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Linux/BQ77915: bq77915

Part Number: BQ77915

Tool/software: Linux

Hi all expert :

                     When we verify bq79915 EVM board, we find out cell balancing behavior is different with datasheet, could you help to check and response ?

                     According to datasheet description, cell balancing work condition as below :

                    1. OV trigger

                    2.VCBTH -VCBTL >100mV 

                    The condition 1 is no problem, but the condition 2 we have some question 

                   (1) We set each bank voltage is higher than 3.8V, when we change Vcell 5 voltage to 3.9V->4.0V->4.1V, the balancing function can not work.

                   (2)We set each bank voltage  is higher than 3.8V, when we change Vcell 5 voltage to 3.9V also add charging current 100mA -> can not work

                                                                                                  when we change Vcell 5 voltage to 4.0V also add charging current 100mA -> can not work

                                                                                                   when we change Vcell 5 voltage to 4.0V also add charging current 200mA -> can work 

                   Seem the test result is different with datasheet description.

                   Could you help to check condition 2 cell balancing work real behavior ? 

                   thanks ~

  

  • Hi Dustin,

    I think I see your issue. At 100 mA charge current, the voltage across your sense resistor is 1.5 mV. This is not high enough to satisfy the charge state comparator. At 200 mA, this condition is satisfied and cell balancing can start.

    Best regards,

    Matt

  • Hi Matt :
    Thanks for your response.
    According to your answer, May I have conclusion as below :
    1. OV trigger-> it can work in rest mode.
    2.VCBTH -VCBTL >100mV -> it should work in active mode, charging current should meet charge qualification threshold.
  • Yes, that is correct.

    Regards,
    Matt
  • Hi Matt :

                  Thanks for your response.

                   Below item is my test result in bq79915 EVM board.

                   Seem it is different even some condition meet charge threshold, could you help to check ?     

                   Attachment is my test video.    

    (1)      VCBTH-VCBTL > 100mV and charging is 200mA->, it will not work.

    (2)      VCBTH-VCBTL > 200mV and charging is 200mA->, it will work.

    (3)      VCBTH-VCBTL > 100mV and charging current is 450mA, it will work

  • Hi Dustin,

    I will check into this and get back to you tomorrow. Sorry for the delay.

    Matt
  • Hi Dustin,

    I think I understand why you are not seeing balancing activated in the case where you set one cell to 3.91V. I think you may also need to set one of the voltages lower than 3.8V for balancing to start. I am attaching some slides that explain the balancing operation in more detail. I think one cell needs to be above VCBTH at the same time another cell is below VCBTL. This is because as one cell rises above VCBTH, both VCBTH and VCBTL both change by 100mV.

    /cfs-file/__key/communityserver-discussions-components-files/196/0576.bq77915_2D00_Cell_2D00_Balancing.pdf

    Best regards,

    Matt

     

  • Hi Matt :

                  Thanks for your response and document.

                   1.In datasheet, cell balancing start only cell voltage exceed 3.8V, if I adjust lower than 3.8V, it didn't meet datasheet description. please help to check this...

                  Datasheet descrption (page20)-> VSTART is set at 3.8 V, therefore,cell balancing starts only when individual cell voltages exceed 3.8 V. 

                  2.Today I see updated datasheet, table 2(page 13) mentioned vstart is 3.5V and 3.8V, why there are two Vstart ? which one is correct cell balancing vstart voltage ?

  • Hi Dustin,

    I know the cell balancing requirements are a little confusing. The cell balancing start does not mean all of the individual cell voltages are above 3.8V. Let me explain 2 cases that might help:

    Case 1) If the balancing window is 3.8V-3.9V and one cell is set above 3.9V while all other cells are within the window, the window will shift up (to 3.9 - 4.0V) and no balancing will occur.
    Case 2) If one cell is below 3.8V when the other moves above 3.9V, balancing will occur. Once all of the cells are above 3.8V, the window will shift up.

    Regarding the datasheet, there are multiple options for different versions of the device (parameters that can be set in production). The device comparison table covers these. However, the VStart voltage is missing from the device comparison table. We will get this corrected in the next revision of the datasheet. The released device (bq7791500) has a VStart of 3.8V.

    Best regards,
    Matt
  • Hi Matt :
    Thanks for your response
    Could you help to list exactly cell windows condition ? How cell windows shift up ? Why cell windows shift up ?
  • Hi Dustin,

    Cell balancing only occurs during charging. Since the cell voltages will be increasing, the cell window should move up to track the rising cell voltages. The window moves up in 100mV steps.

    The window moves up when all of the cell voltages are greater than VCBTL and one or more of the cell voltages rises above VCBTH. No cell balancing is needed in this case because the cell voltages are not too far apart from each other.

    If there are cells with voltages below VCBTL when any cell is above VCBTH, then balancing will occur. The cells with lower voltages need to catch up and and cells with high voltages need to slow down so that the cell voltages are not too spread out. 

    I hope this makes sense,

    Matt