due to lurking). As such, it is helpful to emphasize (in passing) these common oversights, since osmosis often works wonders for learning.

As far as I can tell, barry isn't a newbie and is quite aware of the issues of shorting cells that are in series. Also, we're talking about a pack that all cells are connected in parallel. Discussions about a topic should stay on topic based on what the OP has stated and asked. Not fill the thread full of information that doesn't directly pertain to the OP's thread. If you wish to add extra information, please create a new thread and link to it ("If you'd like more information about such-n-such, please read this thread.....")
Proceed with caution. Knowledge is Power! Literally! Cool 
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#34
(08-12-2020, 04:31 PM)Korishan Wrote: I didn't miss the word "series".

Then it's not at all clear why you replied to my remarks on wrappers for general commercial packs with much more specific remarks about a particular pack topology (such as OP's). Obviously manufacturers must design cells for the general case, not some specific case.

(08-12-2020, 04:31 PM)Korishan Wrote: As far as I can tell, barry isn't a newbie [...]

Again you appear to be (mis)reading general remarks (on safety) much more specifically than intended. No one claimed that the OP was a newbie. Please try to read more carefully. It is such misreadings that are primarily responsible for the thread veering onto tangential meta matters.
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#35
(08-12-2020, 05:26 PM)gauss163 Wrote:
(08-12-2020, 04:31 PM)Korishan Wrote: I didn't miss the word "series".

Then it's not at all clear why you replied to my remarks on wrappers for general commercial packs with much more specific remarks about a particular pack topology (such as OP's). Obviously manufacturers must design cells for the general case, not some specific case.

(08-12-2020, 04:31 PM)Korishan Wrote: As far as I can tell, barry isn't a newbie [...]

Again you appear to be (mis)reading general remarks (on safety) much more specifically than intended. No one claimed that the OP was a newbie. Please try to read more carefully.  It is such misreadings that are primarily responsible for the thread veering off-topic onto meta matters.

I choose to not go into details as it has nothing to do with the OP or his situation. Please stay on target and keep answers within the realm of knowledge of the OP and participants. We don't need super basic replies, or ones that are so advanced that most are lost in them.
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#36
(08-12-2020, 05:36 PM)Korishan Wrote: [...] We don't need super basic replies, or ones that are so advanced that most are lost in them.

Could you please have the courtesy to stop writing "we" when you mean "I". 

Sorry that my posts are either too basic or too advanced for you. But most of them were not targeted specifically at you (or anyone for that matter, since I often strive to consider general matters in order to make my posts as useful as possible).
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#37
It is not "I", it is "we". because several have posted about it. So if it is more than 1, it is "we".

Your content isn't too basic or advanced for "me". What you are doing is not sticking within the boundaries of the thread. Keep your comments in reference and boundaries of the thread. If you have more detailed information you wish to add that goes beyond the thread, post a new thread and link to it.

This conversation is done, over with. The thread has been derailed long enough. It will return back to on topic of the OP and his situation.

As I mentioned earlier, it would be interesting to know if that pack was pushed up against the back of a metal of rack that possibly punctured the cell wrap. We won't know that answer until Barry responds.
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Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#38
just what Korishan said i am not a nooby Big Grin.
yes all my pack are well isolated and rest on nylon L brakets so nothing touches one and other and the skrews are well hidden.

thanks guys for all the info and help!!
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#39
^^^ Not to worry, no one called you a noob. Good job on the quick fix.
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#40
Glad to hear you got it fixed Barry :-)
Nylon pack slides sound like a great idea!
Running off solar, DIY & electronics fan :-)
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