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BQ79718-Q1: BQ79718 Hardware Configuration Support

Mustafa Tarhan
Intellectual 980 points
Part Number: BQ79718-Q1

First of all, I would like to explain the structure we use.

We are using BQ79718. Our battery module consists of 16 cells. Each cell is connected in series with BUSBAR. BMS is fed with BUSBAR from the negative terminal of Cell 1 and the positive terminal of Cell 16. I want to measure the voltage drop across this BUSBAR (Cell 16 positive terminal). I wanted to see this with BB pin on BQ79718.

 

When I examine the datasheet there are limited locations for the BB pin. You can observe this in the BB_PIN_LOC registry below. My question here is BB pin 16. Can I connect it to the positive terminal of the cell? If this is the case, what should my hardware configuration be?

How else can I find a solution if I cannot use the BB pin in the structure I want. What is the BB pin used for? What is its purpose? Why is there a position limitation?

 As seen in the picture below, what are the BBVC_POSN1, BBVC_POSN2 and BBVC_POSN3 registers used for? I didn't fully understand this. Our 16 cells are connected in series with small BUSBARs. How should I configure these registers for this?

I tried to explain the structure of our system in the picture below. I supply power to the board with BUSBAR from the positive terminal of Cell 16. Here, contact resistance, BUSBAR resistance, temperature etc. there is a voltage drop for some reason.

So I ask you to pay attention to the BB pin position.

 

Waiting for your information sharing and solution suggestions.

  • 0
    TI__Expert 8451 points

    Hi Mustafa,

    There are 2 methods to measure busbar voltage--via the BB pin, or connecting the busbar to the VC/CB channels and measuring the busbar voltage using GPIO pins.  When connecting to the BB pin, the busbar MUST be located between the top of cell 6 and the top of cell 9.  If the busbar is not located in these select positions, then it should be connected to occupy a cell channel, and the voltage across it will be measured with GPIO pins.

    I have a few concerns with the diagram you provided as an example.  First, please see section 8.3.3.1 in the BQ79718 datasheet for possible busbar positions using the BB pin.  Additionally, please note that you cannot connect the `busbar to the bottom channel if you plan to connect your busbar to VC/CB channels and measure voltage with the GPIO pins as a workaround.

    Finally, I may be understanding your diagram incorrectly, but it seems as though instead of measuring a voltage of close to 0V (a healthy busbar voltage), you will be measuring the voltage difference between the top and bottom of the stack using either the GPIO pins or the BB pin.  Please see section 7 of the datasheet to ensure you are within device specifications.

    I highly recommend reading through section 8.3.3 of the datasheet to gain a better understanding of how the BQ79718 accommodates for busbars.  Before concerning yourself with the register values, please ensure you have valid hardware connections.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Prodigy 45 points

    Hi Andria,

    First of all, thank you for your feedback.

    Actually, what we're asking is:

    -According to the diagram shared above, do you think the hardware is appropriate? It says there is restricted location for BB pin datasheet. Is it correct to use other than the specified locations?

    -7. section has been read carefully and at voltage applied to pin BB, there is no over spec. BUSBAR, where the BB pin measures, is the positive supply side of the BMS and Monitor board. The goal is not to directly measure the top and bottom of your pile. If the BB pin is not used, we are aware that the specified BUSBAR will be measured by taking the voltage difference with the help of GPIO pins from the bottom to the top.

    Actually problems can be solved for us if you answer the following questions.

    *What is the BB pin used for? What is its purpose? Why is there a position limitation? Is it correct to use it outside the specified position?

    *What are the BBVC_POSN1, BBVC_POSN2 and BBVC_POSN3 registers used for? I didn't fully understand this. Our 16 cells are connected in series with small BUSBARs. How should I configure these records for this?

  • 0 in reply to Tugay Sengul
    TI__Expert 8451 points

    Hi Tugay,

    I will close this thread and continue responding via email.

    Best,

    Andria