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Original question:

BQ77904: sizing the Rsns resistor and how to configure the Vscd voltage threshold

BQ77905: BQ7790509: RSNS

Prathamesh Tadaka
Prodigy 185 points
Part Number: BQ77905

Hello,


Is it correct formula to calculate value of RSNS. I didn't find in the datasheet.

 If it is wrong then how to calculate RSNS Value. I want to calculate RSNS for 10A discharging rate.

  • 0
    TI__Genius 10830 points

    Hi Prathamesh,
    Thank you for your interest in BQ77905.
    Your attachment is missing: can you add it?

  • 0 in reply to David Hien
    Prodigy 185 points

    Hi David,

            I am using bq7790509pwr for 3s li-ion battery protection. In this we can adjust discharging current by using RSNS Resistor. I just want to know how to calculate value of RSNS resistor.




    Regards,

    Prathamesh

  • 0 in reply to Prathamesh Tadaka
    TI__Guru 54715 points

    Hi Prathamesh,

    Look at the protection thresholds of the BQ7790509.  Use Ohm's law to calculate the current V = I x R considering the tolerances and the most important thresholds for your system as whether they are "must pass" or "must trip" thresholds.  

  • 0 in reply to WM5295
    Prodigy 185 points

    Hi WM5295,

    Thanks for reply.

    I calculted from as per this formula:-

    RSNS= OCD1 Threshold / Iout = 50mV/12.5V = 4mOhm                      (Note= for bq7790509 OCD1 is  50mV)

    Testing :-

    3s = 12V (each 4V), 10A capable load is connected

    I used 4mOhm resistor. but as soon as connected load current draw 6A but voltage is decrease at load to 10V. I didn't understand what will be the reason of power loss? How to fix this?

    Regards,

    Prathamesh

  • 0 in reply to Prathamesh Tadaka
    TI__Guru 54715 points

    Hi Prathamesh,

    Yes, 4 mOhm should allow 12.5 A nominal with a 50 mV threshold for OCD1.

    The voltage of the cells with load will depend on the cell characteristics.  See the data sheet, there is typically a family of curves for the cell with different loads, the voltage is lower at higher current since the cell has resistance.  The PCB with the BMS system and the interconnect will also have resistance.  The 4 mOhm at 10A will give a 40 mV loss.  The FETs will have some resistance too, they may be similar depending on the FETs and configuration.  Interconnect is often welded or soldered and should have a low resistance and voltage drop, but be sure to measure.  Find the voltage drop and correct it, or add cell current capacity or change the cell current capability to avoid voltage droop with your load.