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BQ76930: Balancing Accuracy and timing

Jerry Chu
Genius 4915 points
Part Number: BQ76930

Hi Expert,

When should I do the balancing for the cells? charging, discharging or anytime?

When I did the balance cycle, what's the tolerance between each cell? Is the below parameter? Thanks!

  • 0
    TI__Prodigy 385 points

    Jerry,

    Thanks for your question.

    Balancing is typically implemented during charge.  I would recommend you review this and this for basic information on balancing.

    The table you highlighted is simply the accuracy of the ADC. The bq76930 balancing is designed to allow you to develop the algorithm that works best for your system and requirements. What level of tolerance you want between cells depends on your system and algorithm. Please take a look at the linked material and please get back to us if you have further questions that we can help you with.

    Todd

  • 0
    TI__Expert 6085 points
    The table is reflective of the ADC measurement across its entire dynamic range (2V-5V) across Si process range and temperature.

    please have the following in mind,

    (1) large input filter reduces BALANCE current and external circuits are needed for larger currents.
    (2) During BALANCE, internal FET pulls inputs together (i.e the the cell to be "BALANCED" goes to ~ 0V
    (3) Balancing operates on a duty cycle commanded by a Internal Schedule ( refer to SLUA775)
    (4) BALANCING FET is released to measure voltage.
    (5) Balancing every other cell raises voltage on Adjacent cell by 2X ( abs MAX 9V).
    (6) cells with longer time constants will have less time to settle, hence more Error!
    (7) Cells measured long after the BALANCE stop, should have lower ERROR.

    Hope this helps.
  • 0 in reply to Vish Nadarajah
    Intellectual 285 points
    Hey

    I have few questions to ask from above mentioned points,

    (2) During BALANCE, internal FET pulls inputs together (i.e the the cell to be "BALANCED" goes to ~ 0V

    This mean that the voltage measured my AFE will be zero. Am I right ?

    (6) cells with longer time constants will have less time to settle, hence more Error!

    what do you mean by this ?

    Thanks
    Akilan G
  • 0 in reply to Akilan G
    TI__Expert 4640 points

    Hi Akilan,

    To answer your question you will first need to understand how the device works. If you look at this you will see that cell balancing ON has balancing occuring when ADC measurements are not taking place. 

    The second question which is referring to the comment by Vish is referring to the behavior of cells. Li-ion have a relaxation period when you charge or discharge them so you just have to be mindful when you read values of the cells to account for a certain period of relaxation.

    Thanks

    Miguel