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BQ76930: cell 4 value problem

alireza roozitalab1
Prodigy 90 points
Part Number: BQ76930

i am using BQ76930 with 8  cells  . I connected my circuit to the power supply and every thing was ok. then i connected circuit to the inverter/charger (Victron multiples ii) then I saw cell num4 voltage goes up and down between 3.5 to 4.1 volt. all others cells working correctly just problem with cell4 value. I got cell num 4 voltage by multimeter and it was 3.76 

i checked it again with power supply and cell 4 worked correctly . so cell number 4 value changing when i am using inverter/charger .it is very important for me to use this project by inverter/charger  because i want to use it as home storage .

i also change capacitor and resistor according to the slua810 Figure 4 but it didn't worked  . 

r356 connected via wire in battery pack.

  • 0
    TI__Guru 54715 points

    Hi Alireza,

    In SLUA810 Figure 4 note that the bottom of C7 should connect to GND as shown in the data sheet rather than VC0 as shown in the figure.  That should have nothing to do with the cell 4 voltage however.

    It looks like you had an attachment which did not come through.  I don't know your R356 location.

    Common issues with reading cell 4 value include:

    • High impedance to the cells, particularly at the group boundary (since cell 4 connection to the cells includes VC5, VC5B and the VC5X power pin)
    • Using a Schottky type diode at Figure 4 D2
    • Unexpected short or open in the circuit
    • Wrong component value

    The component values in Figure 4 are very common and should work well. Hopefully something here will assist in finding the issue, or please check the attachment method to re-attach.  Knowing the real component values is important.

  • 0 in reply to WM5295
    Prodigy 90 points

    hi thanks for your replay

    I have attached schematic one more time.

    I used 1n4148 for D2  

    R356 connected in battery pack and VC0 also connected to the GND in battery pack.

    I have also changed components value but the result was the same.

    i have just cell 4 problem with inverter/charger . with power supply cell 4 work correctly . there is 50 HZ noise with 2v amplitude in Vout  of inverter/charger, so I have this noise on battery pack in charging time. but at this moment all other the cells work correctly except cell 4.

  • 0 in reply to alireza roozitalab1
    TI__Guru 54715 points

    Hi Alireza,

    R356 is marked as not installed but you indicate it is installed.  Cell 4 must be connected to cell 5 bottom and the junction connected to the filter for the VC5, VC5B and VC5X pins.

    With a power supply only the signals should measure properly.  With cells and a charger the cells should provide a low impedance or stable voltage to the BQ76930 circuit and it should measure properly.  The variation in the charger should be varying current into the cells.  If the inverter can dynamically vary the voltage on the cells provided with a resistor, the BQ76930 won't be able to measure properly at the boundary cells between the groups since the power pin VC5X is filtered heavily while the inputs are filtered more lightly.  

  • 0 in reply to WM5295
    Prodigy 90 points

    so is there any suggestion to reduce noise on vcx5 or changing some component to have the correct value on cell 4? for each cell I paralleled 4 li-ion baterries

    when I reduced parallel batteries to 1  battery cell 4 also work correctly with the inverter/charger (with the same noise on output voltage) . I have this problem when the charging current is more than 3 ampers and only cell4.

  • 0 in reply to alireza roozitalab1
    TI__Guru 54715 points

    Hi Alireza,

    Something must not be connected as you expect.  With more cells in parallel the voltage should be more stable unless the parallel cells are at different voltages and one is charging another causing some current interruption.  A loose or high impedance connection with varying current will also cause voltage change.  More current will cause more voltage variation across a resistance.  It seems your cells connect through the board, and the top of cell 4 connects to the upper group through the R356.  You might be sure there is no unexpected voltage across R356 at the higher currents.