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BQ20Z90 Cell Balancing

zepman
Prodigy 200 points
Other Parts Discussed in Thread: BQ20Z90

I'm using the BQ20Z90 & have a few packs where the cell balancing flag is not coming on when it should while charging, the vast majority are working properly. I'm using the default min cell deviation of 1750. Has anyone seen this before or have any thoughts?

  • 0
    TI__Expert 6470 points
    The condition to start cell balancing is simply: going into charge mode. A time is calculated for each cell to be bypassed based on SOC deviation. There is no minimum value, but cell balancing can be turned off by setting DF.MinCellDeviation to 0. Balancing times are recalculated at the beginning of each charge using the previous OCV reading from the relaxed state to calculate it. If relaxation period was not long enough to take OCV reading, old balancing time values will continue to be used.

    Qmax update is not needed to start balancing, it will just make it more accurate, because Qmax values affect balancing times for each cell. However, max differences are usually smaller than initial SOC differences, and it takes more than one cycle to remove the SOC differences. So even if balancing time has some error, it will keep correcting itself at the beginning of each cycle so error due to Qmax is not critical.

    When charging starts, the algorithm computes the state of charge for each cell block. Then it calculates the deficiency in charge for each cell block compared to the block with the highest SOC. Then a bypass time is calculated for each block that needs it using the formula t = dq * R / Volts * duty cycle. The stored default value of 1750 s/mA just simplifies the math internally and is theoretically more correct in the case where nominal voltage might not be 3.6V. However, we have recently found that there is a wide variation in the actual resistance of the balancing FETs and some customers are reporting better results using a value around 1350 s/mA.