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bq2973: Using protection in a series of cells

Richard Jenkins81
Prodigy 40 points
Part Number: BQ2973
Other Parts Discussed in Thread: BQ76925, BQ77PL900, BQ76930, BQ77905

I have 6 Li-ion cells (unprotected 18650) in series, each using a bq29732 wired as per the ’10.2 Typical Application’ diagram in the datasheet.

On inserting the cells for the first time, each protection circuit has to be individually enabled (as per ‘Caution in section 9.4.1).

In use if the series of cells are operated until one of the cell protection chips trigger (OVP,UVP,OCC,OCD or SCP), automatic recovery is not possible via a charger as there is now in effect an open circuit in the series of cells. The only recovery is as per the ‘Caution in section 9.4.1’ across the individual tripped cell.

Is there a way to use the bq2973x in series connected cells where self-recovery is required ?  

  • 0
    TI__Mastermind 25955 points
    Hi Richard,

    I will look into this issue and respond before Tuesday. Thank you!

    Sincerely,
    Bryan Kahler
  • 0 in reply to Bryan Kahler
    TI__Guru 54715 points
    Hi Richard,
    If each cell has its own protection FETs the part will be overstressed when one protects. For example if you have 6 cells approaching a 2.8V UV the battery will be at approximately 16.8V. If the bottom cell goes to UV and opens the FET with a load present, V- of that part will pull to (BAT + 14) violating the abs max and possibly damaging the part. If you clamp the pin with a diode the clamp will cause leakage current through the limiting resistor and the cells will continue to discharge at some lower rate.
    You would typically need to level shift and combine the outputs of the parts to a common protection switch. Using a device like the bq77PL900 or bq77905 allows individual cell sensing and common control. Neither of those parts provide OCC however. A monitor device such as bq76930 or bq76925 and a MCU would allow you to implement an OCC control.
  • 0 in reply to Bryan Kahler
    Prodigy 40 points
    Hi, do you have any update on this query ?
    Regards
    Richard Jenkins
  • 0 in reply to WM5295
    Prodigy 40 points
    Ok, thanks for the update.
    Regards
    Richard Jenkins