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BQ40Z80: BQ40Z80 Cell Balance Time per mAh/Cell

Battery Manager
Prodigy 25 points
Part Number: BQ40Z80
Other Parts Discussed in Thread: BQSTUDIO,

Hi,

I have many question about to calculate Balance Time per mAh and value of Bal Time/mAh  Cell-1 config in BQStudio.

I will make External Cell Balancing in my system.In Technical Reference Manual document of BQ40Z80, recommended that formula to Balance Time per mAh Cell 1 and Cell 2-7.I will share my parameters and calculation to calculate Bal Time/mAh Cell1, Bal Time/mAh Cell 2-7.

VCELL = 3200mV

RVCx = 200 ohm

Rcb = Fet on Rds = 25 mOhm

DUTY = cell balancing duty cycle %75 typ. I used the typical value.Should I use that value?

Rext = 30 ohm

According to formulas,

Balance Time per  mAh Cell 1 =    39 s/mAh

Balance Time per mAh Cell 2-7 =  32 s/mAh

I will fill in the values which I calculated above to BQStudio Advanced Charge Algorithm Cell Balancing Config section?

Can you check the my calculations?

Thank you.

Sincerely.

  • 0
    TI__Genius 11750 points

    Hello,

    I also got 39 s/mah for cell 1, but got about 42s/mah for 2-7. I believe you should always get a higher value for cells 2-7 since it has 2 series resistors rather than one. As for duty cycle, 75% is fine unless you are trying to speed/slow things for your application. Filling in the values in cell balancing config should be all that is required for setup to work. 

    Best of luck,

    Alex M

  • 0 in reply to Alexander Mazany
    Prodigy 25 points

    Hi Alexander,

    Thanks for reply.You are right.I made a small mistake to calculate for 2-7.It will be 42 s/mAh.If I write those values to cell balance config section,Will it be right?If not,Can you suggest to cell balancing config values to cell 1 and cell 2-7 for my calculation?Because, The default value is 367 for Cell Balancing Config Cell 1 in technical reference manual document.

  • +1 in reply to Battery Manager
    Prodigy 25 points

    Hi Alexander,

    I have question,too on other topics.

    1- I used LifePO4 cells in our sytem.The LifePO4 cells have differences voltages between each other in our system.Should I disable to cell balancing during the learning cycle? or Should I enable to cell balancing during the learning cycle?I used the 6 series in the system.

  • 0 in reply to Battery Manager
    TI__Genius 11750 points

    Hello,

    The default values are higher because they are using a much larger series resistance for the default option; you should be okay just putting the calculated values into those sections. 

    for your second question, cell balancing uses measured parameters like Qmax and RSOC. Charge balancing is actually disabled until the gauge takes the Qmax data, which will happen during/after the learning cycle. Basically, cell balancing is disabled by default when running the learning cycle. One thing that can make things easier, is to identify the CHEMID by using only a single cell, and then do the learning cycle on the whole pack. 

    thanks,

    Alex Mazany