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How much dent is too much?
#1
Yes, I know, any dent is too much but after denting quite few hard to harvest cells the question is a bit more pertinent to me. Some dents are barely visible. Should those be tossed? I pulled some wrappers off some old cells in packs I've used for over a year and see dents in my daily drivers I didn't even know were there. They tested very well and didn't heat.  Why would I throw them now? So, it's got me questioning my stringent policy of throwing away anything dented. Thoughts?

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#2
The general consensus is to toss these (the one's pictured).   I believe the consensus explanation is that even if they work now, they may be subject to an accelerated dendrite damage and short out early in the remaining life cycle.     There have been some 'strong' threads on cell damage in general - https://secondlifestorage.com/showthread...ight=dents and https://secondlifestorage.com/showthread...hlight=4mm

One nit I got from some discussions is that there may be such a thing as a 'max acceptable dent in 18650 can' specification - perhaps if you can find that for your cells you could authoritatively answer this question for your situation.
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#3
The dendrite issue doesn't worry me too much as they form under low current conditions and get "burned" off under high current. Since I build bicycle batteries the loads can be intense but momentary.

I appreciate the links. I'll continue to research.
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#4
The other issue is that where the dent is, the electrolyte could have been damaged, or pushed slightly out of the way. This would allow the two sides of plate to be closer together. This may be why dendrite growth happens more in these instances.

I personally would not use them in pack situations, especially EV ones. Maybe single cell applications
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#5
I would discharge and toss them.
They could be damaged inside, could possibly short circuit and catch fire even in low energy applications.
The forming of dendrite would not be my first concern.
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#6
The question remains. How much is too much? I put the pics up from my trash pile for a point of reference. As I mentioned in another thread, is .005" dent acceptable? How about .015"? Is the dent in the middle cell (pictured) half way up acceptable? Then how about the dent half way up on the one on the right? Two billion cells are manufactured every year yet no first hand reports of dented cells being a problem. I keep reading and searching but have yet to find anything definitive at all.
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#7
(09-27-2020, 03:31 PM)DiggsUt Wrote: Two billion cells are manufactured every year yet no first hand reports of dented cells being a problem. I keep reading and searching but have yet to find anything definitive at all.

It's quite possible that it may not be known if that was a problem or not. When a fire happens and an investigation occurs, they may not look to see if the cells are "dented" or not. A lot of the times it might not be possible to tell if that was the cause or not as well. When those things get that hot, they get deformed. Or during the process of putting out the fire could damage the cells further.
So unless there's scientific specific tests on dented cells being an issue, you may not find the info.

This does not mean that dented cells are fine and problems don't happen from them. The degree of dented is unknown how much is too much. But would be good to know what it is, though.


However, with all that said, generally the amount of cells dented are so small compared to how many are good just about makes it not worth it, or risk it. imho
Proceed with caution. Knowledge is Power! Literally! Cool 
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#8
Dents don't seem to cause immediate problems.   As I understand it, the general idea is that over time, dendrites may accelerate and cause shorts inside the cell.....   so then I have some other questions.

1) Assuming dendrites start causing shorts - is this like an overall cell short - e.g. the fuse wire on DIY packs will protect  OR is this pictured as some kind of gradually increasing thermal heat inside the cell...  and eventually it catches fire but yet the cell works/does not invoke the fuse wire till fire breaks out?   OR is this more gradual and the cell will start losing capacity rapidly?  or other? or all?      Is it like mini-strokes in humans....  and if you get enough you have trouble/fall over?
**I'd be interested in a deeper picture of what 'dendrite shorts' mean exactly...**

2) Setting dents aside, I think I've read that old cells can also fail due to dendrite growth!? 

I ask all this, because I have nearly 10,000 cells in daily charge/discharge operation - and some have minor dents (maybe 100).  The oldest are nearing 1,000 cycles and eventually I will have very old/weak cells..  and so I'm obviously interested if there is some kind of consensus on the risk line - when should DIY packs should be retired?  I mean this whole DIY started by harvesting OLD laptop cells Smile

FYI - here's an example of what I call a 'small dent' created when removing the cell from the battery casing:
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#9
From the initial pic, I'd consider 1 fine and exclude 2 and 3.
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#10
The smart answer is the discard/bin it and not use it.

The wise thing to do is not use it in a large Powerwall pack.

The frugal thing to do is use it for single cell applications like flashlights.
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