Problem-statement:
For
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MS15, some modules had banks with some amount of voltage difference. A few of them were in the range of ~0.5V while others were in the range of 0.2V-0.3V. After taking apart one of the modules with a 0.5V imbalance, we found that there was a single cell in the outlier bank that was dead. We should:
Do a root cause analysis of the dead cell (what caused it to die?)
Determine if the other modules with a smaller imbalance have the same issue, or if they have a different problem (maybe the ones with 0.2V-0.3V simply have a cell that has a lower capacity in the bank instead of a dead one).
Possible reasons:
Manufacturing issue - if a 4.2V and 2.5V battery were placed in-seriesparallel, could it kill the battery?
Testing issue - were defective cells placed in the modules (i.e., did our testing fail to catch them)?
Spot-welding - may have damaged some cells
Resonance - vibrations kills the battery pack
Next-Steps:
Research other-methods for how could a battery could drain that-quickly
Resonance - look into designing the modules to reduce vibrations (padding etc.)
Verify Micah’s test program (ASC also mentioned spot-welding isn’t a great way to connect cells seems like a probable reason)
Over-discharge/undervoltage of cell - Formula Electric over-discharged some cells as a test and said they saw the same sort of weird behavior exhibited by over-discharged cells. We over-discharged 2 modules in the W24 term when testing without a BMS. It seems plausible that the dead cell was in an out of balance parallel grouping, and that particular cell had an outlying DCIR and was itself much lower than the other cells in its parallel grouping, causing it to become over discharged.