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BQ76952: cell balancing

Charles Zhang1
Expert 3080 points
Part Number: BQ76952

Hi Team,

My customer wants to ensure that the minimum cell voltage is above a certain level during cell balancing, for example, if minimum cell voltage drops below 3.6V, BQ76952 would stop balancing. 

According to BQ76952 TRM, BQ76952 would ceases balancing if all cell voltages are within Stop Delta of the minimum cell voltage.

It seems that it would not take the minimum cell voltage as a condition of stopping balancing. To stop balancing, BQ76962 would only take the voltage delta as the condition.

The questions are-

How to prevent the minimum cell voltage dropping too low during balancing?

Is it possible to set two different minimum cell voltage values for entering/stopping balancing? (e.g., 3.8V for entering, 3.6V for stopping)  

Regards,

Charles 

  • 0
    TI__Mastermind 24956 points

    Hello Charles,

    Let me start by saying the device supports autonomous and host-controlled balancing. The customer question looks more inclined to the autonomous feature. 

    Charles Zhang1 said:
    How to prevent the minimum cell voltage dropping too low during balancing?

    The device can allow autonomous cell balancing either during charging or RELAX when allowed.

    Let's say it is balancing when in RELAX mode. There are three cell balancing registers to configure for RELAX. 

    1- Cell Balance Min Cell V 

    While not charging or discharging, automatic cell balancing is disabled if the minimum cell voltage is less than this threshold. 

    2- Cell Balance Min Delta

    While not charging or discharging, the delta between the maximum and minimum cell voltages must be greater than this value for automatic cell balancing to begin.

    3- Cell Balance Stop Delta

    While balancing in relax, this value is used as the target voltage delta to balance to. Once the delta between the minimum and maximum cell voltages falls below this threshold, balancing stops. This provides some hysteresis around the delta threshold. Note that to prevent balancing from draining the pack, the stop threshold is calculated when balancing begins and will not decrease if the minimum cell voltage drops. This means that in some conditions, balancing will stop with a greater delta than this.

    Charles Zhang1 said:
    Is it possible to set two different minimum cell voltage values for entering/stopping balancing? (e.g., 3.8V for entering, 3.6V for stopping)  

    If the Cell Balancing command is sent (meaning the device was informed to cell balance the cells), higher cell voltage cells will balance down to the lowest balancing cell voltage. To answer the question, there's no entering voltage. The device does this automatically.

    As for the stopping voltage, check the Cell Balance Min Cell V and Cell Balance Stop Delta explained above.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    TI__Expert 3080 points

    Hi Jose,

    Thanks for your detailed reply, I could understand the stop threshold you explained above.

    I still want to confirm again, does it mean that BQ76952 will also stop balancing if the lowest cell voltage drops below Cell Balance Min Cell V regardless of stop delta? 

    Regards,

    Charles

     

  • +1 in reply to Charles Zhang1
    TI__Mastermind 24956 points

    Hello Charles,

    For instance, if cell-3 goes below the Cell Balance Min Cell V and Cell Balance Stop Delta is met, then the device will stop balancing cell-3 but will continue the balancing in another cell that needs to be balanced down to the lowest cell. 

    To stop or disable completely cell balancing, a 0x00 should be written to the CELL BALANCING register (0x0083)

    Note: When the cell balancing feature is used in autonomous mode, it will happen whenever a cell meets the balancing requirements. Unless, it is manually forced to stop. Then, to enable it again, the Cell Balancing command must be sent.

    For more details on cell balancing and external balancing circuitry reference, please see the Cell Balancing With BQ769x2 Battery Monitors app note.

    Regards,
    Jose Couso