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Original question:

BQ76920: SOC calculation using bq76920

BQ76920: Monitoring and Gauging of 4S pack during test flights of RC plane.

Joseph Landell
Prodigy 190 points
Part Number:

Hello,

We are designing a battery monitor/gas gauge for use with a 4S LiPo pack that can track, and report, cell voltage, current, impedance, SoC, temp, health, time to empty, and basically all the information we can get our hands on. We would like to have this monitor permanently installed inside a test craft, and used with multiple different LiPo packs, will this cause issues?

The BQ76920 paired with the BQ78350-R1 seems to be a popular combination. Is the BQ34110 also compatible with the BQ76920? If so, why is the BQ78350-R1 more commonly recommended?

We are also trying to determine if Impedance Track (IT) would be appropriate for our application. I understand that "there are certain application conditions that are not ideal [for Impedance Track]: highly pulsed loads and applications that do not allow rest periods." I am curious where this information is documented, and what defines a highly pulsed load, and what defines a rest period. Our test flights typically discharge the battery pack completely, would this mean there is no rest period?

If IT is suitable for our application, we would happily use an IT based gauge, including the BQ40Z80 or the BQ40Z50-R2 as stand alone solutions. 

Are there big disadvantages to using a CEDV gauge? Would impedance measurements be more reliable with an IT based device?

I am spending my days wading through the technical documents and forums that Texas Instruments has, but would appreciate feedback on the above.

Thank you,
Joe

  • 0
    Prodigy 190 points

    Joseph Landell said:

    Part Number: BQ76920

    Hello,

    We are designing a battery monitor/gas gauge for use with a 4S LiPo pack that can track, and report, cell voltage, current, impedance, SoC, temp, health, time to empty, and basically all the information we can get our hands on. We would like to have this monitor permanently installed inside a test craft, and used with multiple different LiPo packs, will this cause issues?

    The BQ76920 paired with the BQ78350-R1 seems to be a popular combination. Is the BQ34110 also compatible with the BQ76920? If so, why is the BQ78350-R1 more commonly recommended?

    We are also trying to determine if Impedance Track (IT) would be appropriate for our application. I understand that "there are certain application conditions that are not ideal [for Impedance Track]: highly pulsed loads and applications that do not allow rest periods." I am curious where this information is documented, and what defines a highly pulsed load, and what defines a rest period. Our test flights typically discharge the battery pack completely, would this mean there is no rest period?

    If IT is suitable for our application, we would happily use an IT based gauge, including the BQ40Z80 or the BQ40Z50-R2 as stand alone solutions. 

    Are there big disadvantages to using a CEDV gauge? Would impedance measurements be more reliable with an IT based device?

    I am spending my days wading through the technical documents and forums that Texas Instruments has, but would appreciate feedback on the above.

    Thank you,
    Joe

    Edit: Fixed Links.

  • 0 in reply to Joseph Landell
    TI__Genius 10830 points

    Hi Joseph,
    Thank you for your interest in our devices.
    Let me try to address some of your questions:

    • BQ76920 is often paired with BQ78350-R1 as BQ78350-R1 was designed as a companion gauge for BQ769x0, taking the data (T, I, V) measured by the BQ769x0 to calculate the State of Charge of the battery (among other things)
    • BQ34110 can be used with BQ76920 but BQ34110 is what we call a top of stack gauge aka it will consider a 10s pack for instance as if it was a 1s back (top of the stack). With BQ78350, you get the information of each cell, measured by BQ769x0.
    • for your question about IT vs. CEDV, I refer you to this training video.

  • 0 in reply to David Hien
    Prodigy 190 points

    Thanks for the reply!

    Did a bit of research on Impedance track, and decided it would work for our application. Since we think IT will work well, we are hoping to use the BQ40Z80 chip.

    One thing we are concerned about is our max discharge current. We would like to ensure that the system can handle a 200 amp load.

    I see that there is a "Scale Factor" that can be used when discharge currents exceed 32 amps. Is there a max value that the scale factor will allow? To accommodate a 200 amp discharge, we would need to use a 6.25 scale factor, am I understanding this correctly?

    Thanks again for your help!
    Joe

  • 0 in reply to Joseph Landell
    TI__Mastermind 42535 points

    Hi Joe,

    Yeah you can use IPScale but I'm concerned that at such high currents, your protections may be affected due to the load on the chg pump that drives the chg and dsg FETs. We are working on a design that can do 40A. As far as gauging goes, I don't think that presents a problem. But we've really not got a clear idea now as to how high the currents can be.

  • 0 in reply to Batt
    Prodigy 190 points

    Thanks,

    Also, 200 A discharges would only occur with multiple 6S packs, for just the one 6S pack, we likely wont see much above 100 A, if that.

    I understand that with the BQ40Z80, the voltage across the current sense resistor should not exceed 100 mV, with a current sense resistor value no lower than 1 mR. This would limit our discharge to 100 A.

    Am I understanding this correctly?

    We have considered different chips to use as a monitor, specifically the combination of a BQ78350-R1 and a BQ769x0. Reading through some info on that, I see the over current voltage range cant be set higher than 100 mV either. My question would be, can the sense resistor be lower than 1 mR on that? 

    As I see it, there are not good options to monitor and gauge a 6S battery pack that may exceed a 100 A discharge? 

    Between the BQ40Z80 and the BQ78350-R1 with a BQ769x0, which would make the more reliable system? 

    I appreciate the help!

  • 0 in reply to Joseph Landell
    TI__Mastermind 42535 points

    Hi Joseph,

    That is correct. At the moment, the bq78350-R1 is the more proven solution. We may come up with an upgraded design or a ref design in the future for the bq40z80.