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BQ76942: Floating/unused cell inputs. Not using top cell input possible?

Quaternion Aerospace
Prodigy 10 points
Part Number: BQ76942

Tool/software:

Hi,

I am currently using the BQ76942 analog front end in a battery management system to monitor cell voltages. I would like to use the AFE to monitor batteries that range from 6s to 10s, which I want to plug into the AFE using the same balance lead connector. However, in Table 16-3 of the datasheet (page74) it says "Cell inputs 1, 2, and 10 should always be connected to actual cells with cells connected between VC1 and VC0, between VC2 and VC1, and VC10 and VC9". For the design I want to use, if you plugged in a 6s battery only the bottom 6 cell inputs would be used with the rest of the cell input pins left floating. 

My question is whether or not it is actually required to be using cell input 10 or whether this will just disable other features of the AFE. If it is required to be using cell input 10, what is another way I can monitor 6s to 10s batteries and have all the required cell inputs populated.

  • 0
    Intellectual 416 points

    Hello,

    As mentioned, there should a minimum of 3 cells connected and the 3 cells should be connected between VC1 and VC0, between VC2 and VC1, and VC10 and VC9. 

    For example, if you are have a 4S pack, then:

    • Cell 1 - VC1 <> VC0
    • Cell 2 - VC2 <> VC1
    • Cell 3 - VC2 <> VC3
    • Cell 4 - VC10 <> VC9

    So, if you have a 10pin connector, then Pin 1, 2, 3 and 10 MUST have a connection to the appropriate cell with Pin 4 to 9 left floating (if shorting is done on the BMS PCB side) or shorted to Pin 3 (if no change on the PCB).

    Please see the FAQ thread here which will go into detail on how VCx pins are shorted.

    Best regards,
    Abhijith V Narayan

  • 0 in reply to Abhijith V Narayan
    Prodigy 10 points

    Listen here buckaroo,

    I get that the bottom two cells and the top most cell must be connected, but what happens if the top most pin IS left floating? Will this affect any other behaviors of the AFE or just the measurement? 

    Thanks,  

  • 0 in reply to Quaternion Aerospace
    TI__Intellectual 1140 points

    Hello,

    The topmost pin will affect cell balancing if left floating. It is not as critical for any other operations. The bottom two pins remain critical for operation. Additionally, keep in mind that at least three cells must be connected at all times in order to operate.

    Again, we do recommend shorting the unused pins instead of leaving them floating as mentioned above.

    Best Regards,
    Rohin R Nair