Over the last few years, I've had two Lithium battery packs fail. One was an Ego 5Ah pack with two banks of 14 x 18650. The other was a Craftsman with one bank of 5 x 18650.
In both cases, every cell in the bank went to zero volts at the same time. The 14 batteries in the other side (non-failed bank) of the Ego battery were still good.
In the case of the Ego battery, the failure appeared to be in the circuit board, which had visible burn damage. For the craftsman, there was no obvious failure point.
What kind of failure would hit all cells at once? Or, would there be circuitry designed to disable all of them if even one goes bad?