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BQ20Z655-R1: Using the device as a Battery Monitoring System with 2S2P config; are the following measures suitable?

Rizqi Wahyudin
Prodigy 60 points
Part Number: BQ20Z655-R1


My team and I are building an avionics system for a student-led rocketry project in which we will be using a 2S2P configuration (we had recently switched from 4S1P) for our batteries. Is it right to take our measurements for each 'battery cell' at the following points in the cell configuration? 

Readings will be taken from the 'most positive node' at vBat, then through a parallel connection at Bat_term_1, then a connection to the most negative node at GND. These will be respectively fed into VC2, VC3, and VC4 (unsure about this as well, perhaps VC5 should be used as the default 'most negative' node reading regardless of configuration)? 

In addition, kelvin connections will be made as recommended by the manual. 

  • 0 in reply to Shirish
    Prodigy 60 points

    Thank you for replying! But why is that the case? I assumed that the manual recommended that we start connecting the highest potential from VC2 and then connecting VC4 to the lowest potential in the battery packs. On the VC1 pin description, it says to 'connect to VC2 in 2-3 cell stack applications', and on VC2's pin description it states the same instruction just replaced with VC3. What do these actually mean? And if I may, being that Bat_Term_1 is half of the potential at VBat, why would that be measured at VC1? Shouldn't it follow consequently from VC2 where VBat is? 

    Wouldn't mind any theoretical explanation if you would bother, we're always up for learning. 

  • 0 in reply to Rizqi Wahyudin
    TI__Guru 57697 points

    Hello Rizqi,

    For the cell stack side: You want to short the unused cells to ensure that those are always correctly read as 0V. The cell selection settings ensures that these 0V cells are not used in calculations for gauging.

    If you see something different in the manual, then please point me to the specific section.

    Update: The document description is correct. It refers to pin numbers and not cell stack. Comments above have been modified to avoid confusion.

  • 0 in reply to Rizqi Wahyudin
    TI__Guru 57697 points

    Hello Rizqi,

    I see that you are referring to the Pin designation. I misunderstood it as the cell designation in the cell stack. I will update my previous comments to reflect that.

    I would recommend that you use this document as a reference for your design https://www.ti.com/lit/ug/sluu697/sluu697.pdf

    See page 7. For your use case: 4P,3P and 2P will be connected together.