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High Capacity Battery Fuel Gauge

Javier Vara
Expert 1625 points
Other Parts Discussed in Thread: BQ34Z100

Hello

I am using BQ34Z100-G1 to monitor the state of charge of a truck battery. The battery is 225Ah with max current of 1200A, although in practice it does not go beyond 300A. It is a 12-cell lead acid battery with a nominal voltage of 24V.

My question is related to the scaling factors. Since it is bigger than 32A, I thought that I could use a 10x scaling factor (300/10 = 30 < 32). But when it comes to calculate the design energy my understanding is that it should be: 225 000 mAh * 2 V = 450 000 mWh, and using the mentioned scale factor it would be 45 000 mWh which is beyond the limit (32 000 mWh). Should I use a new scaling factor of, lets say 15, so that the energy design is below 32000 mWh or use the energy scale factor to account for that?

Thank you

Javier

  • 0
    Expert 1625 points

    Another question I have is related to the Flash Update OK Cell Volt.

    In the datasheet it says that the value should be: 2800*Number of Cells (12) * 5000/ Voltage Divider

    But in the application report of the SLUA664 where the parameters are explained it says that the value should be:
    Desired Cell mV for no flash updates*Number of Cells (12) * 5000/ Voltage Divider

    In my case the Desired Cell mV for no flash updates I guess it should be around 1750 mV (end-of-discharge voltage of the lead acid cell). As far as I understand if I where to use 2800mV as desired voltage for no flash updates it would never update because the voltage would always be below that threshold. Am I right? Which value should I use?

    Thanks

  • 0 in reply to Javier Vara
    TI__Guru 73830 points
    Javier
    The important thing is that that pack voltage be at least 3.3V to 3.5V at the minimum cell voltage. If you have a 12S pack, then you can set it to 1000mV and you will be okay.
    Tom
  • 0 in reply to ThomasCosby
    TI__Guru 73830 points
    Supporting 1200A is going to be difficult. You should size the sense resistor to keep the voltage drop across SRN/SRP is less than 125mV, but definitely less than 300mV. That would put the sense resistor in the 125mV/1200A = 100 uohm. That means that there will only be a 3.3mV drop across the interface when the current is 32A. What are the conditions where the interface will be exposed to 1200A? You may have to consider using a Hall sensor device, instead of a sense resistor. I have not tried this, but other have implemented this.
  • 0 in reply to ThomasCosby
    Expert 1625 points

    We are indeed using a 100uOhm sense resistor though the maximum current we expect is around 300 Amps. That's why we have added an operational amplifier with adjustable gain before the bq34z100. Although right now is not being used.

    As it is right now since the maximum current we expect is around 300 Amps, the drop across the interface would be around 30mV. Is that enough? or should I make use of the amplifier?

  • 0 in reply to Javier Vara
    TI__Guru 73830 points
    Javier
    If your maximum current is 300A, instead of 1200A, then I would use a larger sense resistor to provide better resolution and accuracy.
    Tom
  • 0 in reply to ThomasCosby
    Expert 1625 points

    The sense resistor used is embedded within the enclousure used itself and there is no way to use a different one that is why we added an amplifier.

    Should we then make use of the amplifier or leave it with less resolution as it is right know? Right now we are inclined not to use it because it adds extra offsets and drifts with unpredictable behaviour but we have the option to do it if necessary.

    If we leave it as it is right now, based on my first post, should we increase the scaling factor to 15 or make use of the energy scale factor?

  • 0 in reply to Javier Vara
    TI__Guru 73830 points
    Javier,
    If you increase the scale to 15, then that is going to reduce the range on the current even more. I guess that you will have to try running it with the 10x scale and see how it will perform. This is going to be a challenge.

    Tom
  • 0 in reply to ThomasCosby
    Expert 1625 points
    Thomas Cosby,

    In that case, since the design energy (with the 10x scale factor) would be 45 000 mWh and it is higher than 32 000 mWh (the maximum), should I set the design energy scale factor to 2, and set the design energy to 22 500 mWh? Is there any other change necessary before running the calibration cycle?


    Javier
  • 0 in reply to Javier Vara
    TI__Guru 73830 points
    Yes, you could use the Design Energy Scale to bring the Design Energy within range.