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Original question:

BQ77PL900: Battery Pack Charging/Cell Balancing

BQ77PL900: Cell 1 Balancing and Capacity

Barry Carroll
Prodigy 70 points
Part Number: BQ77PL900
Other Parts Discussed in Thread: BQ76942

BQ77PL900 Cell Balance

A couple of our Batteries (8-Series) have issues with Cell 1 Balancing; this is the first Cell (of the 8-Series) to complete charge, and the first to reach the low end/cut-off voltage during discharge.  We have made a few changes to the layout and FW both with limited improvements, however, this remains to be an issue in the overall capacity of the 8-Series pack (due to Cell 1 reaching the cutt-off prior to the remaining Cells).

We have been scoping the newer BMS (BQ76942) as it has more/better features (tighter tolerances, ability to calibrate individual Cell voltage readings, etc.).  While we believe the ability the ability to calibrate the individual Cell voltage readings will help minimize the Cell 1 Balance issue we experience, we also notice the filter caps on the "VC" inputs are larger on the BQ76942 (0.22uF vs. 0.1uF on the BQ77PL900).  Additionally, the resistor is much smaller value (20 Ohm).  Is this recommended for the BQ77PL900 as well (we currently utilize 1K and 0.1uF with external FET balancing).

Has TI received feedback of a similar issue with the Cell 1 Balance/Discharge from other customers/users of the BQ77PL900?  What is the recommended path/solution to resolve this issue using the BQ77PL900?

Thank you for your inputs!

Best Regards,

Barry Carroll

  • 0
    TI__Expert 4465 points

    Hi Barry, 

    There are definitely a lot of advantages to move to newer generation BQ769x2 over the BQ77PL900 as you stated. In addition, the BQ77PL900 is a really old device, so we normally see customers using either the BQ769x0 (previous gen) or the newer BQ769x2. 

    Regarding your questions: 

    • I checked the history, and we are not aware of other customers having issues with Cell 1 balancing on this family.
    • For the input filers, having too big of an input resistor can results on voltage errors. For the PL900 we recommend having a maximum of 1Kohm, so you are right at the limit. I would recommend to start with matching the EVM input filters of 510 ohm and 0.1uF for the PL900 device.
    • In addition, I would suggest looking into the BQ769x2 for newer projects. 

    Best regards, 

    Arelis G. Guerrero 

  • 0 in reply to Arelis Guerrero
    Prodigy 70 points

    Hi Arelis,

    Thank you for your comments and inputs!

    I assume you are suggesting to change the filter resistor to 510 Ohm for all inputs (not just Cell 1)...correct?

    Best Regards,

    Barry Carroll

  • 0 in reply to Barry Carroll
    TI__Expert 4465 points

    Hi Barry, 

    Correct, change it for all input cells. 

    Regards, 

    Arelis G. Guerrero 

  • 0 in reply to Arelis Guerrero
    Prodigy 70 points

    Thank you Arelis, will do!

    Best Regards,

    Barry Carroll

  • 0 in reply to Barry Carroll
    Prodigy 70 points

    Arelis,

    I have a couple other questions;

    1. I realized while our schematic using the BQ77PL900 indicates the filter as 1K and 0.1uF, the BOM (and the component installed) is 1K and 10000pF (vs. 0.1uF). Can you provide your comment on the capacitor value (not using 0.1uF) with respect to errors in the voltage measurements...will this potentially lead to read errors in addition to the higher value resistor (1K vs. 510 Ohm)?

    2. The BQ77PL900 filter capacitor locations are a bit different than the BQ76942 per below screen shots. Has this changed on the new BMS due to architecture differences...or provides a more accurate reading?

                            BQ77PL900                                                                                            BQ76942

                                   

    Thank you!

    Best Regards,

    Barry Carroll

  • 0 in reply to Barry Carroll
    TI__Expert 4465 points

    Hi Barry, 

    1. Yes, too small of a capacitor would be a filter with higher cutoff frequency, so it wont be as an efficient to filter out unwanted signals which could generate errors on the measurements. I would recommend increasing that capacitance to 0.1uF. 
    2. Yes, this is due to architecture differences between devices.

    Regards, 

    Arelis G. Guerrero  

  • 0 in reply to Arelis Guerrero
    Prodigy 70 points

    Hi Arelis,

    Thank you for the responses and continued support!

    Best Regards,

    Barry Carroll