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BQ77915: Charging only up to 4,0V, No Balancing observable

Marv In
Prodigy 40 points
Part Number: BQ77915

Tool/software:

Hello guys,

I am experiencing some issues with the BQ7791500PWR.

I am charging a 7s-Batterypack. The charger is charging with ~2.7A. As soon as one or more cells reach ~4.05V the CHG-FET is turned off. Shouldn't it be charging until 4.2V?

Sometimes I observe it turning the CHG_FET on and off multiple times. Another weird behavior is that sometimes it doesn't open the FET again at all and I am unable to discharge the pack with the electronic load.
on-off


Balancer_7s3p_v2.pdf
Also I tried charging Cells with a difference in cell-voltage of 0.1V and 0.2V and I am not observing any of the typical balancing plots on my oscilloscope. Is 33R too small for R_in?

I am very thankful for any advice or suggestion.

  • +1
    TI__Expert 4320 points

    Hello,

    For the OV protection to cause the CHG FET to turn off then yes the voltage would need to be 4.2V. Since the cells did not reach that threshold something else is causing it to turn off.

    33Ω input resistance shouldn't be too low. 33Ω is the typical value.

    When comparing your schematic to the schematic in the Stacking Implementations with bq77915 (Rev. A) your capacitor values may need to be changed. It is recommended to use 1μF capacitors for internally balancing.

    Can you review this table and make sure you are not meeting any of these fault trigger conditions?

    Best regards,

    Thomas Rainey

  • 0 in reply to Thomas Rainey
    Prodigy 40 points

    Thank you for your reply. the status is stuck at CHG off, DSG on. even with charger turned off. and I think there is no "CTRC disabled" condition and also Tempereature is not a problem. So the only thing which makes sense to me is the OV condition, since the hysteresis is also 200mV. 
    The Datasheet also says " The device assumes an OV fault after reset, and clears automatically after an OV delay if all cell voltages are below the OV threshold minus hysteresis." and also " The device assumes an OV fault at POR"  so i thought, maybe the devices reset themselves for some reason. Maybe there is some noise on the PRES Pin coming from the charger. but the CHG FET already goes into off state, even if the charger wasn't turned on yet. 

    Could there be another reason, why the devices reset themselves, or why they are not ending the OV-Protection mode?

    Edit: what is even weirder, is that sometimes I see negative Gate voltages at the CHG FET of -10V or even -25V. PACK+ to PACK- is only 3,5V in this case. 

  • +1 in reply to Thomas Rainey
    Prodigy 40 points

    actually this seemed to have solved the Problem: "It is recommended to use 1μF capacitors for internally balancing"

    Many Thanks

  • 0 in reply to Marv In
    TI__Expert 4320 points

    Hi Marv,

    Glad to hear!

    Best regards,

    Thomas Rainey

  • 0 in reply to Thomas Rainey
    Prodigy 40 points

    I just tried again with a second board using bq7791508 and unbalanced batteries. 3 Batteries in the stack have 3,9V, four have 4,0V. started charging, observing this behaviour:
    supply
    is this normal, expectable behaviour?

    the OFF-durations are getting longer each time.

    while the charger is on, the "4,0V" cells are at ~4,15V


    ... i think maybe the 50mA internal cell balancing are not enough, when using a 3A charger.

  • 0 in reply to Marv In
    TI__Expert 4320 points

    Hi Marv,

    I am closing this thread and moving to the new thread you posted: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1387482/bq77915-bq7791508-turning-on-and-off

    Best regards,

    Thomas Rainey