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BQ76920: VC5-VC4 incorrect reading.

Tom Eaton
Prodigy 170 points
Part Number: BQ76920

Hello, I have recently received my new PCB which uses the BQ76920. The schematic is below.

The problem I am having is with VC3 and VC5 readings. I am using a 4 cell battery, so I have placed R15. I think the problem is something to do with my PCB layout, so I have attached the a pdf of the top view of the pcb. Hawkcell JobFile.PDF

The Voltage readings are as follows:

VC1: 4.184559

VC2: 4.184182

VC3: 6.050332

VC5: 1.683541

Total: 18.366875999999998

Please help me fix this issue. 

Many thanks.

  • 0
    TI__Guru 54715 points
    Hi Tom,
    I would suspect something with the connection. The cells 3 and 5 (from the device standpoint) sum to less than 8V probably because cell 3 has reached its maximum. This is common when the wire to the cell between the high/low reporting cells is not connected. Since you installed R15 you should be connecting the 3rd cell to J2-2 or J2-3 with the appropriate resistor (R4 or R6) installed. Be sure the pin you have connected matches the installed resistor. Check the solder on the resistor and J2 solder tail, be sure there is no contamination on the J2 mating pin, and check the crimp on the wires of the J2 mating connector. Check the wire harness connection at the cell. Also check the board for continuity but one would think the other connections might be more suspect.
  • 0 in reply to WM5295
    Prodigy 170 points

    Hello,

    Thank you for the reply. I think that the issue is that while I have added configuration jumpers to configure the appropriate shorting of VC pins (as in Table 8-1), the cell connections are incorrect. If I connect a 4 cell battery, the highest cell of the battery would be connect to VC4, which is incorrect. It should be connected to VC5. A 3 cell battery will have a similar problem.

    Do you have any idea how I could make this cell connection configurable when building the board?

    Many thanks,

    Tom

  • 0 in reply to Tom Eaton
    TI__Guru 54715 points
    Hi Tom,
    You appear to have done it right, you just need to connect using J2 pins 6,5,4,3, and 1 skipping pin 2 (or pin 3). If you battery harness for the smaller battery must remain sequential you could use an alternately populated resistors to go from pin 2 to VC5 and wherever else VC5_R must go, or a shorting jumper to connect J2 pins 1 and 2.