That is a cell that is offgassing because of damage or slow failure. My guess is the internal resistance on that particular cell is much higher now than the others, and it can't handle the load anymore
This is the beginning of the runaway train if you continue to use that battery, as that cell will start to drag the others down with it, and then the pack starts to puff all over and overheat
You need to keep that pack outside of your house until can dispose of it if you don't plan to use it any further
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