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Is this a magic pot, or a self-healing Li-Ion battery?

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RoGeorge:
Found last months a 10Ah Li-Ion cell.  It was swollen, like a pillow, about 2-3 times its normal thickness.  Charged it to 4.2V, out of curiosity, and it seemed to take charge just fine.  Then I've stored it in a thick metal pot with a normal lid, for just in case the battery will vent.

Next day measured its voltage again, still 4.1V, and the swollen was almost gone.  Back then I thought the battery's plastic bag cover might have developed a microscopic puncture/rupture somewhere and the gasses vented.

Stored it back inside the pot, put the lidded pot on the balcony for just in case more gases will vent.  Today looked at the battery again, and it is very, very flat.  The external cover looks like there was some suction from the inside.  :o

The battery measures now 4.06V.  Temperature was about the same at all times, ~25*C.

In phones battery, I've never seen a battery to recover by itself.  This 10Ah is not from a phone, looks like a bulk Li-Po cell somebody discarded because it was so bulged.

Is the bulging reversible?  What conditions are needed for that to happen?
How come that the mobile phones batteries never recover once bulged?

AVGresponding:
Maybe the heatsinking effect of the iron pot allowed the gases to recombine into the electrolyte by cooling it to condensation point?   :-//

RoGeorge:
It was all at room-temperature.  :-//

The battery being that bulged when I found it, I was afraid it might vent.  Charged it at first at 10mA for a few hours, then let it at about 100mA for 10 more hours or so, then charged it at 1A for another 10-20 hours.  I don't recall the exact charging time, but I've kept charging it for a whole weekend, until it finally reached 4.2V.  Then finally disconnected it at 4.2V, after the charging current dropped under 0.1A or so.

It didn't heat during charging, and it was stored in the pot only many hours later after charging.

james_s:
I've never seen that happen, but if the cell is self-discharging in such a short period then it's obviously damaged and you really don't want to use a damaged LiPo cell, they have a nasty habit of failing spectacularly at some random later time. The self discharge should be negligible, a few percent a year perhaps.

RoGeorge:
For now it is store inside that 3mm thick cast iron pot with a lid, in a concrete balcony with nothing else around.

I've write down the voltage a few times.  Measured max thickness of the pillow-like shape at first was 2cm and under small pressure inside (now it is 1.25cm everywhere, flat as a plank and under vacuum inside).
4.1873V - right after disconnected from the 4.2V charging, bulged as a pillow, some pressure inside but not much
4.1813V - 30 minutes later
4.1599V - 5 hours later
4.1390V - 15 hours later
4.1062V - 3 days later  <--  here it was de-bulged but no signs of internal vacuum, only a flask plastic cover
4.0935V - 5 days later
4.0663V - 20 days later  <--  today, as seen in the pics

The plan was to discharge it later and measure its capacity.  Because back then I've thought the plastic cover has ruptured, I've just let it there and never measured the capacity.

Maybe the bulging gasses did escaped, and something else entered and started another chemical reaction, and that absorbed the remaining gases and made the plastic cover stick to the battery material, thus self-sealing whatever micro-puncture might have been there before.

Another observation is I remember the internal material softer 3 weeks ago than it is now.  Not sure if this is because of the vacuum inside, or if the battery material started to harden itself.

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