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BQ29209: What is the balancing current path and what is the function of the resistor RCB1 in DS

Part Number: BQ29209

Hi Ti,

Just want to find out what the balancing current path is, in my opinion, should be something like:

There should be no current run through RCB1. And I guess the resistor RCB1 and Cin will provide a low pass filter for the balancing chip, however I can measure 0.5V voltage drop across the 260ohm resistor RCB1, don't know what the reason is.

Any comments will be appreciated.

Thanks!

Fan WANG

  • Hi Fan,

    The path you have on the schematic is not correct. The correct Kirchoff current law equations for cell balancing are listed in section 8.3.5 of the datasheet. These equations outline the path for each of these currents. Note that RCB1 is included in the ICB1 equation.

    Please let me know if you have any additional questions!

  • Hi Shawn,

    Thanks for the reply. However I still can't have the clear picture of the discharging path in my mind, cause what I found in the block diagram is that VC1 is just a input to the comparator and Schmidt trigger, could you provide more details?

    Thanks!

    Fan WANG

  • Hi Fan,

    VC1 is the voltage sensing pin for the bottom cell. For cell balancing, the value of cell 2 is determined to be the difference between the VC2 and VC1 pins. The value of cell 1 is the difference between VC1 and VC1_CB. Cell balancing will start when the mismatch between the cells is greater than about 30 mV. It will stop when the cells are about equal.

    If you are concerned about the discharge path, it may be helpful to take a look at Icd(dsg). This is the over voltage detection discharge current. This current is 60uA in the datasheet, and with the 4.3V over voltage protection, you can calculate the resistance of the path. For this protection, the value of the cells is compared with an internal reference threshold to determine if a fault has occurred.

  • Hi Shawn,

    Thanks for the further explanation.

    I thought the value of cell 2 is determined by the difference between VC1 and GND. As I can see from the functional diagram in DS 8.2, there is a mosfet between VC1_CB and GND.

    Another question is I still don't understand how RCB1 and RCB2 are involved in the balance discharge process, I mean the formula in DS 8.3.5.

    Thanks!

    Fan WANG

  • Hi Fan,

    Section 8.3.5 of the datasheet also mentions that the value of cell 2 is determined by VC2-VC1. The difference between VC1 and GND is the value of cell 1. 

    Since GND is essentially zero, the cell balancing current for cell 1 is determined by dividing VC1 by the resistance of the path. For cell 1, the path from VC1 to GND includes Rcb and Rcb1. The cell balancing current for cell 2 is calculated in a similar way, except the voltage is the difference betweeen VC2 and VC1. This section also describes what these resistors are. Rcb1 is defined as the resistor between the top of cell 1 and VC1, Rcb2 is defined as the resistor connected to the top of Cell 2 and VC2. 

  • Hi Shawn,

    Thanks for the reply,

    My question is I don't understand how Rcb and Rcb1 build up the discharge path for cell 1 and Rcb, Rcb2 and Rvd build up cell 2 balancing path. Can you provide a drawing of components outside the chip along with the internal block diagram(like what I did when I raised this question)?

    Thanks so much!

    Fan WANG

  • Hi Fan,

    I'm going to dig into this some more and get back to you Monday afternoon (central time).

  • Hi Fan,

    Actually, the original cell balancing current diagram you posted is correct. I also agree that there should be no current in Rcb1. This voltage drop may be from contamination, leaking capacitor, or leaking/damaged IC pin.

  • Hi Shawn,

    Thanks for the feedback.

    Can I assume that the formula provided in the DS is incorrect?

    Thanks!

    Fan WANG

  • Hi Fan,

    Yes. Assume that formula is incorrect.