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Locating Self Discharging Cell(s) in Packs
#1
So I created a few "standard" sized packs of 100 paralleled cells in order to power a project of mine recently. I created four of them about two months ago and they've essentially been sitting, not connected to any loads or in parallel or series to each other since they were completed (I needed to create a tray, plexiglass cover and ensure they could fit in my enclosure in the time being hence the storage period).

They were all close to full at that time, hovering around 4.1V or so. Just tested them again and three of them are still above 4.0V, however one, of course, is now down to 3.2V. Obviously I have one or more bad cells in the pack (really disappointed in myself that I didn't weed them out during the cell harvesting and testing phase but I likely just mixed a handful up during one of those late nights testing or it possibly went bad on me since then).

So, my question is, now that I've got all these packs welded up with fuses and tabs soldered to busbars, essentially all completed with not so cheap terminals and materials, can anyone think of a way to locate the self discharging cell(s) in the pack of 100?

The only things I can think of, without having to undo the entire pack, is to maybe point a thermal camera at it and see if maybe one cell is hotter than all the others? But I'm also pretty dumb so I'm not sure that the cell would register a higher temp, even if I happened to have a nice FLIR camera (which I don't).

I suppose I could also undo the fuses on the positive end one by one, checking the current running to each cell with a multimeter then (I do have a DC clamp meter but no way in hell I'll get that clamp just around one fuse leg). I'd also rather not make a brand new pack of 100 but, honestly, that may be the quickest solution. I'm hoping against hope there's a better solution that's been mentioned on this board that I just can't seem to find, so if anyone knows of it, I would greatly appreciate it. I shall send you a 12 pack in the mail.

And sorry for a new thread for such a specific, selfish question, I tried searching for a like minded existing thread or post but came up with squat. Thanks for the help and time guys!

TLDR: Any ideas on how to find one cell in a pack of 100 that's self discharging/parasitic without taking apart the pack?
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#2
Could you upload pictures of the + and - sides of the pack so we can see how you have it wired?
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#3
What I would do is charge the pack back up to 4.2V, then disconnect 1/2 the pack. Watch what happens. Then the half that went low again, split it half, or there abouts. Depends on how you have it wired up.
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#4
(08-21-2019, 12:13 AM)Bubba Wrote: Could you upload pictures of the + and - sides of the pack so we can see how you have it wired?

But, of course. Here's the positive side of the pack in question. Actually, tried to get the entire thing in frame here so it's a weird angle:



Then here's the negative side. This is actually two packs just resting on top of each other but they're not electrically connected in any way. So just the first four top rows are of the pack in question again:



(08-21-2019, 12:22 AM)Korishan Wrote: What I would do is charge the pack back up to 4.2V, then disconnect 1/2 the pack. Watch what happens. Then the half that went low again, split it half, or there abouts. Depends on how you have it wired up.

That's not a bad idea at all. So just kinda whittle it down in halves basically, trying to find the correct part with the drainer in it instead of taking the whole thing apart? And I could add cells back in one at a time or so if it the part I left connected doesn't self discharge again. Better than ripping the whole thing apart definitely.
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#5
Yeah, problem is the way the design is. Hard to cut it down unless you cut the copper bus bar in sections.
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#6
I think what Korishan meant:
1. Charge whole pack to 4.2V (voltage drop will be more noticeable at high V)
2. Let it rest for an hour or so for all cells to settle
3. Disconnect 1/2 of the fuses
4. Wait for say 24h
5. Measure voltage of disconnected cells. Voltage drop -> leaker. There could be more than one.
6. Measure voltage of the still connected cells. Voltage drop -> return to step 1.
7. Replace leaking cells (match voltage first) and reconnect fuses.
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#7
(08-21-2019, 01:09 AM)Korishan Wrote: Yeah, problem is the way the design is. Hard to cut it down unless you cut the copper bus bar in sections.

Yeahhhhhhh... I haven't exactly made these modular or easy to disassemble by any means. I realized that while making them but just kept forging on. With those cheap 4x5 cell holders, you can break off the tabs that hold each cell in to slide a cell in & out but, other than that, not a lotta options to swap things. I wish I just had the foresight to make a 5th back up pack of 100 that I could substitute in. Oh well. Looks like there's no other option but to start tearing fuses off.

(08-21-2019, 01:12 AM)ajw22 Wrote: I think what Korishan meant:
1. Charge whole pack to 4.2V (voltage drop will be more noticeable at high V)
2. Let it rest for an hour or so for all cells to settle
3. Disconnect 1/2 of the fuses
4. Wait for say 24h
5. Measure voltage of disconnected cells. Voltage drop -> leaker. There could be more than one.
6. Measure voltage of the still connected cells. Voltage drop -> return to step 1.
7. Replace leaking cells (match voltage first) and reconnect fuses.

Perfect, that's kinda what I thought he was referring to but you thought it out much better than I could.

I suppose I could also start with the cells that I have an intuition might be the culprit based on the stats I got during testing. I was smart/dumb enough to record everything in a spreadsheet and some cells are undoubtedly "healthier" than others. Though none show any visible signs or issues, I might get lucky isolating and checking those. But most likely I'll have to do half and half to ensure I get em all. Goddamn you recycled cells, you've bested me again!
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#8
I recently did this to my first packs. and What I did was Charge up to 4.2 then remove the Positive Side fuses and Bus bar.

Let it sit for 2 weeks and then Tested the voltage of each cell. I found 7 cells out of 560 that were Really Bad. (3v) and another 30 that were kinda bad and I replaced those too. (got down to 4v where the rest of them were 4.18)
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#9
(08-21-2019, 02:11 AM)jdeadman Wrote: I recently did this to my first packs. and What I did was Charge up to 4.2 then remove the Positive Side fuses and Bus bar.

Let it sit for 2 weeks and then Tested the voltage of each cell. I found 7 cells out of 560 that were Really Bad. (3v) and another 30 that were kinda bad and I replaced those too. (got down to 4v where the rest of them were 4.18)

I have a feeling thats exactly what's going on. Which is weird cause I had waited at least two months before even choosing my cells to make these packs, taking the ones with the smallest voltage drops usually. Previously had you noticed that your packs were draining significantly without a load attached or were they just draining rather quick when connected to a load?
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#10
Not trying to be a pain in the butt here but where these cells checked for IR before the pack was assembled?
If they were and there still was such a failure then it would be good to know which cell(s) causes this issue once you find them and recheck the IR to see how different it is now from when the cells were assembled.
Certainly looks like something odd has happened. 

Best of luck finding the bad cells and hope you get the pack up and together quickly.

Wolf
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