BQ76940: Random Cell Connect

Part Number: BQ76940
Other Parts Discussed in Thread: BQ76972

Tool/software:

Hi,

We're using the BQ76940 IC as the AFE for our BMS. During operation, abnormal cell voltage readings were observed (0V and 6.0V). This was seen on adjacent cells, and this issue appears usually when cell connectors are attached. On replacing the IC the issue does not appear until we reattach the cell connectors again. This was concluded to be a problem due to Hot Plug because of a Random Cell connect. The datasheet mentions that the IC is random cell connect tolerant, but this issue appears randomly and unpredictably on attaching the cell connector. 

The balancing algorithm does not allow adjacent cells to be balanced as well (external FETs used for balancing).

What may be the reason for this and how can we mitigate it? (Apart from connecting VSS first)
We are also using the BQ76972 IC for our upcoming design and we do not want to replicate such an issue again, are there any suggestions to avoid this from happening?
It would be helpful if the Hot Plug random cell connect limitations for this series of ICs could be explained as well.

Thanks!

Akshat Madaan

  • Hello Akshat,

    I may be a little confused by your questions here so please correct me if I am misunderstanding them. 

    1. The cell balancing algorithm does not allow adjacent cells to be balance. What is the reason and how can you mitigate it? Are there any suggestions to avoid this from happening?

    We do not recommend balancing adjacent cells simultaneously as the differential input voltage of the next cell (the one after the adjacent balancing cells) will double. This could in turn cause the cell pins to exceed their absolute maximum conditions. 

    For more details, I would refer to Section 9.5 of the FAQ. This will also demonstrate a method to appropriately determine when it is okay to balance adjacent cells if it is required for your application.

    This should not be an issue with BQ76972 since the device will support adjacent cell balancing through host-controlled balancing.

    2. Hot plug random cell limitations explanation

    The device is hot plug tolerant meaning it is designed to have pins that are not extremely sensitive to transients which may occur from cell connection. However, we do have recommendations/preferences for cell connection. One suggestion would be connecting VSS first and then going up from there. Another suggestion would be to connect VSS then follow it up with connecting the cell that provides the group power. This will be safer for the device during hot plug.

    For more details random cell connection and its limitations, I would suggest looking at Section 10 of the Design Consideration App Note.

    Are you using N-channel or P-channel balanced FETs? N-channel FETs are preferred because for random cell connection as they will not pull unconnected inputs up. It seems you have some unused inputs so this could be occurring here.

    Perhaps sharing a schematic here would allow me to better help diagnose the issue.

    Regards,

    Rohin Nair