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BQ34Z100-G1: Gas Gauge Sense Resistor Calculation

Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQSTUDIO, BQ34Z100EVM, EV2400, GPCCHEM, BQ34Z100

Hi All,

I am just about to add the BQ34Z100-G1 Gas Gauge to a new PCB design, but I cannot see on the datasheet how to calculate the sense resistor. The datasheet does quote:

"The value of the current sense resistor should be entered into both CC Gain and CC Delta parameters in the Data Flash Calibration section of the Evaluation Software"

So my next question is, where do I get a copy of the Evaluation Software?

Look Forward to your reply.

  • Hi Rocketman46,

    To physically size the hardware:

    The standard current sense resistor that comes with the EVM is 10 mOhm. The device supports up to +/- 125 mV across the sense resistor. With a 10 mOhm sense resistor this works out to maximum of 12.5 A.  What is the maximum current are you expecting for your design?

    You're absolutely correct, the CC Gain and CC Delta parameters are how the device is set to the current sense resistor selected for your design. The easiest way to set these values is to apply a known load to the EVM and utilize the current calibration feature in bqStudio.

    The evaluation software, bqStudio, may be downloaded here: www.ti.com/.../bqstudio

    Please use the 'bqstudiotest' version, as it is the latest and greatest.

    Sincerely,
    Bryan Kahler

  • HI Bryan,

    Thank you for your reply.

    I will require this chip to be used on a 24V battery and currents upto 40A.  Could you offer a starting sense resistor for the evaluation board?

    Kind regards,

    Rocketman46

  • Hi Bryan,

    I have downloaded the The evaluation software, bqStudio, and I selected the below options - note dev board not yet plugged in:

    # Gauge

    # 0100_0_16-bq34z100G1.bqz

    # Auto Detect Device: None

    An error is displayed " The detected device is not compatible with this application"

    Does this therefore mean I need different software for the BQ34Z100-G1 chip?

    Thanks,

    Rocketman46
  • Hi again Bryan,

    A couple more questions for you:


    1) With the above resistor selected, how do i connect the BQ34Z100EVM board to my PC which is running the bqStudio software. I have only been supplied a 4-wire header with this development board.

    2) Do you have a bullet point step by step guide to development?


    Thanks,

    Rocketman46

  • Hi Rocketman46,

    The standard current sense resistor that comes with the EVM is 10 mOhm. The device supports up to 125 mV across the sense resistor. With a 10 mOhm sense resistor this works out to maximum of 12.5 A.

    Knowing the maximum allowed drop across the sense resistor, and that your application must support currents up to 40 A, a quick application of Ohm's Law will tell us that a sense resistor valued at ~ 3 mOhm may suffice. Please make sure to select a current resistor with low PPM and appropriate power rating.

    The evaluation board is only rated up to 7A. Either a custom board will need to be created or a separate board attached to your EVM will be required for your high current path. For more information on the EVM, please refer to the EVM User's Guide found here: www.ti.com/.../sluu904

    With respect to bqStudio, everything is working properly. Just click through the dialog menu to select the gauge you are interested in. To use the software with the gauge, please connect the gauge to the EV2400 via I2C using the keyed 4 wire connector, plug the EV2400 into the PC using the provided USB cable, and set the jumpers and power the board with an external voltage source as described in the EVM User's manual, linked above.

    As for a bullet-point list for development, please refer to Section 8 of the Datasheet for more information on how to configure the hardware and bring up a device initially. Once brought up, and the parameters in the gauge are adjusted for your specific application, the ChemID of the battery will need to be determined using the GPCCHEM tool (this will require generating a log - this is described in the GPCCHEM documentation). The ChemID returned from the tool will then be added to the gauge using the CHEMISTRY tab in bqStudio.  With the guage now fully set up and with the proper chemistry, IT_ENABLE is set and a learning cycle is run.

    Sincerely,
    Bryan Kahler

  • Hi Bryan,

    Thanks for your reply.

     I don't have the EV2400 so I will have to miss this step, so I will be writing the code straight to the micro and communicating to the BQ34Z100-G1 using I2C.  

    Just to confirm I have a few more questions:

    1) How do I work out the battery parameters without using the EV2400.  

    2) Does the EV2400 create all the variables for the c source code and add into a header file.

    3) Do you have example c source code for communicating with the BQ34Z100-G1 using I2C and a detailed list of all the parameters.

    4) Why would you use HDQ over I2C

    5) If the current requirement of my project ranges from 10A - 40A, do I pick the worst case sense resistor of 3mR?

    Thanks,

    Rocketman46

  • Hi Rocketman46,

    1) I strongly recommend using the bqStudio + EV2300/EV2400 toolchain to development, especially if you are a first time user of these products. The toolchain simplifies the process significantly, especially calibration. The easiest way to develop with a MCU is to configure the gauge with this toolchain, and then export the DFFS file or BQFS file. The DFFS (data flash only) and BQFS (instruction + data flash) contain i2c commands that may be easily implemented on your MCU to program the gauge. In addition, bqStudio + EV2400 are incredibly helpful in logging the gauge. These tools are here to enable and expedite development. Having to recreate these functions on an MCU may be cost prohibitive.

    2) The EV2400 is a communications interface to allow bqStudio to communicate with the gauge over i2c. Think of it as an adapter.

    3) There is example c code available in the gauge communication app note found here: www.ti.com/.../slua801.pdf

    It provides a good example of how program the DFFS file to the gauge with a MCU.

    4) i2c is recommended, but some customers use hdq for a single wire interface. It is slower than i2c and we recommend programming your gauge via i2c and if switching to hdq, to do so as one of the last steps during production.

    5) Yes.

    Sincerely,
    Bryan Kahler
  • Hi Bryan,

    I am getting upto speed now. I have a few more questions:

    1) Do you recommended going for the EV2400 over the EV2300?

    2) with the BQ34Z100-G1 fitted to my PCB and connected to the battery. Do i just connect 3 wires to the BQ34Z100-G1, SDA, SCL and 0V. Therefore the EV2400 is not powering the BQ34Z100-G1, the Micro/ Battery PCB is powerering the BQ34Z100-G1?

    Thanks,

    Rocketman46
  • Hi Rocketman46,

    1. The EV2300 is NRND and the EV2400 is recommended, however, both devices work assuming they have been updated with the latest firmware.

    2. Correct. In addition to the I2C connection above, please connect a voltage supply to Bat + and Bat - on the bq34z100 EVM and increase to 4 V for initial device bring-up.

    Sincerely,
    Bryan Kahler
  • Hi Bryan,

    I have been studying the datasheet and I may of found a problem.  I will be using a 2mR resistor due to the current peaking at 50A.  But the datasheet quotes below on page 29 the the sense resistor should be between (5mR to 20mR typ).  Does this mean this chip is not suitable for a 24V, 40-50A appliacitons?

    Thanks,

    Rocketman46

    7.3.5 Fuel Gauging

    The bq34z100-G1 measures the cell voltage, temperature, and current to determine the battery SOC based in the Impedance Track algorithm (refer to Theory and Implementation of Impedance Track Battery Fuel-Gauging Algorithm Application Report [SLUA450] for more information). The bq34z100-G1 monitors charge and discharge activity by sensing the voltage across a small-value resistor (5 mΩ to 20 mΩ typ.) between the SRP and SRN pins and in-series with the cell. By integrating charge passing through the battery, the cell’s SOC is adjusted during battery charge or discharge.

  • Hi Rocketman46,

    5 mOhm to 20 mOhm is the typical range for the sense resistors with this device. The device is able to support larger currents and smaller sense resistors, such as a 2 mOhm sense resistor, but as a tradeoff at low currents, may require modification of some parameters, such as CC deadband and offset to prevent 'ghost currents' from being read.

    As this application will be in excess of 32 A, please also refer to this app note: www.ti.com/.../slua760

    Sincerely,
    Bryan Kahler