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BQ29200 CELL BALANCING QUESTION

WhiteHorse
Genius 4430 points
Other Parts Discussed in Thread: BQ34Z100

I already know that our VC1 and VC2 are connected to the wrong cells, so, please ignore that part of the design.  It doesn’t have any effect on my question. Please see attached schematic.

 What if the enter balance and exit balance thresholds are both at their negative most tolerance ( -9mV for exit and 15mV to enter)?  If one cell becomes more than 15mV above the other, balancing will turn on until that cell is 9mV below the voltage of the lower cell.  At that moment, what was the lower cell is now 9mV above what was the higher cell.  There is only 6mV of margin for the original lower cell to now trigger balancing as the higher cell.  Could noise or reading accuracy cause this condition to begin balancing?  If so, wouldn’t the balancing alternate between the cells until they are both fully depleted?  I am concerned that this condition may happen.  Is there some aspect of the chip design/functionality not mentioned in the datasheet that prevents this scenario from occurring?

Thanks for your help!

 4062.QB29200 Cell Balancing Question Schematic.docx

 Regards,

John

  • 0
    Prodigy 5 points

    Interesting question John.
    I am looking into it and hope to have a reply posted soon.

  • 0 in reply to Ed Walker
    Genius 4430 points

    Hi Ed,

    Thanks for your help.  Very much appreciated.  I have an additional question.

    How likely it is that the chip can be damaged by not following the recommended sequence for connecting the cells.  That is a risk for us right now.  Are there recommended design changes that can help prevent damage if the recommended sequence isn’t followed? 

     Regards,

    John

  • 0 in reply to WhiteHorse
    Genius 4430 points

    I have still another question.

    In the data sheet there is a specification for Iin for the VCx pins.  For VC1 it seems to be negligible.  VC2 however is 2.5uA max.  The one thing that is not clear is whether this 2.5uA goes to ground, or if it just bleeds the upper cell.  This is critical to understanding if the IC is creating a 2.5uA imbalance between the cells or if it is just bleeding an additional 2.5uA from the whole pack.

    Regards,

    John

  • 0 in reply to WhiteHorse
    Prodigy 30 points

    hey gusy anybody have which ic can i us efor my project? i was making circuit for two 3.2v lifepo4 cells connected in series.so im looking for an ic which can balance the

    the two cell and also works the charge and discharge process of the battery.i would appreciate if some body tell me

    thanks

  • 0
    TI__Guru* 76230 points

    If I understand your question correctly,  the minimum cell mismatch detection threshold voltage for turning on  is 17 mV and not 15 mV. The margin is 8mV.  Also,  since it is a voltage difference i.e delta  between (VC2-V1) and (VC1 - GND) that the device uses to detect  cell balancing and there are filters on the input pins, the concern on noise triggering  balancing and eventual cell depletion is eliminated.

    Thanks

    Onyx

  • 0 in reply to lealem bishaw
    TI__Guru* 76230 points

    Lealem,

    I would recommend the bq34z100. This supports 1s to 16s LifePO4 cells

     

    thanks

    Onyx

  • 0 in reply to WhiteHorse
    TI__Guru* 76230 points

    John,

    The data sheet says, while not advised, connecting the cells in a sequence other than that described therein does not result in errant

    activity on the OUT pin. In order words, this doesn't damage the ic.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Genius 4430 points

    Hi Onyx,

     

    Thanks for your replies.   Could you respond to this one as well:

    In the data sheet there is a specification for Iin for the VCx pins.  For VC1 it seems to be negligible.  VC2 however is 2.5uA max.  The one thing that is not clear is whether this 2.5uA goes to ground, or if it just bleeds the upper cell.  This is critical to understanding if the IC is creating a 2.5uA imbalance between the cells or if it is just bleeding an additional 2.5uA from the whole pack.

    Regards,

    John

  • 0 in reply to WhiteHorse
    TI__Guru* 76230 points

    Hi John,

    refer to figure 1 which is the block diagram of the device and you will see where that current is going to. Also,you should know that 2.5uA is max current possible into vc2 and not the minimum. The device has been tested and that current is neglible enough not to cause an imbalance.

    thanks

    Onyx 

  • 0 in reply to Onyx Ahiakwo
    Genius 4430 points

    Thanks Onyx for your help!

    Regards,

    John