• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 389 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77915: BMS of 7 cells

SHASHANK KUMAR49
Prodigy 155 points
Part Number: BQ77915

Dear E2E Team,

I am working on BQ77915 for 7 Cells in series. and when i connect both the cells only DCHG pin is high and the CHG pin is low. But in normal mode both the pins should be high.

Attached is the schematic for reference.2605.7 Cell BMS Schematic.pdf

  • 0
    TI__Guru 54715 points

    Hi Shashank,

    I don't recognize an issue with the schematic connections.

    This is a common observation when fully charged cells are connected or the part exits hibernation with fully charged cells.  See section 9.3.2.1 of the data sheet: "The device assumes an OV fault after reset,...".  With a BQ7791500 and cells in the 4.0 to 4.2V range, CHG will not turn on at connection since the part sees the cells in the OV hysteresis range.   The pack can discharge through the body diode of the charge FET.  When a load is applied which activates the state comparator the CHG will turn on to protect the charge FET from heating during the discharge current.  When the cell voltages remain below the OV recovery threshold for the delay the CHG will be set high.

  • 0 in reply to WM5295
    Prodigy 155 points

    Thanks a Lot.

    But facing one more problem.

    When start discharging the cells at 19.8V DSG pin is Low but CHG pin is high that's the why through the body diode of DSG mosfet cells getting discharged.

  • 0 in reply to SHASHANK KUMAR49
    TI__Guru 54715 points

    Hi Shashank,

    With 7 cells totaling 19.8V, if they match the cell voltages will be 2.83V, below the 2.9V for UV of the BQ7791500 configuration. If the cells don't quite match one or more will be lower. The part should go into the UV mode, in table 5 of the data sheet DSG goes low while CHG remains on.   Of course if you have a load on PACK, when DSG goes low PACK- likely pulls up and R31 will pull up CHG toward its clamp.

     If you have a different version part the threshold could be different. 

  • 0 in reply to WM5295
    Prodigy 155 points

    Dear Sir,

    When start discharging the cells with a load of around 1A, at 22.8V (3.26V per cell) BMS is cut off but i want it should be cut off at 19.25V (2.75V per cell).

    How to change this low cut off level of BMS.

    3716.7 Cell BMS Schematic.pdf

  • 0 in reply to SHASHANK KUMAR49
    TI__Guru 54715 points

    Hi Shashank,

    The part must be ordered with different protection thresholds. Options are shown in table 1 of the BQ77915 data sheet.  Available configurations are shown in the Device comparison table in section 6.  A 2.75V OV threshold is not presently available.  If you have sufficient volume to support a new configuration please contact your local TI sales representative for assistance.