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BQ76940： Equilibrium problem

Luo XiuFeng
Intellectual 676 points
Part Number: BQ76940

Dear Technician:
    Our product is 14 strings, using BQ76940 for management, now I want to verify the question about balance:

    Schematic:
   

  1. The manual mentions that adjacent units should not be balanced at the same time. What is adjacent is:
        a. can not open CB2 when CB1 is turned on? But can you open CB3 at this time?
        b. Can I open CB2 without CB1? Can't open CB3, can you open CB4?


   2. What is the equilibrium battery current flow? After turning on CB1, VC1 and VC0 start to balance? If my first string needs to be equalized at this time, I will open the 8th string or other high voltage string, will the current flow to the first string?

 

  • 0
    TI__Guru 54715 points
    Hi XiuFeng,
    1. Adjacent means the one next to it. Considering "open" as the FET on:
    a. If CB1 FET is open, CB2 should not be opened, but CB3 would be OK. For balancing with cell 1 or CB1 open, the register settings 0x01, 0x05, 0x09, 0x11, 0x15 are OK. 0x03 should not be written.
    b. Yes, CB2 can (and should) be opened without CB1. For the register 0x02 is OK, 0x0A is OK. 0x03 or 0x06 should not be written.
    For the top group since 4 cells will be used cell numbers 13 and 15 from the perspective of the IC are now adjacent in their connection and should not be balanced simultaneously. When balancing 2 cells in this group only 0x05 and 0x12 are suitable.
    2. See figure 5 of www.ti.com/.../slua749a.pdf for example of current flow. Current flows in the pins and FET associated with the cell balanced. Current does not flow between groups, so cells may be balanced in one group (CB1) at the same time as another group (CB8).
    In your schematic the 100 ohm resistors are in series with the inputs to the filters. I expect this will operate but may reduce the Vgs available to the external balance FET as it turns on and may operate the FET to limit current. Normally we recommend and expect the connection in figure 5 of the application note so that the FET can be fully turned on to reduce heating.
  • 0 in reply to WM5295
    Intellectual 676 points

    Thank you for your reply, but I am still confused:
       If my current second string voltage is too low, I need to balance him, I need to open the CB3 in the register to open the balance? (discharge to the second string)


  • 0 in reply to Luo XiuFeng
    TI__Guru 54715 points
    Hi XiuFeng,
    I may not understand. If cell 2 voltage is too low after charge you would want to balance down all other cells since the balance technique is dissipative. You would want to implement an algorithm, if balancing the most cells possible you might open cells 1, 3, and 5 simultaneously and alternate with cell 4 on some cycle. The register sequence might be 0x15 for several seconds, then 0x08 for several seconds. The upper groups would need a similar sequence but could alternate between 0x15 and 0x0A.