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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?

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  • \$\begingroup\$ Overcurrent triggering the CID in all cells? \$\endgroup\$
    – winny
    Commented Jun 10, 2022 at 21:40
  • \$\begingroup\$ Are these bare cells or protected (per cell)‽ \$\endgroup\$
    – Passerby
    Commented Jun 10, 2022 at 21:43
  • \$\begingroup\$ In the one case I had it was a bad BMS that gradually drained all the cells. \$\endgroup\$ Commented Jun 10, 2022 at 22:32
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    \$\begingroup\$ If a Li-ion cell is reading 0 V, it means there's an open circuit, probably caused by the BMS. Even a fully discharged Li-ion battery will not go down to 0 V easily. Did you test each cell individually? \$\endgroup\$
    – forest
    Commented Jun 10, 2022 at 22:49
  • \$\begingroup\$ Pretty sure they are protected cells. I tested them individually, and am guessing the protective circuit tripped on all of them. Just because I would guess they would have some minimal (or slightly different) voltage if just slowly drained. So, maybe short circuit in the BMS? \$\endgroup\$ Commented Jun 12, 2022 at 15:46

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A bad BMS often kills the pack. I had a series of scooter battery packs, where the software in the BMS had bugs and could crash. The bleeding transistors of the balancing circuit were turned on and drained all cells. The bug was in the deep sleep part of the software and the problem came up after a longer period without pack usage.

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  • \$\begingroup\$ Do you recall if all batteries were at exactly zero in your case? I would think a slow drain to zero would leave them testing each with slightly different (including negative for li-ion) millivolts of charge. \$\endgroup\$ Commented Jun 12, 2022 at 15:48
  • \$\begingroup\$ @HumanJHawkins After some time they were all zero, intermediate states were as you say. Only thermal fusing inside the cells can turn off all at once. Fortunately I have never seen that. \$\endgroup\$
    – Jens
    Commented Jun 12, 2022 at 16:51

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