Last night I thought I should recharge my pack of 18650 cells after a 2-summit weekend. It was late and I fumbled as I plugged in the charger. The two cells popped out and one landed on the tiled kitchen floor. Picking it up I saw that the wrap at the bottom had been split and the protection board (these are protected cells) had come astray. After some more fumbling (I should have put the right glasses on!) there was a puff of smoke and the cell started to get too hot to handle. With yet more fumbling I struggled to unlock and open the back door as quickly as I could - exit one cell pursued by an oath! Half an hour later the cell was still hot, and spent the night under a flower pot in case of fireworks.
So, what happened?
The protection board is there to prevent over current, over charge and over discharge. To measure cell Voltage obviously it needs a connection to the tip as well as the base. This is provided by a metal strip running down the side of the cell to the protection board. A plastic spacer keeps this from shorting to the base. When it all came loose evidently there was a short at this point. Perversely the protection circuit actually made this unsafe!
The cell was still shorted when I recovered it this morning. Removing the short the Voltage came up to 80mV, so I think this cell will be heading for recycling.
I measured the strip with my multi-meter: 0.0 Ohms. Surely this could just as well be tens or even hundreds of Ohms, greatly improving the safety. Oh well!