The self discharge test at full state of charge, may not be a good one to detect subtle defects since the li-ion chemistry has the highest self discharge at full state of charge. - Characterize self discharge versus storage time for each cell manufacturer/design to differentiate between normal self discharge and that due to a subtle manufacturing defect. The various soft short test methods indicate that if this test is carried out at full discharge (0% SOC) with all capacity removed (by lowering the current load in a stepwise manner to the same EODV), more than 72 hours are required for cell stabilization. - But a valid pass/fail criteria needs to be worked out with the cell manufacturer (fail may be greater than 10 or 20 mV drop?) If the cells are to be charged up even to a small percentage (ex. 1%), 72 hours may be sufficient to determine gross defects but not subtle ones. - Less than 10 mV voltage decline is not a good method to detect subtle defects. - Self discharge is a competing reaction when a charge is introduced and hence a characterization of the test protocol vs storage time is required to fully correlate voltage decline to any defect. Capacity cycling of failed cells did not show any changes in performance or result in catastrophic failures. Soft short test methods tested here cannot be relied on completely for defect detection because cells with and without voltage decline seemed to have similar defects and characteristics.
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