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BQ76907: BQ76907: Design Questions

Savas Sahin
Prodigy 230 points
Part Number: BQ76907
Other Parts Discussed in Thread: BQ76942

Tool/software:

Hi, 

We are currently designing 4S-8S BMS using BQ76907 and have 2 questions.

1- In 8S Lifepo4 battery pack, Can we use 8th cell for BAT pin, for supply? There will be 2-4 volt difference between VC7 and BAT pin. Will this difference cause any problem?

2- In a 4S configuration can we use VC1-2-3-4 pins and short VC5-6-7 to VC4?

All the bests,

Savas

  • 0
    Prodigy 230 points

    Hi,

    There is one more question.

    3- In any configuration, can BAT pin be lower than VC7 pin if yes what is the limit? Let's say if VC7 is 24V, can BAT pin  be 10V?

    All the bests,

    Savas

  • 0 in reply to Savas Sahin
    TI__Intellectual 2030 points

    Hello Savas,

    1. Unfortunately, you cannot use the 8th cell for the BAT pin. The BQ76907 is meant for 2-series to 7-series cells. Cell balancing and monitoring for protections will not properly work with an 8S battery pack. If you are interested in monitoring an 8S battery pack, I would recommend looking at the BQ76942 which allows 3-series to 10-series cells.

    2. VC6 and VC7 should be connected your top cell. VC1 and VC0 should be connected to your bottom cell. The other cells must be connected in ascending order from VC1, and unused pins should be shorted. For more details, please refer to the unused pins Section 8.2.7 in the datasheet.

    3. The BAT pin voltage could be theoretically lower than the VC7 pin, but it should still be relatively the same voltage. It must fall into a specific range depending on the supply voltage. Refer to section 6.3 in the datasheet.

    Regards,
    Rohin Nair

  • 0 in reply to Rohin Nair
    Prodigy 230 points

    Hi Rohin,

    Thank you for your replies. Please see my comments.

    1- My intend is not use BAT pin as 8th cell and monitor it using BQ76907 we have a seperate monitoring for it. As i read from datasheet, BAT pin is supply pin, so i want to learn can i use (VC7+4V) as supply voltage. In my operation i am not using cell balancing features and only using for voltage measurement.

    2. Ok i see that, top and bottom cells should be connected.

    3. YEs i have already checked that and there is nothing stated for my question.  As 6.3 says BAt pin is between 3 and 38 and nothing about VC7 -BAT pin relation and  limits in terms of voltage difference. Could you please verify what is the maximum and minimum voltage difference can be  between (BAT - VC7),

    it is very important and very urgent.

    Waiting your return.

    All the bests,

    Savas

  • 0 in reply to Savas Sahin
    TI__Intellectual 2030 points

    Hello Savas,

    I am not quite sure if there is a designated maximum and minimum voltage difference between BAT and VC7 allowed or suggested for operation. Let me check with someone on my team and get back to you as soon as I can.

    Regards,

    Rohin Nair

  • +1 in reply to Savas Sahin
    TI__Intellectual 2030 points

    Hello Savas,

    I have double checked with the team, and there is no direct relationship between the BAT pin and VC7. As long as both voltage values fall withing there required parameters, the device will be operational. So as long as Vbat falls between 3V and 38.5V, and VC7 falls within the ranges pasted below where the left side is the minimum and right side is the maximum:


    For example, theoretically Vbat can be 3V and VC7 can be 35V.

    Please let me know if there is anything that is unclear. Again, you can find this information in section 6.3 of the datasheet.

    Regards,
    Rohin Nair

  • 0 in reply to Rohin Nair
    Prodigy 230 points

    Hi Rohin, 

    Thank you very much, this is very clear right now.

    All the bests,

    Savas