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BQ76940EVM: Unexpected ADC readings when shorting a cell

Nikola Felkaroski
Prodigy 40 points
Part Number: BQ76940EVM

Dear support team,

My project requires that at certain times I need to physically short a cell (through a MOSFET switch), while keeping the required block voltages for normal operation. When I try to read the ADC in the new state, I get some unexpected artifacts. The shorted cell shows some small voltage in the order of a few hundred mVs (which is not a problem), but this also affects the ADC reading of the next cell, showing a value of circa 1V instead of 2.2V (the input to the EVM reads out 2.2V using a voltmeter). Are there any workarounds for this case? Should I perhaps make some physical modifications to the board or is this an ADC issue?

Thank you for your time.

  • 0
    TI__Guru* 81034 points

    Hi Nikola,

    I have not seen this type of requirement before - shorting a cell through a MOSFET. How is the block voltage maintained during this short? If the cell stack voltage remains the same while the external components are still present at all of the cells, the voltage may be redistributed during the short so you may see strange values. Ideally, you want to short the device pins in addition to shorting across the cell.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 40 points

    Hello Matt,

    Thanks for your quick reply.

    As I understood, each block needs at least 6V for normal operation, so I never short more than 1 cell in a block simultaneously. The MOSFET acts as a physical short at the inputs of the EVM, so the cell block voltage is (Vtotal - Vcell). I will try to use a second switch, controlled by the same MCU pin, directly at the device pins and see the results.

    EDIT: I added a screenshot from the readings (obviously these are not Volts, but the actual ADC values (~6000 = ~2.2V)). I am using a 12-cell configuration so cells 3, 8 and 13 are naturally 0 (removed resistors and shorted as instructed in the manual). In the first block (cells 0-4), I am shorting Cell 1 using a MOSFET and as you can see it works without affecting Cell 2. But in the second block (cells 5-9), I do the same to Cell 5 and it has those weird readings (especially the following Cell 6).

     

    Best regards,

    Nikola