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BQ76930: 9 cell pack - Last Cell in Pack over charges.

Part Number: BQ76930

I have designed a 9 cell Battery Pack using the bq76930.  It pretty much works except I am running across one problem.  Cell 9 will charge faster (3 different battery packs).   I can see the bleed resistor turn on (100 ohms) when it tries to balance and the cell 9 doesn't seem to care it just keeps charging.  I tried 50 ohms too.  This is the last cell in the pack and I know there are a few items that must be taken care of to make it work.  I am using the EM as a design reference.   I have removed the cap between VC9 and VC8 and replaced it with a 0 ohm resistor, I have removed R23 and R33, I have shorted C9 and C8 together and have cell 9 connected between C8 and C10.   Most of the cells get to 3.35V to 3.36 and stop, but cell 9 will go up to 3.6 where the Micro will stop the charging.  Am I just trying to over charge the cells?  LiFePo3  A123 2665.  Nominal is 3.3V Charging Voltage is 3.6.  

  • Hi John,

    We haven't seen this specific issue before. Can you try moving the top cell to another position to see if the problem follows the cell? 

    Just thinking through this, if the top input were damaged, it might report a low voltage causing the MCU to balance down the other cells causing the top to overcharge. However, the MCU is controlling the balancing and is seeing the higher voltage reading on this cell, right? It seems strange that cell 9 would be the only cell to overcharge since the charge current should go through all cells. 

    Regards,

    Matt