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BQ76940: 14S system, middle group draining excesive current

Alejandro Veloz Savino
Prodigy 55 points
Part Number: BQ76940

Hi, I have a system of 14 cells and the group of 5 cells (vc5 to vc10) is draining more current than the other two groups.

In the beginning, I had bad measurements on this group of cells, I replaced the CAP2 to VC5X and the measurements are ok, but now the draining on this group is excessive.

I connected an array of 14 2.2k resistors in series to simulate the cells and the voltages that the BQ76940 is reading in mV are the following

Cell 1 3905
Cell 2 3819
Cell 3 3819
Cell 4 3849
Cell 5 3853
Cell 6 2627
Cell 7 2793
Cell 8 2775
Cell 9 2787
Cell 10 2751
Cell 11 3810
Cell 12 3825
Cell 13 3800
Cell 14 3857

 As can be seen, the voltage from Cell 6 to Cell 10 is considerably lower as if there were a resistor in parallel of around 6k for each of these cells or a 30k resistor connected between VC10 and VC5

This means that the BQ76940 is damaged or it could be any of the external components like the thermistors filter capacitor or the RC filter for VC10x or VC5x?

  • 0
    TI__Guru 54715 points

    Hi Alejandro,

    The VC10X and VC5X pins are intermediate power pins, normally the supply current for these is on average low as shown in the data sheet.  The measurements above and as you suspect indicate more current on the middle group.  You might confirm this by measuring the voltage across the power filter resistors Rf for VC10X and VC5X.  These are commonly 1k ohm for the BQ76940 and so 1 mV measured = 1 uA consumed. Look for the average value not the peak which may occur if using external thermistors. You may see excess current into the VC10X net and out the VC5X net. This could be through the Cf filter capacitor (leakage, contamination), contamination between the nets on the board, or into the IC.  If it is in the IC it could be damage to the IC or leakage of the CAP2 capacitor.  Since you mention replacing the CAP2 capacitor check for contamination or leakage at this capacitor or surrounding traces.  Pins 11 and 12 also connect to CAP2 and may be opportunities for contamination.

  • 0 in reply to WM5295
    Prodigy 55 points

    Hi, I just received another part with the same problem and I noticed that the capacitor on CAP2 was broken, also, I measured the current thru the Rf for VC10X and VC5X and there was a current on the Rf(VC10X) of around 800uA and around -800uA  on Rf(VC5X) this means that there is a loop current from VC10X to VC5X, this current is present even when the IC is shutdown.

  • 0 in reply to Alejandro Veloz Savino
    TI__Guru 54715 points

    Hi Alejandro,

    It is curious that CAP2 is broken and current is excessive on the middle group also when off.  If the current went away when it was shutdown it could be current into the broken CAP2 capacitor.  Check if the Cf capacitor for the middle group might also be broken causing high current.  If not it is likely a leakage through some contamination or through the IC.  You might check history on the board to see if it may have had some high voltage. High voltage across the middle cell group exceeding its abs max could cause the IC to be damaged and draw high current, I'm not sure of the failure modes of the capacitors.