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BQ29209: BQ29209 OUT pin high when balancing?

Part Number: BQ29209
Other Parts Discussed in Thread: BQ24650, BQ29200


I'm considering using the BQ29209 in my 2 cell lifepo4 charger circuit. However, I would like the charging to stop while the cell is being balanced to avoid overloading (in case of constant current) the balancing resistors. I could do this by switching the same MOSFETS that are used by the under/overvoltage protection IC with the help of the BQ29209 OUT pin.

The datasheet states that the OUT pin will go HIGH when overvoltage is detected at one of the cells. Will the OUT pin also go HIGH when there is imbalance (>30mV) between the cells?

  • Hello,

    A cell imbalance will not trigger the OUT pin HIGH. Cell balancing will be stopped if a fault (OUT goes HIGH) is detected.

    It is possible to alternate between charging and cell balancing during the constant current cycle using the CB_EN pin. Once the charger enters the constant voltage cycle cell balancing can always be enabled. 

    Best regards,

    Thomas Rainey

  • Thanks, thats unfortunate. I was hoping to be able to disable charging when unbalance is detected. I'm using the BQ24650 to charge the lifepo4 cells, but even though the cells are brand new and of the same make/age/type with the same start voltage, at the 'end' of the charge cycle one cell is already on 3.65v while the other lags behind on 3.5. This causes the total voltage to remain too low and the charging cycle not to end properly.

    So that means I misinterpret 8.3.2 of the datasheet?

    When one or both of the cell voltages rises above VPROTECT, the internal comparator is tripped, and the delay
    begins to count to td. If the input remains above VPROTECT for the duration of td, the bq2920x output changes from
    a low to a high state, by means of an internal pull-up network, to a regulated voltage of no more than 9.5 V when
    IOH = 0 mA.

    The output pin does not go high in some usable way?

    do you have any suggestions for other IC's I can use to solve this problem? Effectively I need a way to pull down TERM_EN on the BQ24650 as soon as cell imbalance is detected.

  • Hello,

    The OUT pin is just for over voltage fault detection. 

    Also, charge current is not what determines the balancing current. The balancing current is determined by the cell voltage. This should allow for cell balancing while charging. 

    BQ29200 and BQ29209 are the only IC's that support 2s and have cell balancing.

    Best regards,

    Thomas Rainey