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:
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).
Possible reasons:
Manufacturing issue - if a 4.2V and 2.5V battery were placed in-series, 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 (ASC also mentioned spot-welding isn’t a great way to connect cells seems like a probable reason)
Next-Steps:
Research other-methods for how could a battery could drain that-quickly
Vibrations - look into 1. designing the modules to reduce vibrations (padding etc.) and 2. making sure the next-design of the enclosure is as-tight as possible to fit the modules Verify Micah’s test program
Other things to think about:
Vibrations - dead cells were found after manufacturing, not while modules were in the car - vibrations kill the battery pack - severe vibrations can cause the internal components of the battery to become damaged - if we are doing testing in the field this is a cause of concern
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