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CCS/BQ40Z60: How does cell balancing achieve?

Emma Wang_DELL EMC
Intellectual 695 points
Part Number: BQ40Z60

Tool/software: Code Composer Studio

Hi Batt:

    1, If there is 3 series battery pack and one cell is overcharged, how does cell balancing function? Is there still charge current when doing cell balancing for one cell? And is this overcharged cell charged with smaller charge current? Why is additional resister for cell balancing 100ohm to 1kohm?

2,What is condition when CC charge changes to CV charge stage?

Thanks very much!

Emma

  • 0
    TI__Mastermind 42535 points

    Hi Emma,

    1. When overcharged, if the cell voltage exceeds threshold, the protection will turn on and chg will stop even for one cell. However, cell balancing does bypass current and reduces the chg of the cell with a higher voltage. We need external resistor in series with the cell balancing FET, that's why they are needed.

    2. CC to CV mode happens when the current starts to taper off, it means the cell is still capable of charging but will do at a slower rate of chg.

  • 0 in reply to Batt
    Intellectual 695 points

    Hi Batt:

         I still don't understand them.

    1, When cell balancing, charge current is same with original one and doesn't reduce. Current will go through cell1, cell2 and resisters FET not go through cell3(that needs to do cell balancing). Is above understanding right? And I still don't understand what adding external resisters does for?

    2, Yes, I know current will start reduce until taper current to terminate. However, what condition makes charge current decreased? a voltage threshold? like 4V, 4.1V or 4.2V? In other words, how does charger know to reduce charge current and change to CV stage?

    Thanks very much!

    Emma W

  • 0 in reply to Emma Wang_DELL EMC
    TI__Mastermind 42535 points

    Hi Emma,

    1. The cell balancing is done by a circuit that's internal to the bq40z60. It has an AFE that bypasses the other cells directly to GND and that's how the cell current is diverted. The external resistors are needed for biasing the transistor.

    2. The voltage catching up to the charger reduces the delta between the battery and the charger. This reduces the ability of the charger to push current at a high rate.