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BQ76940: OV fault occurs when charging, depending on charging current

Robert Nunez
Intellectual 570 points
Part Number: BQ76940

Good day, 

I am making a BMS for a 9S battery pack. The battery I use has a rating of 3200mAh (but actual is just about 3050mAh), and the quick charging current from its datasheet is 3A. The voltage levels of each of my cells are about 3.35V (whole pack is 30.11V). Note that I have my OV_TRIP at 4.2V. Using the charging conditions of 37V and 3A, there is an immediate OV fault. I tried the same with 2A, and the fault still occurred. Again I tried lowering the charging current to 1A but this time the fault didn't occur.

Is this a known issue, OV fault occurring depending on the charging current? Any suggestion I could do to resolve this issue?

  • 0
    TI__Guru 54715 points

    Hi Robert,

    It is unusual.  Three possibilities come to mind which you might check:

    1. Look for high resistance in your cell connection such as a weld which is "stuck" but has resistance.  If you have cell holders check for contact resistance.  Voltage across the connection resistance will show up in one cell or the next depending on where the tap to the strap is connected. With the cell holder it would be on the cell. Read the cell voltages from your BMS or the BQ76940 if you have a debug cable.  OV takes time for the part to recognize and should be apparent in the readings.

    2. Check for open wires from the cell tap to the BMS board.  This seems less likely since the filter capacitors should divide any increased voltage due to charging. Again cell voltages should show in values read from the device.

    3. Check if your BMS controller may be balancing and the voltages are reported high.  If the Cc values are large the filter may still be settling and you may measure a larger voltage than actual causing the OV to trip.  Again voltages should show up when read from the device.  This condition may cause OV if near the OV limit, less likely to vary with current.