For clarification: crude means "it does not have any protection circuit attached".
Short history long:
I extracted 6 18650 (Sanyo) batteries from an Acer battery pack. They had more than 3V each and accepted charge and load.
Unfortunately a pair of them stood (unused) next to the hot air output of a laptop.
When rescued from the heat they were, obviously, hot, maybe more than if they were just a piece of something else – not sure.
Hours after that they had almost no more charge (0.8V and falling) and, strangely, extremely high internal resistance (a load of 100Kohm was sufficient to turn 0.8V into 0V).
When trying to resurrect it, there was no current flowing through it, no matter how high the voltage applied (I tested only until 15V).
A normal behavior would be a low resistance after discharge. My conclusion is that something disrupted inside with heating. Does anybody know if these 18650 have such "thermal fuse"?
Best Answer
They don't have that sort of passive protection.