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BQ77915: How to determine the cell voltage sensing capacitor value

Shivani Ghorpade
Prodigy 190 points
Part Number: BQ77915

Hello,

I want to understand the calculations that are needed to determine the cell voltage sensing (VCx pins) filter capacitor value. As per the Design example, the resistance value Rin is calculated from the charge taper current but how to determine the filter capacitor.

Also the recommended value for filter capacitor with internal cell balancing is specified as 1microF and the capacitor value with external cell balancing is specified as 0.1microF in the datasheet. But, the design example having internal cell balancing is set to have filter capacitor of 0.1microF. This is confusing.

Request to explain this in more detail.

Regards,

Shivani Ghorpade

  • 0
    TI__Guru 54715 points

    Hi Shivani,

    When balancing the part will duty cycle between balancing and measuring to determine if other values are safe and if balancing should stop.  See figure 6 in the BQ77915 data sheet.  

    With internal balancing a high internal balancing current can be used, see the limits and recommended values in the data sheet (RINI and CINI) which you note.  The larger filter cap with the small resistor provides a simple time constant (VC0) of 33 us.  With external balancing lower current can be used internal to the IC, see (RINE and CINE).  The simple time constant is 100 us.  If the filter time constant is made much longer the filter will still be settling when the voltage sample is taken and OV or UV and balancing control may not occur at the threshold expected.  When selecting a different resistor value to tune the balancing current calculate a time constant and select a capacitor to give a time constant in the range shown.  Always test to confirm performance.

    . 

  • 0 in reply to WM5295
    Prodigy 190 points

    Hello,

    Can you please explain more by giving an example?

    Thank you.

    Regards,

    Shivani Ghorpade

  • 0 in reply to Shivani Ghorpade
    Prodigy 190 points

    It would help more if you can explain how the filter capacitor Cin is calculated as 0.1microF in the design example.

    Regards,

    Shivani Ghorpade

  • 0 in reply to Shivani Ghorpade
    TI__Guru 54715 points

    Hi Shivani,

    Perhaps you decide you want about 20 mA balance current at 4.2V before duty cycling. So you calculate you want a total resistance of 4.2/.02 =  210 ohms.  RIN x 2 + RBAL = 210, RBAL is 12 typical, so you select 100 ohm for RIN.

    RINE x CINE = 1k x 0.1 uF = 100 us.  Using this time constant & calculating a capacitor for your selected resistance: 100 us/100 ohm = 1 uF.

    RINI x CINI = 33 x 1 uF = 33 us.  Using this time constant & calculating a capacitor for your selected resistance: 33 us/100 ohm = 0.33 uF.  

    So now you have a range, both are common values as is 0.47 uF, and there are some others which are readily available.  You may decide you already are using 1 uF caps elsewhere in your production and the added volume for this build will get you a better rate, so you pick that one, or another value based on your particular interest. 

  • 0 in reply to WM5295
    TI__Guru 54715 points

    Hi Shivani,

    I think I see a point of confusion for the BQ77915 design example.  In Table 8 of the design example RINE and CINE values are listed for the RIN and CIN for the design.  Later in 10.2.2.1 step 1 it discusses selecting RIN as 75 ohm but does not comment on CIN leaving it at 0.1 uF.  This would make the time constant shorter than the range above.  There could be more noise in the sampling than with the other designs, but the part should not see incorrect samples as it might with long time constant values. With the process above 100us/75 = 1.3 uF or 33us/75 = 0.44 uF, so 0.47 or 1 uF may have been more consistent choices.

  • 0 in reply to WM5295
    Prodigy 190 points

    Hello,

    Thank you for your response, It solved many of my open queries. But I have one more question here regarding the cell balancing current.

    As you mentioned, the cell balancing current of 20mA in your example and we can use internal cell balancing in this case. But In my case, my battery pack is 10Ah with charge current of 0.5C which is 5A. So if I consider the termination current of 10%, my balancing current shall be 500mA which is high for internal cell balancing and external cell balancing FETs shall be used. Please correct my understanding if wrong. The example considers 0.05C charge current which is very low. Is this feasible.

    Regards,

    Shivani Ghorpade