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BQ40z50 Balance Time

David Thompson2
Intellectual 685 points
Other Parts Discussed in Thread: BQ40Z50

I'm currently trawling through the settings to configure my BQ40z50 for use with my cells. I'm intend to use the internal cell balancing and have got to the "Bal Time /mAh Cell 1" value. I'm not convinced my value is correct. For a cell with nominal voltage of 3.65V, I get 16274 which seems much larger than the default value of 367. Is the formula correct (is my calculation correct)? 

Vcell = 3.65; RVCx = 100; DUTY = 66; Rcb = 150

I suspect the formula isn't quite right as the V/R portion gives a result in Amps which is the used to divide a value in mAs.

Additionally, the datasheet says Rcb is max 200 but the Tech Ref says to use 150.

Thanks

  • 0
    Intellectual 685 points
    Comparing to another device (i.e. BQ20z95) it would appear the equation is correct but is poorly written.
    The BQ40z50 Tech Ref gives the equation as: 3600 mAs/(VCELL/RVCx + Rcb) × DUTY/1000
    This suggests that The result of the division 3600 mAs/(VCELL/RVCx + Rcb) is multiplied by the duty cycle. It should (I think) be the term VCELL that's multiplied by DUTY. That leaves the division of 1000 which applies to the 3600 value (in the BQ20z95 the constant is 3.6). That also solve the conflict of mA and A.
  • 0
    TI__Guru 73830 points

    David,

    There is a parenthesis error on the first equation that needs to be fixed. The structure of the equation has also caused questions. Here is a better format for the equations.

    Tom

  • 0 in reply to ThomasCosby
    TI__Guru 73830 points
    I did not answer the question about the resistance. While 200 ohms is the worst case statistical value, we recommend using 150 ohms as it will be more representative of the actual resistance.