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Problem Statement:

For 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:

  • Determine what caused this issue

  • 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).

Questions to Answer:

  1. What caused the cell to die? Was it because of:

    1. Spotwelding

    2. Connecting a fully charged cell to a fully discharged cell in parallel

    3. any other reasons?

  2. Why is there a 0.2V imbalance between banks in some modules?

    1. Is this also caused by the dead cell, a capacity issue, or something else?

Research:

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