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2 pack faling behind with charging and get unbalanced (update 9/07/2020)
#21
The OP's issue is clearly cell a critical failure in the middle of the cell with nothing to do with the ends of the cell.
As not2bme pointed out above, the most likely cause is physical impact.
A dent, etc might have been concealed if the cell's shrink wrap sprung back to the "normal" shape after impact.
Replacement of that cell will likely fix the issue for the OP.
The damaged cell should be put somewhere fireproof asap & discharged to zero (safe then) with eg a 100 ohm resistor.
As mentioned already, while is has any significant charge it is a high fire risk.
Given the shape & orientation of the heat/damage, my opinion is nearby cells would be unharmed & at most got a few degC warmer (harmless).
If the burn had faced other cells, then agree adjacent cells might have suffered.
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#22
(08-11-2020, 08:04 AM)Redpacket Wrote: The OP's issue is clearly cell a critical failure in the middle of the cell with nothing to do with the ends of the cell [...]

As I explained briefly in post #15, the reason for a post-mortem examination of the top is to check for any evidence that the cell attempted to vent through the top vent holes (or see if they are blocked). Knowing that helps to narrow down the possible modes of failure. In particular, since this cell did not appear to vent on top, that increases the probability that the event was caused by an internal short nearer to the can sidewall (away from the central vent tube (mandrel) - see images 1 & 2 below), where gases are more likely to get trapped -  unable to reach the central vent - resulting in sidewall ruptures - as in image 3 (in 1 second thermal runaway can generate gas having 50 times the cell volume).



excerpted from Computed Tomography Analysis of Li-Ion Battery Case Ruptures (2020), by L. Kong et al.

(08-11-2020, 08:04 AM)Redpacket Wrote: Given the shape & orientation of the heat/damage, my opinion is nearby cells would be unharmed & at most got a few degC warmer (harmless). If the burn had faced other cells, then agree adjacent cells might have suffered.

Since 18650 cells have been reported to reach up to 700°C in thermal runaway (cf. 2020 paper above), there is certainly a chance that it caused thermal damage to neighboring cells (no matter the location of the rupture). Thermal damage can wreak havoc on cells.

(08-11-2020, 08:04 AM)Redpacket Wrote: [...] As not2bme pointed out above, the most likely cause is physical impact. A dent, etc might have been concealed if the cell's shrink wrap sprung back to the "normal" shape after impact.

Yes, as I remarked in post #9, mechanical damage is one possibility, but usually it takes a significant dent to the sidewall to cause a short since the jelly role has opportunity to deform a bit without damage under such radial forces  (unlike axial compression on the top). Below is an image excerpted from a 2012 paper discussing shorts caused by sidewall dents.

But there are also many other possible causes of internal shorts, e.g. copper/lithium dendrites from over (dis)charges, metal contaminants (shrapnel) introduced during manufacture, thermal damage, etc. 
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#23
gauss163 thanks again for another informative post.
I'm just looking at the simple side of it as shown in the OP's picture.
We love all the detailed info you bring but please allow the practical & pragmatic side some space here too.

The OPs pic shows some localized spot heating which when combined with the other info is not likely to have been deep in the cell.
Compared to other typical failure result pics this is very mild - OP was lucky!
Even the cell's own sleeve isn't melted much apart from the hole.
The OPs cell cap was "normal" after this.
The cell is even (mostly) holding voltage & taking a charge! This suggests much of the internals are not destroyed.
It's as though this event was a"slow blow resistive" not a "fast blow full short"
Thermal runaway obviously did not take hold.
So temps are not likely to have gone to 700degC & pic shows only a wisp of smoke on other cells.
All this points to something in the outer layers not the cell middle.
In this case, pressure venting or winding design doesn't seem to be that relevant (although still interesting).

I agree it could have been caused by a dendrite too and since the OP said they can't see a dent maybe this is a likely cause.
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#24
(08-12-2020, 02:01 AM)Redpacket Wrote: [...] please allow the practical & pragmatic side some space here too.

I have no idea what you mean by "please allow". As I stressed in the past, my view on safety education is that we should provide users with as much pertinent information as possible. Only then do they have the best chance possible to make wise decisions. Too many folks have suffered serious injuries from Li-ion batteries only because no one properly warned them of the inherent risks.

(08-12-2020, 02:01 AM)Redpacket Wrote: The OPs pic shows some localized spot heating which when combined with the other info is not likely to have been deep in the cell. Compared to other typical failure result pics this is very mild - OP was lucky!

We'll have to disagree on that. That gaping hole likely required a serious thermal event.

(08-12-2020, 02:01 AM)Redpacket Wrote: Even the cell's own sleeve isn't melted much apart from the hole

That implies very little without knowing the wrapper material and its melting point. Even PVC has formulations with very high melting points - up to 500°F (260°C) - which is far more than needed to thermally damage a (neighboring) cell.

And there is good reason for top-tier manufacturers to use a wrapper with a high melting point because it helps to prevent runaway chain reactions (if a thermal event melts the wrappers between two side-by-side series cells then their cans may short together, which shorts the neighbor, then that goes into runaway, which shorts another neighbor ... (domino effect). 

In the OP, we don't have anywhere near enough evidence to rule out thermal damage to neighboring cells. In such cases the safe way to proceed is to assume the worst and replace the neighbors too.

(08-12-2020, 02:01 AM)Redpacket Wrote: In this case, pressure venting or winding design doesn't seem to be that relevant (although still interesting).

Since you seemed to think that my asking for photos of the vent was not relevant, I replied explained why this is a standard step in such post-mortem analyses. Further, understanding such "under the hood" matters is relevant since it may help one to use cells more safely. An educated Li-ion user is a safer user.
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#25
That cell cant have been that hot. The cells that go full thermal have melted the sleeve totally and the casing Will be discoloured all the way around.

Remove the sleve and check.

Edit; it almost look like a melted hole due to some Contact on the outside?
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#26
^^^ Obviously it did not go full thermal, but even localized thermal events like this can generate enough heat to damage neighboring cells. The hole needn't be from external contact since sidewall ruptures like that are characteristic of internal shorts - see post #22 and the paper linked there.
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#27
*update*

I changed the cell and neighboring cells with the same capacity all the packs are in balance now and works how it is designed.
and just in time Big Grin last night we had now power 
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#28
(08-12-2020, 03:57 AM)gauss163 Wrote:
(08-12-2020, 02:01 AM)Redpacket Wrote: [...] please allow the practical & pragmatic side some space here too.
I have no idea what you mean by "please allow". As I stressed in the past, my view on safety education is that we should provide users with as much pertinent information as possible. Only then do they have the best chance possible to make wise decisions. Too many folks have suffered serious injuries from Li-ion batteries only because no one properly warned them of the inherent risks.
Please allow = There's a difference between helping this forum's readers understand Li-Ion issues & risks vs hammering the safety issue every time with a sledge hammer.
We as a group want to build packs & power systems.  Dumping a ton of bricks on every possible risk isn't helping, it's loosing people's interest.

I note you have not provided links to injuries/damage from DIY Li-Ion.  Yes people have been hurt, etc but not from DIY so far.
Before you jump on that - yes we want to understand & learn & be as safe as reasonable.
This very forum helps people learn & follow reasonable practices.

I stand by what I've said, it's plain obvious, as I explained based on the details the OP provided.
It was (fortunately) a mild event & cell burst/explosion clearly did not occur so in THIS case internal venting design is not of primary relevance it's secondary.

A wrapper stopping thermal run away? I don't think so. Even in this mild case (yes quite bad enough but there's much worse) the wrapper was burnt through.
Cell wrappers are only heatshrink plastic & that doesn't stand a chance. Kapton tape would do better, but I can still burn that with a soldering iron.
As many videos & results show, full thermal runaway = most bits crispy.

(08-12-2020, 03:57 AM)gauss163 Wrote: An educated Li-ion user is a safer user.
Agree.
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#29
(08-12-2020, 09:52 AM)Redpacket Wrote: A wrapper stopping thermal run away? I don't think so.

It seems your above comment refers (too briefly) to my remark below:

(08-12-2020, 03:57 AM)gauss163 Wrote: And there is good reason for top-tier manufacturers to use a wrapper with a high melting point because it helps to prevent runaway chain reactions (if a thermal event melts the wrappers between two side-by-side series cells then their cans may short together, which shorts the neighbor, then that goes into runaway, which shorts another neighbor ... (domino effect). 

In that case, yes, wrappers - as electrical insulators - can prevent shorting of neighboring cells/packs. As a consequence they may help prevent those neighbors from also reaching runaway. Such electrical insulation is the primary purpose of the wrapper.

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!)

If you wish to further discuss the meta topic of how to best discuss general safety matters then I think that would be better placed in a thread devoted to that purpose.
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#30
(08-12-2020, 09:52 AM)Redpacket Wrote:
(08-12-2020, 03:57 AM)gauss163 Wrote: An educated Li-ion user is a safer user.
Agree.

Double Agreed

(08-12-2020, 02:33 PM)gauss163 Wrote: If you wish to further discuss the meta topic of how to best discuss general safety matters then I think that would be better placed in a thread devoted to that purpose.

Triple agree
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