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BQ76952: CELL BALANCING

Govinda Giri
Prodigy 95 points
Part Number: BQ76952

 Hi Expert

I'm working on bq7695202 based 16S BMS, In cycle test with different batteries I observed that

1)Cell 1 voltage becomes a little lesser than other cell voltages after a few charging-Discharging cycles.
ex CV1 = 3.69V while CV2 to CV15 = 3.86 volt (approx).

2) Cell balancing is slower, although based on answers provided on this forum for more current  I have changed series Resistance to CV pins from 100 Ohm in the past to 51 Ohm now

In the past I had shared my Register setting with You so I guess there is nothing from the software side. I'm attaching a schematic of my BMS kindly review it if there is anything that I missed.
3463.Cell balancing.pdf

Regards
Govinda

  • 0
    TI__Guru* 81034 points

    Hi Govinda,

    I do not see anything in the schematic that would explain why Cell1 would discharge more than other cells. Is this observation the same on more than one battery or is it possible that you have a weaker cell on one specific battery?

    You should use 100 Ohms on the VC pins since you are using external FETs with 20 Ohm resistors to set the balancing current. If the VC pin resistance is too small, there will not be enough voltage on the gate of the cell balancing FETs to turn them on. The 20 Ohm resistors set your balancing current. According to the datasheet for your FETs, the RDSON is very high until the VGS is around 1.5V.

    This is not related to the issue, but the capacitor on REGIN should be 22nF. There should also be a 1uF capacitor on the collector of Q1.

    Best regards,

    Matt