No, 140*F. It it’s too hot to touch, it’s too hot and doing damage. Different Lithiums have somewhat different heat tolerance but all of them deteriorate with excessive heat.
FWIW a large battery will have more problem shedding heat than a small one: surface area to volume ration issue.
Stuffed in a light it may have even more issues. Worked hard in a light with a high performance LED (more heat) + poor thermal sinking will just compound the problem.
Good batteries have a thermal vent. Blow that > dead cell.
The battery should have handled the 3A just fine. I sure sounds like the safety thermal vent blew, (that pop you heard), from overheating.
A cheap light won’t have a good thermal path for shedding light a zoomie, one of those cheap Chinese lights that I modified with a 3A driver. That’s pretty much the definition of a light NOT designed to be thermally efficient.
To do that you need excellent, conductive contact with the LED+a large thermal mass to act as a heat sink, and a good way to radiate the heat, like fins or water. Think of the mega-coolers put on high performance CPU’s now. Those suckers won’t last long without it, and most of them have thermal throttling to prevent burn up.
Often, very high performance lights also have thermal throttling. You may get very high output for a 30 seconds, a minute, couple minutes, then it backs off to keep from cooking itself. I’m sure your’s does not. Reducing the draw to 2A will lower the heat production, but it won’t do anything to improve the thermal efficiency. Run it for awhile. When it starts to get nice and warm immediately shut it down, pull the battery, and check the temps.
I strongly suspect the culprit was your light, not the battery. I’ve been using those same cells for years, almost every weekend, and they are still fine. They ARE in a DIVE light and COLD water so heat is not an issue. Their instructions specifically say to not use them above water.
Well, in my defense at least I tried to improve thermal conductivity. I dremel’d a 20 cents of € till it fitted snugly in the pill to make it a a somewhat “solid” pill.
I would have thought the thermal conductivity from the LED to the pill is decent, since the whole enclosure gets pretty hot pretty fast. I don’t know.
I also run a few tests, measuring the lux output as it heated up, then used the Cree Product Characterization Tool to estimate the temperature, but I can’t find the results I got.
Sort of irrelevant I guess, since the IR thermometer is probably more accurate.
I’ll run the test you suggest there in a while and will post my temps
Let me throw in some wise words among all of this theorizing:
dazz said “I reckon the battery still had more than half of it’s expected cicles in it, measured about 4200mAh at 500mA discharge tests, down from some 5000mAh”. 42 out of 50+ is an already run-down battery in my honest opinion. I have much older laptop batteries in better condition than that. Ageing also affects internal resistance and even self discharge, this last attribute being the most meaningful with respect to remaining service life estimation because of accumulated internal micro shorts. Do I think or believe the battery was the culprit? Yes but not exactly. dazz probably needs to give his cells a better life with more care (charging li-ion at maximum voltage and letting it rest there is bad), or buy new and send the older ones for recycling more often.
You guys are probably right and I’ve unknowingly been giving my batteries a hard time. In fact I just checked and this dead cell was only 2 years old. I always store my li-ion batteries at some 3.7V so it’s probably not that. My 18650’s have withstood the test of time much better. I had no idea that these cells were so temperature sensitive.
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