I was always under the impression that when a cell gets overly hot that it’s either worn out or defective. With laptop pulls I had a number that got fairly hot like yours and never charged over about 4.10 or so. They were 2600mAh Sanyo’s and only discharged about 1000mAh. Does your 16340 cell lose voltage overnight? My bad ones dropped about .05 volts in around a day or so.
To be on the safe side I tossed those batteries.
Hey, that’s exactly the same thing that happened to me!
A few short months ago, I couldn’t find where to source 18650 batteries (I had just bought a generic XML T6 flashlight that uses 18650 cells).
After looking around forums, I note that these could be harvested from laptop battery packs, so I asked some relatives and friends if they have old laptop battery packs that may be bad and they’re not using anymore. I got 2 laptop battery packs from them (both were deemed ‘bad’ already). One contains 6 Samsung ICR18650-22F (4 were still usable, 2 were 0volts and nothing I did could recover them). The other contained 6 Sanyo UR18650A.
I bought and used an XTAR VC4 charger (that was my only Li-Ion battery charger at that time; aside from the cheap single-18650 charger that came with the generic XML T6 light) to charge the Sanyo UR18650As.
I left them for a few hours, when I came back, the capacity reading on the VC4 were already 2500+ or 3000+ mAh (which shouldn’t be possible since the rated capacity for them are 2200mAh — I was largely ignorant about 18650s at that time). Also, they were very warm or HOT (for some). The voltages were in the 3.93-4.03v. I removed the 18650s to let them cool. Then after they were cooled a few hours, I charged them again, and this time they charged to near or around ~4.20v but they don’t seem to terminate charging (I was monitoring [I touched the batteries every now and then to ‘check’ temperatures] the charging process at this time when I learned the 18650s could get quite HOT).
If I remove those batteries and then place them again in the XTAR VC4, they will now terminate charging properly.
They also don’t seem to discharge fast (leaving them for several days, only got them to around 4.10-4.15v (is this normal or is this fast self discharge?). A discharge capacity test (using EBD-USB+ load meter and an 18650 battery holder) showed they had around 900-1200mAh.
My experience (I’m new to 18650s, less than 6 months ago since I first recognize what is an 18650 battery) appears to exactly mirror the OP’s experience 3 years ago.
All the 6 Sanyo UR18650As had this overheating problem (but the lowest capacity one [around 900+mAh] is most likely to heat up first, the 2 higher capacity ones [around 1200+mAh] seems to be slightly less likely to heat up)