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Not a duplicate, just to separated a good question from Does a crude Li-ion 18650 battery have internal thermal (mechanic?) protection?

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.

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The protection measures against thermal runaway in some Li-ion batteries include

  • Thermal shutdown separator, pore closure

See Failure Modes & Effects Criticality Analysis of Lithium-Ion Battery Electric and Plug-in Hybrid Vehicles

The cells can also contain

  • CID or pressure valve, will disable the cell permanently if the pressure is to high in the cell

See The Anatomy of a Protected LiIon Battery

Those 18650 cells also contain a protection circuit at the negative end.

Protection circuits are likely to include protection for

  • Excessive temperatures will cause all cells to fail eventually. Most protection circuits therefore incorporate a thermal fuse which will permanently shut down the battery if its temperature exceeds a predetermined limit.

See Lithium Battery Failures

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