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BQ40Z80: External Cell Balancing Example Schematic question

Part Number: BQ40Z80

Dear TI Experts,

My customer want to design a 7S pack with BQ40z80. They need external cell balancing function and find the example schematic in BQ40z80 Technical Reference as below.

They found that every capacitance from Vcell Pin is connected to GND, however,

in the BQ40z80RevA Schematic from bq40z80EVM, the internal cell balancing schematic demo, the capacitance is connected to adjacent Vcell Pins.

Are these two schematics both right,external cell balancing need Vcell PINs capacitance to GND, and internal cell balancing need Vcell PINs capacitance to adjacent PINs?

Thanks a lot.

the demo of external cell balancing schematic from BQ40z80 TRM:

the demo of internal cell balancing schematic from BQ40z80EVM :

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  • Dear Batt,
    Got it. We checked several cell ballancing reference schematics, and not sure about the difference of these 2 capacitance circuit . Please kindly help to advise which one is the right circuit for these cell capacitance and how it works when you're available. Thanks a lot for your attention.
  • Dear Batt,
    Could you kindly help to give some advice for this question? Thanks a lot.
  • You don't need the caps that go to GND in the external balancing circuit. They don't hurt having, but are probably not needed. So long as they go to adjacent cell inputs you should be OK.
  • Dear Batt,

     Thanks for your reply. How about the capacitance connected to adjacent Vcell Pins? Is it required for external balancing circuit as internal cell balancing circuit ?

  • Hi Bruce,

    Separate comment - the figure you have pasted above from the TRM has been changed since when you pulled this one. The latest figure 8-4 is slightly different in the connections near the 7th cell vs the one you show here. However, it does still show the caps to gnd at each cell input. This is not necessary, it is ok to instead change the caps as being between adjacent cell pins. The only exception is the 7th cell, since the device is measuring that cell using a divided-down version of the 7P voltage, so there is no need for a cap between the VC6 pin and where the 7th cell gets measured (VC7SENSE), you instead have a cap at the VC7SENSE pin to gnd.

    When using external cell balancing, it is recommended to reduce the series R going to the VC pins from 1k to approximately 200-Ohms, to avoid impacting the cell voltage measurement. You will need to ensure this still provides enough Vgs to turn on your external cell balancing FETs. This needs to be corrected in that figure as well.

    Thanks,

    Terry
  • Dear Terry,
    Got it, thanks a lot for your support.