Fire, fire, fire...

18650 MJ1 taken out from charger was 37 degrees C.

30 minutes later, cell laying on the table reached over 60.

Over 70 went to the concrete drive yard. We’ll see how it goes.

Old cell with lots of cycles? Dropped/dented? Extended deep discharge time? Washing machine? Malevolent apparitions?

Charger is fine.
Cell was fine.
More or less 50 cycles.
Not abused.
Used only in ZL sc600w mk3

Best argument against onboard charging. Might not have caught it.

Wow, not good. Glad you caught it when you did, let us (me) know how it ends up.
Interesting you were able to rule out malevolent apparitions.

Good point.

Charging was fine. It gets hot half an hour later.

If you’re sure the charger is fine, I’d be recycling that cell after draining it slowly. The daredevil in me might give it some testing since it didn’t reach or surpass the magic 80C suggested temp limit…but. Better to play it safe and get rid of it.

Could it have been a fake MJ1? Very strange that a battery would start heating up like that on its own - scary!

Hard to say.
Bought from reputable seller in my country.
3 others bought the same day still working, but I keep an eye on them.

That’s great that you checked your battery 30 mins after charging, I usually just put them aside after that. I will keep a better eye on them now!

Couldn’t agree more

Charging in a light or outside the light is not going to change this issue as I see it. I am likely to touch a new battery or a new light the first time while charging. After the first time I may randomly touch things while charging with the back of a finger but not on a regular basis. Certainly you are touching a battery or a light when done charging. So you’re going to feel that heat (37c) whether it’s in a light or loose at that point. It would probably be warmer than that if it was in a light in this case. Although the body of the light would have been cooler to the touch. I don’t ever go back and check things 10 minutes after charging. I don’t ever leave them unattended while charging. When charging “in” lights or loose I always use one of two different USB testers to watch the amp load and I’ll typically check it multiple times while charging. With loose batteries as I normally use a folomov A1 magnetic charger.
The only advantage to charging outside of a light that I see is the ability to inspect it more closely for those that are going to bother. At least one advantage of keeping it in a light is less handling. I suspect at this point any evidence of any abnormal heat during charging would be grounds to take things immediately outside. @sp5it, at some point it would be good to take the wrapper off and inspect the whole thing and see if there’s any evidence of any problems from the outside.

Did the cell still have full charge voltage, or was it depleted after the delayed heating event?

Good that you caught it. I wonder how high the risk of a fire is, if you tried charging it afterwards?

Is there a market for a cylindrical cell holder with temp sensors to monitor temperature post-charge? Or even a during-charge temp sensor you can stick to cells, for chargers without this function?

Thinking about it, I’m surprised more chargers don’t advertise an over-temperature (60C?) alarm.

Never felt comfortable with liions at home. What makes it bearable is the low probability they biite.

I didn’t measure it after self heating.

It is also a strong argument against gifting li-ion flashlights to friends/relatives/colleagues.

On reddit in particular I have noticed an extremely disturbing trend, whereby posters who themselves know little about li-ion safety are gifting cheap junk (onboard charging flashlights with “free” 18650 cell of dubious origins) to friends who know even less.

I keep everything with LiIons in a roll cab. Lift top has an electrical passthrough for power and charging gets done inside of it with the lid closed. If something blows up it will be contained.

I have hundreds of these batteries and the peace of mind is worth it. It only takes one having a malfunction to burn your house down.

This is actually an interesting reason for using tabless cell designs over tabbed ones.

As it turns out, doing precision tabbing is time intensive, expensive, and even with modern tools, one of 2 cells out of 10000s can still have imperfect welds, resulting in subpar performance and the like.

Tabless cells improve on this by being far more efficient, cheaper on the manufacturing side, and increase safety further because welding tabs is not necessary. It also improves thermal handling, etc.