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BQ77915: Why the Vfc threshold discarded and balance trigged by only OV event?

József Hermann
Expert 1040 points
Part Number: BQ77915

Dear Forum

I built my own designed 5-cell pack utilizing '77915-01 device. I experienced the unusal balance operation:

staring conditions: cells are: 3,58 ; 3,58 ; 3,58 ;3,58 ;3,72V (lowest cell in the string)

The balance started only tandem with Overvoltage (4,25V) event and did not start if no longer OV occured but a lower Vfc threshold reached despite the charge current developed enough voltage for Vstate_comp (>1,875mV) (on the shunt).

Therefore the balance worked only partially: altough it could eliminate the Overvoltage apperence, it was not able to effectively balance the cells even not if multiply cycles applied.

The '77915 shall balance any time if full charge voltage reached (Vfc) regardless of cell relationships (>Vstep etc). I think for -01 variant the Vfc = 4,15V (=Vov-Vhyst=4,25-100mV). This is confirmed by balance off: at 4,15V occured prior to turning ON the CFET (ca 4,05 which is OK because Hyst_ov=200mV). Because in case of OV-trigged balance, it Will be OFF when dropping cell voltage below Vfc or Vov-HYST_ov which reached earlier.

Note, I use external balance NFETs. Balance ON/OFF info taken by VC1-GND measurement.

I have graphical data: there were many chg/dchg cycles but I show the last one when balance was estabilished. /*Unfortunately the current curve inverted because I tied the shunt voltage with opposite directon to the recorder input :( */

CBI pulled down by 10K. I use series diode at LD pin (and JP8 shorted):

Joseph

  • 0
    TI__Expert 7465 points

    Hi Joseph,

    If you have not already, I would highly recommend taking a look at this video which describes the cell balancing feature of the device. It is a very complicated feature that is best explained in the video.

    It is important to note that cell balancing will not start until one cell is below the given threshold and one is above. For example, the VSTART of the BQ7791501 is 3.8V and VSTEP is 100mV. If your cells are at 3.8V, 3.8V, 3.8V, 3.8V, and 3.9V, cell balancing will not occur. This is because none of the cells are below the threshold. Similarly, your device will not balance if your cell voltages are 3.8V, 3.8V, 3.8V, 3.8V, and 3.7V. 

    In order for the cells to start balancing, they would need to look something like 3.8V, 3.8V, 3.8V, 3.7V, and 3.9V. In this scenario, one cell (3.7V) is below the lower threshold (Vstart in this example) and one cell (3.9V) is above the upper threshold. 

    Yes, there is also a second balancing mode where all cells above Vfc will balance. You are correct, Vfc for BQ7791501 is 4.15V. If any of the cells go above that voltage, they will be balanced out. But keep in mind, if all cells are very similar, there is no where for the cell balancing current to go and they will not balance. There still has to be some difference in voltage. 

  • 0 in reply to Shawn Hinkle
    Expert 1040 points

    Hi Shawn,

    I have already took an account to the video, very informative.

    Yes you have right: if cell voltages are very close and charging voltage CV=n*Vfc, then only small charge current Will flow (CV part of the charging): below the Vstate level->no balance

    I found the problem:

    I used external shunt and FETs and the 77915 current sens points were not connected to this shunt. Therefore 77915 always measured Ichg=0A. I Will use the on board (77915 connected) shunt (unfortunately, the on board FET was small that's why I used external and the connection was easier to use external shunt too but I forget to connect to the '77915)

    Thanks for triggering the Ichg related investigation!

    Joseph