Hi Folks,
Our customer has an issue affecting some of their bq20z95 based battery packs in the field.
There is a leakage current path that can develop in the cell interconnects within the battery pack. In some cases this leakage exceeds the bq20z95’s ability to balance during charge so a State Of Charge imbalance develops over time. The bq20z95 gas gauge appears to effectively track the loss of capacity this causes, so no problem – just reduced run time.
However, we are now finding that, at high cycle counts the IT algorithm starts to lose track of the deteriorating condition of the battery, leading to over-reporting of remaining run time. Failure analysis has shown that the lowest voltage cell has a much lower state of health than the other 3 cells in the 4S3P configuration. This is expected as the lowest voltage cell is continually cycled to a much lower voltage than the other 3 cells, which may accelerate its aging. CUV is set at 2500mV.
We would like to make an adjustment to the IT settings, if possible, to improve the gas gauge reporting under these conditions. However, we don’t know the details of the IT algorithm and what might be causing it to lose accuracy. I have attached a .gg file exported from one of our field returns. The Resistance Tables contain some strange looking values towards the end of the discharge characteristic. It would be great if we could get some feedback from one of your gas gauge experts on what we could change to minimise the over-estimation of remaining capacity under these conditions.
Thanks.
