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BQ76940: FOR 48V 100AH BATTERY

Gudelli Thrishool
Prodigy 80 points
Part Number: BQ76940
Other Parts Discussed in Thread: CSD13381F4

Hello

i would like to make bms using bq76940 for my 48v 100AH  13s battery pack . but the maximum current is given as 20A.can you give me clarity on this

 thanking you

Thrishool G

  • 0
    TI__Guru 54715 points
    Hi Thrishool G,
    The bq76940 does not have a maximum current. It has a maximum measurement range and maximum current thresholds in mV. The protection limits have 2 ranges. See the bq76940 data sheet register definitions for Protect 1 and Protect 2.
    For high current, select an appropriately small sense resistor. Note however that the dynamic range is limited, if you have a large value to sense mA, you will not be able to handle lots of amps. If you have a small resistor to handle lots of amps, you will be limited in the resolution in current.
  • 0 in reply to WM5295
    Prodigy 80 points

    hi

     Tq for the response i have another query. i would like use  an external balancing .So what should be the rating MOSFET rating i need  to choose  to place across each mosfet. I have seen some document which recommended to use CSD13381F4 but the maximum current rating is 2A. MY doubt  is my charging current is 30A ,so during balancing if any of my cell exceeds the  voltage my mosfet starts to discharge the that excess but during this time my other cell will charging right,so during this current my does that my charging current of 30A will be passing through this balancing mosfet or not.in case it diverts then mosfet rating should be high.

    thanking you

    Thrishool G

  • 0 in reply to Gudelli Thrishool
    TI__Guru 54715 points
    Hi Thrishool G,
    The cell balance current will be low compared to the charge current. The current bypassed will heat the components and must be dissipated in the battery pack. Determine how much imbalance you expect, plan how much time you plan to allow balance and look at how much heat you can get out of your battery. Then you can determine how much current you need to bypass. Currents will normally be low, < 1A, but it depends on your design. The FETs should handle the current at the low Vgs voltage available. The resistors to dissipate the power should be selected similarly.
  • 0 in reply to WM5295
    Prodigy 80 points
    my charging current is 25A How to choose my external balancing mosfet.
  • 0 in reply to WM5295
    Prodigy 80 points
    is cell balancing current effect my battery charging current?how?
  • 0 in reply to Gudelli Thrishool
    TI__Guru 54715 points
    Hi Thrishool G,
    The balancing MOSFET would be chosen for the balancing current. If you decide to balance at 100 mA, you might pick a FET with good margin, maybe 1A. Of course the RDSON at the available gate voltage is also very important.
    Cell balancing current will not affect your charging current. Cell balancing will bypass some charging current around the cell being balanced. If you are charging at 25A and you are balancing one cell having implemented a 100 mA balance bath, you are charging most cells at 25A and the balanced cell at 25 - (0.1 x 0.70) = 24.93A. The balanced cell's voltage should rise more slowly and it will have fewer Ah in the cell. the 0.70 comes from the 70% duty cycle of the bq76940 balancing.
    Similarly if all your cells all discharge at 2 uA faster than one cell in the pack, every 1000 hours you will have a 2 mAh imbalance. You would expect to balance that high cell 2 mAh/(100 x 0.70)mA = 0.29 hours.
    You must do the calculations for your system, there is no easy answer. Also the bq76940 measures voltage, so your algorithm will likely balance based on an observed change in voltage.