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Do I have to worry about Over Discharged Cells?
#1
Hi all,

I have 40 18650 which were 0V and I recovered all of them, the capacity is 3400mAh and the SD is ok.

So recently I stumbled over this Article:

Under-voltage / Over-discharge

Rechargeable Lithium cells suffer from under-voltage as well as over-voltage. Allowing the cell voltage to fall below about 2 Volts by over-discharging or storage for extended periods results in progressive breakdown of the electrode materials.
  • Anodes
    First the anode copper current collector is dissolved into the electrolyte. This increases the self discharge rate of the cell however, as the voltage is increased again above 2 volts, the copper ions which are dispersed throughout the electrolyte are precipitated as metallic copper wherever they happen to be, not necessarily back on the current collector foil. This is a dangerous situation which can ultimately cause a short circuit between the electrodes.
  • Cathodes
    Keeping the cells for prolonged periods at voltages below 2 Volts results in the gradual breakdown of the cathode over many cycles with the release of Oxygen by the Lithium Cobalt Oxide and Lithium Manganese Oxide cathodes and a consequent permanent capacity loss. With Lithium Iron Phosphate cells this can happen over a few cycles .


Now, do I have to worry about that? And should I not use the Cells for my Powerwall?

If anyone has experience with recovered low voltage cells please tell me what you suggest.

And, am I right with that, a fully charged cell which gets a internal short circuit, will probably end in a thermal runaway?

Best Regards
LEDSchlucker
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#2
This is really from "continuous" over-discharge. Not from a one off from being discharged. If they have that kind of capacity after recovering, I don't believe you'd have any issues.

It takes quite awhile for dendrites to form, this is what makes the short circuit in the cell.
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#3
Ok. Thanks a lot!

So in generally we could say, if one time the battery wasn't "handled" correct we don't have to worry at all? Except for cells with less percent of orig. capacity and or high impedance or SD?

And what would happen if one gets a short circuit inside?



Okay, I think after that all my questions will be answered for now. Smile
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#4
Yeah, pretty much. That one time stress isn't going to destroy the cell. If it did, we wouldn't be using reclaimed laptop/ebike/etc cells to make our packs Wink
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#5
I just measured the voltage of my recently charged cells to find them with high SD and four of the ones I recovered, had around 3.1V Dodgy

So there are two possibilities, either because of these cells, the whole pack was at 0V or they got damaged because of ODC. So, what do you guys think?


Anyway I will try charge them again and see if the voltage sinks again so fast.

Edit:
The cells are LG MJ1 (LGEBMJ11865)
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#6
If those 4 cells were in parallel with the others, then they would have brought the whole pack down.

But, it's also possible the bms drained them down to nothing.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#7
It was a 10s4p E-bike battery, so they couldn't not be in each parallel pack.
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#8
@LEDSchlucker, from the research/testing that I've done, I've found that most of the damage occurred in a cell below 2V is the result of anode breakdown collecting on the separator.

Therefore, what happens at normal charging rates is that this can lead to physical damage to the separator due to heightened internal resistance below 2,5V, and can cause a short.

The cathode breakdown is less severe in terms of consequences, mostly losing capacity depending on how long.

However, normal charges rates can still lead to chemical degradation, and capacity loss.

Therefore, this is why charging rates below 100mA are recommended below 2,5V, and below 50mA below 1V.
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#9
So it was a big mistake, that I bump charged some of them by paralleling with a full cell and some wit 0,6A Sad
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#10
Yeah. Down the line, you will be losing capacity without noticing it at all.

I personally realized it back in 2017 when tearing down some powertool packs containing Samsung 20Rs.

Ended up charging them at 0,5A like normal at about 1,8V. Got around 1900mAh, which was good. Internal resistance was also around 22-25mOhms DC, so still good.

After a few cycles, capacity went down to 1500mAh, and internal resistance shot up to 45mOhms. That was a massive difference.
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