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Original question:

BQ29200: BQ29200 balancer question

BQ29200: BQ29200 cell not working.

Prathamesh Tadaka
Prodigy 185 points
Part Number: BQ29200
Other Parts Discussed in Thread: BQ76200

Hello, I am trying to make 2s Li-Ion Battery charger with cell Balancing. I am using charger IC BQ2057wsn and for cell balancing BQ29200. 3731.SCHEMATIC1 _ PAGE1.pdfCharger is working fine but cell Balancing is not working. battery charged with one cell 4.35V and other cell 4V.
Charger stops charging because one cell reached above threshold voltage(I am used here transistor to turn ON & OFF Charging using OUT pin of bq29200). Here is cell difference between two cell 0.35V.
so I want BQ29200 cell balancing should work here even after charging stop. but cell balancing is not working.
1)Please check schematic. I uploaded the schematic. is there anything wrong with schematic? BQ29200 is doesn't balance the cells.
2)Please verify Value of RCB1=RCB2=1KOhm, RVD= RCB=100 Ohm. CCD=0.33uf, CVD=0.1uf.

3) How much current consume BQ29200 to balance the cell  & how much time required to balance the cell.

  • 0
    TI__Genius 10830 points

    Hi Prathamesh,
    Thank you for your question. 
    One of our engineers will reply once we are back at the office.

  • 0 in reply to David Hien
    Prodigy 185 points

    Hi David, 

      OK, No problem.

      But please reply ASAP.

    Regards,

    Prathamesh

  • 0 in reply to Prathamesh Tadaka
    TI__Guru 54715 points

    Hi Prathamesh,

    1. The schematic looks ok.  How did you determine it is not balancing?  To see if it is balancing inspect the VC1_CB pin.  If it is pulled down it is balancing the bottom cell, if pulled up it is balancing the top cell.

    2. Values seem ok.

    3. Balance curent is shown in the BQ76200 data sheet both in the section 8.3.5 description and the 7.6 recommendations.  RCB1 and RCB2 are not shown in the data sheet but you could calculate out from the 7.6 recommendations table at about 260 ohm with 120 ohm RCB (3.8V/.01A - 120 ohm =  260 ohm). Using that in the equations gives about 4.35/(100 + 260) = 12 mA if it is the bottom cell balancing, or 4.35/(100 + 260 + 100) = 9.4 mA if the top cell is balancing.  0.35V is a lot of imbalance, if it were to represent 20% of your cell capacity and cell capacity is 2600Ah, it could take (2600 mAh * 0.2)/9.4 mA = 55 h to balance.    The BQ29200 is better at maintaining balance of matched cells than establishing balance of mismatched cells.

  • 0 in reply to WM5295
    Prodigy 185 points

    Hi,

    Thanks for replying.    

    1) As i said earlier when one of the cell reaches at 4.35V charging stop but BQ29200 Cell balancing still remains on to balance the cell. But after charging stop current reaches at 0.0A (No current flowing through it). So i consider that it doesn't balancing the cell.

    2)when i start charging the difference between two cell is 0.15V.  But when charging stop the voltage difference 0.35V. This is one of the reason that to consider cell balancing circuit not working.

    3)Can you please suggest me cell Balancing IC for mismatched cell?

    Regards,

    Prathamesh

  • 0 in reply to Prathamesh Tadaka
    TI__Guru 54715 points

    Hi Prathamesh,

    1. The charge current for the battery does not control balance current for the BQ29200.  The balance current will flow in the cells and BQ29200 circuit.  The easiest way to observe balance current may be the voltage of the VC1_CB pin or the voltage across RCB.  

    2. Since the voltage difference across the cells is larger while charge current is flowing it seems one cell must have a significant resistance.  With balancing enabled the BQ29200 should eventually reduce the voltage difference at rest to VMM_DET_OFF shown in the data sheet.  It will take a long time.  Adding an external circuit such as Figure 11 of the data sheet will increase the current and reduce the time, but it could still be a long time. 

    3. The BQ29200 is the device we offer dedicated to OV and balancing for 2 cells.  Another method may involve measuring cell voltages with a MCU and determining which cell to balance, the loading of the cells could be selected by the MCU according to the design.

  • 0 in reply to WM5295
    Prodigy 185 points

    Hi,   

    BQ29200 Cell balancing ic takes too much time to balance the cell. Can you suggest me quickest Two cell balance ic for 3.7V 2200 mAh Li-Ion cell?

    Regards,

    Prathamesh

  • 0 in reply to Prathamesh Tadaka
    TI__Guru 54715 points

    Hi Prathamesh,

    The BQ29200 is the battery IC we have available.  Other options may be switching regulator circuits to charge the low cell either from external source or from the higher cell. If you can parallel the cells you might control a resistance  between them to keep the currents within acceptable limits for the cells.