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BQ3050 Cell Balancing

Richard Baum
Prodigy 140 points
Other Parts Discussed in Thread: BQ3050

I have a 4S1P pack with 3400mAh 18650 batteries.  I am trying to test the cell balancing feature.  I noticed that most of the reference designs have a 100 Ohm resistor into the Vx pins of the BQ3050.  If I have a 4V cell, and I read that the Rdson of the bypass FET is 125 Ohms, the 100 Ohm resistor would produce a current around 16mA.  This exceeds the 10mA max current for this pin.  I am not seeing much response when the Cell Balancing flag is active, but 10mA is not much compared to the 1500mA of charging current.  I am wondering if I damage the part by using the 100 ohm resistors and that is why it is not seeming to work, or is the current so low compared to the charging current that it does not make a difference.

Will it take many charge/discharge cycles to balance something like a 200mV cell difference?  We try to match them within 10mV when installing but sometimes my customers do bad things like remove the batteries from a few units and then put them back randomly.  

Thank you,
Richard

  • 0
    TI__Expert 6470 points

    Hi Richard,

    Looks like the datasheet is a bit unclear about the balancing FET Rdson values. I believe the upper row is the accurate value because the drop across the external 100 ohm resistors will insure that the differential voltage on the IC inputs will decrease to way under 4V.

    The current flows through two 100 ohm resistors, as well as the Rdson of the IC balancing FETs. You may have been thinking that the current is routed to ground, but thats not the case - it is routed from the top of a given cell back to the bottom terminal of that cell, traveling through 200 ohms plus the Rdson. From the values in the upper row of the electrical char. table, the current will not exceed 10 mA or so.

    We have had excellent results with this balancing scheme in packs with up to three cells in parallel. The key to understanding is that the cells will never get very unbalanced if they are rebalanced a little bit with every charge cycle.

    Regards,

    Doug