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Incorrect Vcell, 13S cell connections

Pranit Pawar
Intellectual 520 points
Part Number: BQ76940

Hi experts,

I have made a 13s-15s BMS using BQ76940 (3.3v version). I’m fetching the cell voltage data from the BQ using atmega328p. The ckt has jumper pads which can be used to configure the board for 13s-15s battery configurations.

Kindly check the schematic

There are 6 jumpers pads in total – SJ6, 7, 8 and SJ10, 11,12

SJ7 shorts VC13-VC14

SJ10 shorts VC8-VC9

So the jumper states corresponding to 13-15s config are

13s: SJ6 open, SJ7 close, SJ8 open

        SJ10 open, SJ11 close, SJ12 open

14s: SJ6 close, SJ7 open, SJ8 close

        SJ10 open, SJ11 close, SJ12 open

15s: SJ6 close, SJ7 open, SJ8 close

        SJ10 close, SJ11 open, SJ12 close

I’m using the board in 13s configuration. I’m using Bluetooth for debugging, pushing the data on a Bluetooth terminal on my phone I get the following output. Kindly check the output image(black background with green text)

C1-5 are reported perfectly  

C7 is reported perfectly

C12 is reported perfectly

Rest all cells’ voltages are incorrect. In my ckt, VC0 is connected to gnd, VC15 is connected to BAT+. So the cell connections are

C1: gnd-VC1

C2:. VC1-VC2

C3: VC2-VC3

C4: VC3-VC4

C5: VC4-VC5

C6: VC5-VC6

C7: VC6-VC7

C8: VC7-VC8

C9: VC9-VC10

C10: VC10-VC11

C11: VC11-VC12

C12: VC12-VC13

C13: VC14-VC15

I checked the datasheet and the sample ckt shows VC0 is grounded. However in TIDR773, TIDR00792,

VC0 isn’t grounded directly. Is this what is causing improper cell voltage issues?

 What is the cause of the improper cell voltage readings? If my connection is wrong, what is the correct connection?

  • 0 in reply to Pranit Pawar
    TI__Guru* 81034 points
    Hi Pranit,

    The VC0 connection you have is correct. The datasheet method is the right way.

    There are a few issues in the schematic:
    - The BAT pin needs to be connected to the top-most voltage through a filter resistor. I don't see this connection in your schematic.
    - The power filter resistors (R30, R31) are too small. 1k is recommended.
    - D23 should be a higher voltage value (15-18V)
    - A blocking diode is needed in the REGSRC circuit. See Section 9 (page 14) of this app note for a full explanation: www.ti.com/.../slua749a.pdf
    - The jumper states you described appear to have problems. For example, both SJ6 and SJ8 should not be open since this would result in no connection to the battery cell. One of these should be closed. And then a short is needed across the cell connectors as well, otherwise the cell balancing circuit will try to adjust the cell voltages. I think you also wanted SJ10 closed.

    I hope this helps.
    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 520 points

    Thanks Matt!

    Yes you pointed it out right that jumper states are incorrect. I'll be shorting all the jumpers and by doing so VC13-14 and VC8-9 will get shorted and also the neighbouring cells will get connected to the IC.

    I wont remove the balance circuitry for the undesired cells, rather I'll not connect the undesired cell. So in short, VC13-14, VC8-9 will be shorted and no cells will be connected across those two pairs of terminals. However, the balance circuitry related to those two cells will still be kept on the ckt as it is.

    I will change the power Resistors R30, 31 to 1k

    And yes there is a filter resistor of 100ohm between BAT and Pack+/Load+ terminal. Its shown in the "THERMISTORS" section of the schematic.

    I'll also change their diodes as you mentioned and add the protection ckt for regsrc