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BQ76940: Cell is "discharging" itself in adjacent cell

Nurgak
Prodigy 140 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQSTUDIO

We have build many BMS's based on the BQ76940 for a 12S pack. Sometimes we observe a behaviour where one cell "discharges" itself into an adjacent cell. We use a "battery emulator", a simple resistor array, to calibrate the BMS, so actually all cells are perfectly balanced.

When measuring before balancing the cells are all at 4V, when measuring at the capacitor between the "faulty" cells we observe the same voltage as reported by bqStudio, however as soon as the multimeter leads touch the capacitor the cell voltage decreases and continues to decrease under the 4V threshold... Basically it seems like it's a faulty cell balancing issue. I've replaced the capacitor between the cells, checked all the components (MOSFET, resistors) all seems fine.

This happens slowly, starting at around 4V and keeps going up, eventually reaching a state where one cell is under-voltage and the other over-voltage and goes into permanent fail mode. See cells 9 and 10:

Has this been observed before? What else can I do short of replacing the balancer chip?

  • 0
    TI__Genius 10830 points
    Hi Karl ,
    One of our BMS experts has been assigned to your issue and will answer shortly.

    Regards,
    David
  • 0 in reply to David Hien
    TI__Guru 54715 points
    Hi Karl,
    This observation is typical of a broken wire or solder joint to the cells or battery emulator. Attaching the meter to the monitor terminals discharges the capacitor for the input. The sum of the voltage reported for the 2 cells remains 2 cell voltages until the ADC saturates for one cell. Moving the meter to the other cell input changes which cell is low.
    Missing solder on the IC pin causes a similar effect but the meter shows correct voltages on the PCB. If the IC pin is probed the movement of the reported cell voltages may be much faster since the capacitors are not attached, or the meter probe pressure may cause contact with the PCB and allow the part (and meter) to report normal voltage.
  • 0 in reply to WM5295
    Prodigy 140 points
    Hi David,

    Thank you for the quick and in-depth reply.

    I made absolutely sure all the connections were properly soldered and there were no solder joints. I also replaced the caps just in case, however I keep getting the same behaviour. I just tested another BMS with the same battery emulator and it was working perfectly fine. I am now suspecting a faulty chip.
  • 0 in reply to WM5295
    Prodigy 140 points
    After some more poking around I found the culprit broken solder joint, it was at a connector level. The IC is working fine now.