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BQ78350-R2-DEVICE-FW: Cell balance question for bq78350

Beginner
Intellectual 2261 points

Part Number: BQ78350-R2-DEVICE-FW

Hi Team,

My customer is testing CB in bq78350-R2. There are some interesting investigate and need your comment to clarify. Btw, this is 10S application.

Here is CB setting for your reference.

  • Cell Balance Threshold : 3800
  • Cell Balance Window:100
  • Cell Balance Min:15
  • Cell Balance Interval:5

Test Method and Result

  • Using power supply to charge cell #3 , #6 ,# 8 to cause their cell voltage is higher 50mV than ones.
  • Charging to whole system with different “Cell balance window ”setting – 100mV,50mV , Logging by bqStudo to observe the changing.

Attached #1 – Cell balance window = 100mV --> By checking CB in operation status B, we notice CB is activated in the beginning , however it stop balance after 1 min even if the deltaV is higher 50mV

Attached #2 – Cell balance window = 50mV --> We saw the cell#5 voltage is changing a lot.

Could you kindly take a look this symptom?    

 Cell Balance window 50mV.xlsxCell Balance window 100mV.xlsx

  • 0
    TI__Guru 54715 points

    Hi Beginner,

    Check the resistance between the board and the cell.  When cell 6 balances it will create a voltage drop on the interconnect which can be measured by cell 5.  Also check the trace width on the board and where the components are connected.  Sometimes a shared route has been used on the board which develops a drop.  Routing from the balancing resistors and FET terminal directly to the board's cell connection is preferred.

    The BQ78350-R2 balances only during charge.  In the 50 mV log when current stops at blue line balancing stops, the cell 5 voltage stabilizes.  See the technical reference manual for the description of the current needed for balancing.

  • 0 in reply to WM5295
    TI__Intellectual 2261 points

    Measure the resistance between the board and the cell as below. The max value is for cell #8 , not #5 or #6. 

    Meanwhile, as you can see, only there is higher rpple voltage on ch#5. Do you have any idea for symptom? 

     

  • 0 in reply to Beginner
    TI__Guru 54715 points

    Hi Beginner,

    See https://www.ti.com/lit/pdf/slua749 section 4 figure 8.  The resistance for the other cell connections such as cell 8 is not such a concern since the part does not balance and measure at the same time.  At the group boundary one cell may be balancing while the other cell is measuring. In the multiple cell plot it is difficult to be certain which cell is high, but it looks like one of the orange cells is high, perhaps cell 6 which would be balancing.  If the resistance in the path is 57 mOhm and the cell balance current is 100 mA, there will be 5.7 mV effect on cell 5. Consider the current for their design and the voltage, since balancing is resistive the magnitude will go up with cell voltage. If there is common path resistance on the board also that can add to this value. Figure 8 is a schematic but shows separate paths to the board terminal.