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BQ76940EVM: What are the conditions required to enable cell balancing on the BQ76940EVM

Pa P
Prodigy 20 points
Part Number: BQ76940EVM
Other Parts Discussed in Thread: BQ76940, BQSTUDIO, BQ78350-R1A, BQ78350-R1, EV2400, BQ78350

We are using the BQ76940EVM as a cell monitor for a 15S Pack we are cycling. (Please find attached a block drawing of our set-up for additional context)

Would it be possible to make use of the cell balancing features with our current set-up?

If so, what would be the conditions to enable the BQ76940 AFE internal and external balancing features?

Would you also please confirm which FETs on the EVM are used for the higher current external balancing?

  • 0
    TI__Guru* 81035 points

    Hi Patrick,

    If you are controlling the BQ76940 device directly using the monitor software, cell balancing is enabled and disabled directly through the device registers. If you are using the BQ78350-R1 gauge to control the BQ76940 using BQStudio, this device has its own automated algorithm for cell balancing - you can read how this works in the BQ78350-R1A TRM.

    The circuit for balancing is described in Section 4 of this application report: https://www.ti.com/lit/an/slua749a/slua749a.pdf

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hi Matt,

    Could you please clarify what are the conditions required for balancing to be enabled via the 78350?

    Based on the TRM, as you mentioned, we expected it be enabled by default and we tried to observe the underlying behavior in the two following ways:
    - querying the AFEConfig (0x59) via SBS while connected to the SMBus port
    - observing the registers with the BQ769x0 eval software while connected to the I2C port with a EV2400.

    In both cases, we were charging an unbalanced pack externally (image of our setup attached below) and did not see any of the cell balancing registers toggle. In both instances, we were able to see changes in cell voltages via both SMBus and I2C while charging.

    Is it a requirement for charging to happen through the BQ board in order for cell balancing to be enabled? E.g. are there certain hardware connections the board is looking for and/or settings that need to be configured?

  • 0 in reply to Pa P
    TI__Genius 11315 points

    Hey Patrick,

    Make sure your Cell Balance Threshold and Cell Balance Min are set appropriately for your application.

    Charging current needs to flow through the BQ sense resistor so that the device knows you are charging. The device can see a changing voltage on the cells while charging externally and may not want to balance. Try depleting the cells and leaving it idle (no charging or discharging) to see if balancing occurs after a while. 

    Thanks,
    Caleb.

  • 0 in reply to Caleb Perez
    Prodigy 20 points

    Hi Caleb, 

    Thanks for the reply. We have tried to adjust our cell voltages in accordance with the default cell balance threshold/min settings.

    Which resistor is the BQ sense resistor?

    Also we were under the impression that charging is supposed to only be enabled by charging. Is it possible for the Gauge to perform balancing while it is neither charging nor discharging?

  • 0 in reply to Pa P
    TI__Genius 11315 points

    Hey Patrick,

    On the EVM, R3 is the sense resistor. The schematic is in the EVM user guide. Yes, the gauge can perform balancing while the device is idle or charging.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 20 points

    Hi Caleb, unfortunately we are still having issues getting the balancing to work on our setup. Would it be possible for us to schedule a call with someone regarding this issue?

  • 0 in reply to Pa P
    TI__Genius 11315 points

    Hey Patrick,

    The BQ78350 needs to measure the charging current to enable cell balancing.

    Can you make sure that all the conditions are met for balancing and then click "start log" in BQStudio. The log file contains a lot of information that will help me debug the situation. Can you also export all of your register settings by clicking "export" on the data memory screen. 

    Thanks,
    Caleb