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Prototyping mods for bq24z100-g1 EVM

Peter Upczak
Intellectual 965 points
Other Parts Discussed in Thread: BQSTUDIO

BAT_SHUNT_04-0134_A-vis.pdf

Hi, I've attached the datasheet for our product battery shunt resistor. We use 2 in parallel for effective 0005 value @ 6W, our current from K-2 Energy LFP26650P can be > 80A. 

We believe for quick proto purposes that we can remove R30 shunt from the EVM and tap from across our product shunt right to EVM TB3 pins 2,3.

But what are the gain implications to the bq? Do we need additional external conditioning or would board & CC  calibration through bqStudio result in proper input / offset configuration in the chip?

Please advise, as best you can with what I'm able to give here.

Thanks,

  • 0
    Intellectual 965 points
    Any thoughts on this?
    I overstated the max current through our sense R: should be 60A on discharge only, not > 80A as originally stated. The charge current is limited by battery-pack's cell spec at around 2A, which our charger adheres to. 2A fits the limit on the EVM so I'm not too concerned with charging now.

    1. So let's add 10% margin on that discharge max, for (-)66A through 0.0025Ohm R = (-)0.165mV worst case, but still in operating range of our discharge spec. That's (-)40mV out of spec with SLUSBZ51 paragraph 9.2.2.1.4. At that load with our battery, it's ~ 10min run time.
    Is (-)40mV for 10min an issue for the SP/SN input on the EVM ?
    2. SLUSBZ51 paragraph 9.2.2.1.4. also states "the value of the current sense resistor should be entered into both CC gain and CC delta parameters ..." but I don't see these anywhere in data mem via bqStudio. How do I do this, or is it just a software thing for another program?

    thanks!
  • 0 in reply to Peter Upczak
    TI__Guru 73830 points

    Peter

    I would use a 1.5 mohm sense resistor to allow some margin on the ADC voltage range. You can use bqStudio to calibrate the gauge for the best accuracy. You can also enter the value directly in the CC Gain and CC Delta parameters in the Calibration section of the Data Memory. You can replace your .bqz with this one to enter the smaller value.

    Tom

    https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/6087.0100_5F00_0_5F00_16_2D00_bq34z100G1.bqz

  • 0 in reply to ThomasCosby
    Intellectual 965 points
    Tom - thanks much for the feedback. We'll try it out.
  • 0 in reply to Peter Upczak
    Intellectual 965 points
    Tom - I've learned there are a number of conditions where I > 90A for some number of seconds is considered normal operation for us. At 1.5mohms, the power cap requirement on the resistor package is prohibitive for the design, so now we're thinking about feeding the BQ sense input circuit from an op-amp.
    Is there anything non-obvious for the bq we'd need to consider in that?
    thanks,
  • 0 in reply to Peter Upczak
    TI__Guru 73830 points

    Peter,

    I have not used an OPAMP circuit to feed into the coulomb counter circuit, but it should be possible. I have had a couple of user use hall sensors for current sense, but I did not receive feedback as to the accuracy of the current measurement. You will need to consider the voltage range of the ADC and I would recommend maybe +/- 100mV for the range and the Input Offset of 10uV typical.


    Tom

  • 0 in reply to ThomasCosby
    Intellectual 965 points
    Interesting about hall sensors, we'll investigate that too.
    Thanks!