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BQ77915: bq77915 balancing algorithm

Lenna Yan
Intellectual 1960 points
Part Number: BQ77915

Hi expert,

I have a question about the balance algorithm of BQ77915. Please read the following manual first. 

Why is the balancing enable condition of the passive balance scheme based on the individual average? In this way, the overall PACK capacity is always limited by the lowest node cell, CV2. What is the purpose of such a strategy?

Thank you for your help.

  • 0
    TI__Guru 54715 points
    Hi Lenna,
    The balancing is based on cell voltage, not on average voltage. There are 3 phases or modes of balancing when on:
    1. When cell voltages are rising there is a delta voltage within which the cells will not balance. If they have a large differential they will balance.
    2. Once the battery reaches full voltage the cells above that VFC average will balance, while the charger keeps the voltage supplied the pack will continue to charge.
    3. If a cell is pushed into the OV condition by the charger, charging will be stopped by the OV condition if protection FETs are used in the pack and the OV cell will be balanced.
    Figure 7 shown above is intended to show that cell 5 would be balanced because a cell is above VCBTH and a cell is below VCBTL, the differential voltage condition. Charging would not stop in this condition, all cells would continue to charge. While the picture does not say it is to scale, if charge continued to an average of VFC without significant adjustment of the voltages, it looks like cell 5 may be pushed to OV.