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High Voltage Battery Pack Fuel Gauge IC Usage

Carlos Ramos21
Prodigy 240 points

Other Parts Discussed in Thread: BQ76PL536A, BQ34110, BQ34Z100-G1, BQ78350, BQ34Z100, EV2400, BQSTUDIO

Hello,

I am trying to add and design a fuel gauge capability to a 96 count battery pack. Are there any IC I can use for this kind of application? Can I use one of the fuel gauge IC you guys have and used them in series?

Thanks,

Carlos

  • 0
    TI__Guru* 76230 points
    Carlos,
    We do not have any gauges that can support this many batteries. But we do have the bq76pl536A or the bq76pl455 which are stack-able battery monitors. You will have to write your own gauging algorithm, using the data provided by these gauges.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 240 points
    Cam you recommend me an algorithm to do fuel gauging (papers or documents)? Also, what would be the most effective method for battery fuel gauging in EV vehicles? What is the trend you have seen in the market on this kind of applications?

    Thanks

    Carlos
  • 0 in reply to Carlos Ramos21
    TI__Guru* 76230 points
    that is not going to be a simple answer. Battery gauging is complex and if you will be writing your own algorithm, you will need to thoroughly understand how they operate.

    we have some materials here that may be of help:
    www.ti.com/.../battery-management-products-support-training.page

    For will want to use a combination of voltage look up and coulomb counting for the battery gauging.
    thanks
    Onyx
  • 0
    TI__Expert 5300 points

    If you do some scaling on the results of the gauge, you can consider the bq34z100-G1 or bq34110.  They can measure the top of a high voltage battery stack, divided down with a switched resistive divider, and can provide gauging for the full pack.  The devices will natively support stacks up to 65V, but you can then do scaling to handle higher stack voltages (i.e., tricking the gauge into thinking the stack is lower voltage, then multiplying the results it generates accordingly).  The resolution of the gauging will get worse as the divider ratio gets larger, so depending on your resolutiion needs, this may not be acceptable.

  • 0 in reply to -Terry
    Prodigy 240 points
    Terry,

    Have you done this before? And if you can, could you please provide a schematic on how to do this? How about the current sense shunt to perform coulumb counting? It also says that the limit is 29Ah, how do I scale that?

    Thanks,

    Carlos
  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points
    Look at www.ti.com/.../slua760.pdf which describes the procedure (it can be used with both devices). The schematic can be based on our EVM schematic, you just modify the divider resistor settings so the voltage after divide is still in the acceptable range (0V-1V). For the current, you basically use a smaller sense resistor (such as 1mOhm) but program the gauge as if you are using a larger resistor (such as 10mOhm). So the gauge will measure the IR drop across the sense resistor and report a current, then you will scale that number in your system by 10X to get the actual current. A similar scheme is used for the voltage scaling, as the document describes.
  • 0 in reply to -Terry
    Prodigy 240 points
    Will that work with both devices aforementioned in the past post? Would that work with other devices you guys have?
  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points
    Current scaling can be used with any gauge. Voltage scaling can only be used with these two products.
  • 0 in reply to -Terry
    Prodigy 240 points
    Speaking of the gauging resolution, what would I lose if I use this in comparison to other gauging schemes? Could you tell me about a case where you have used either of the devices mentioned in a HV application? Can you tell me about the BQ's ability and resolution to generate gauge data in terms of granularity in HV compared to LV?

    -Carlos
  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points
    Assume your system is 96 cells in series, so max voltage of the stack is approx. 400V. Think of it as if the gauge measures this voltage with a divide-by-96, which gives the average voltage of one cell in the entire stack. Then the gauging algorithm operates on this voltage and the measured current to calculate capacity, health, etc.

    This is a simplified explanation, actually the divider needs to reduce the applied voltage down to <1V rather than just the voltage of a single cell, and the gauge calculations internally take into account the scale factors to model it as a single cell. In your case, you will have a resistive divider that takes the 400V down to <1V, then the gauge measures that divided down voltage with resolution better than 50uV. You can also calibrate the gauge so the exact divider value is used in the calculations. The measured voltage divided down will have some noise due to the resistor dividers, but as long as your resistances are not too large, this should not be a problem for you. We have had multiple customers using these devices with stacks in excess of 65V, which is why we published that application note I pointed you to.
  • 0 in reply to -Terry
    Prodigy 240 points
    Basically, by using really good resistors in my divider and good cooling I can reduce that thermal noise. How about the thermistor? Do I need the thermistor for SoC-SoH compensation (Is it crucial for the operation)? For a battery pack of many cells, where do you recommend to put the thermistor?
  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points
    You will get thermal noise even at room temperature (recall a resistor gives an rms noise voltage of sqrt(4kTR*BW) or 4nV/sqrt(BW), so a 1kOhm at room temp across a 10kHz BW has around 400nVrms noise.

    You will definitely want temperature measurement of the pack, since cell impedances change significantly with temperature, and these are modeled extensively by the gauge to calculate capacity. The gauge can either use a single thermistor or the internal die temperature, selected by a flash setting. You will also require a protection solution which uses temperature to determine when to disable the pack from charging or discharging if outside allowed temperature limits based on the cell specs, that will probably require multiple thermistors measuring different places in the cell stack, and then at least one thermistor should be near the hottest place in your pack, wherever that may be.
  • 0 in reply to -Terry
    Prodigy 240 points

    What is the purpose of using the internal die temperature? So explaining myself, I should put the single thermistor coming from the BQ device on the hottest place of the battery pack so I have a better impedance tracking compensation, is that right? Also, can I measure the temperature using other methodologies (micro to BQ via i2c) and send a battery pack average temp data to the device?

  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points
    The internal die temp is an option if a customer doesn't want to use a thermistor, but the gauge PCB is placed so closely to the cells that it can get an accurate measurement of cell temp. Since you are calculating the capacity of an average of many cells, ideally for gauging you have a temp that is the average of all the cells. However, your protection solution will generally need to know the maximum temperature, so that will probably require separate/multiple temp sensors. This gauge does not support writing a temp into the device.
  • 0 in reply to -Terry
    Prodigy 240 points
    Thank you so much for the help! Should any problem arise while developing this solution, I'll contact this forum again. I fell more confident about this device.

    -Carlos
  • 0 in reply to Carlos Ramos21
    Prodigy 240 points
    Hello,

    Is there any workflow to get this device functional or the devices will learn how to calculate SoC and SoH on their own with each charge and discharge cycle?

    Thanks
  • 0 in reply to Carlos Ramos21
    TI__Guru* 76230 points
    hi Carlos,
    What gauge is this. If it is the bq78350, pls refer to our online tool gpcedv. It has detailed instructions on what to do.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 240 points
    Onyx and Terry,

    I was referring to bq34z100-G1 or bq34110. I would like to know the workflow to get the SoC and SoH running on a high voltage (96S, 403V) battery pack. Is it possible to use the BQ tools and burn the BQ firmware to the device using the BQ tools. Is it possible with the eval boards and usb converter? If not, is there a way to calibrate such high voltage battery with the BQ tool and a upgraded eval board and whatnot?

    -Carlos
  • 0 in reply to Carlos Ramos21
    Prodigy 240 points

    Onyx and Terry,

    Any suggestions?

  • 0 in reply to Carlos Ramos21
    TI__Guru* 76230 points
    bq34z100 or bq34110 will not support such high voltage
  • 0 in reply to Onyx Ahiakwo
    Prodigy 240 points
    Onyx,

    You are posting contradicting information. Your coworker Terry, posted in this thread that in fact bq34z100-G1 or bq34110, I could use both devices for high voltage application. I mean, even he posted an application note on the fact. Who is wrong then?

    Again, I just want to know if I can use any of the evaluation board to test my application or if I have to manufacture one myself. Also, I want to test the BQtool with my high voltage application if possible.
  • 0 in reply to -Terry
    Prodigy 240 points
    Terry,

    Can I use/modify the EVM for 96 cells or do I have to modify the EVM for that. Also, these devices would require a firmware reflash (bQ studio or bQtools) , how do I do that, if possible, using the tools available on the TI store?

    -Carlos
  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points

    Hi Carlos,

    I haven't checked for certain, but I'm doubtful you can just modify the EVM.  Your voltages are so much higher that even if you changed the divider settings, you may still have other areas breaking down, such as the control FET not handling the new voltage range, or the traces and dielectrics handling the voltage.  You can always try if you'd like, but I would not be surprised if it didn't work.

    The EVM will already have the instruction flash (i.e., program code) and data flash (i.e., settings) as default.  You may not need to reflash the instruction flash, but you will need to change the data flash values to match your configuration.  That will require you to read through the TRM carefully, there are many settings that typically need adjustment for a particular application.  I'd recommend using bqStudio for this initially (together with ev2400), since you may make many changes to the parameters before you finalize them.

    Terry

  • 0 in reply to -Terry
    Prodigy 240 points
    Terry,

    For my application, do I need to create my own bq34z100-G1 that will handle those voltages, including my scaled down resistor ladder for 400V and shunt? If so, would the bqStudio see the battery as one with 65.xxxV or so?

    How about the EV2400? Is it safe to use this box to comunicate with the bq34z100-G1 using a PC with these voltages?

    Are there other workarounds for this, like for example characterize a smaller battery and use this data for the larger one?

    Can you give me a quick guidance on the workflow to program the bq34z100-G1 if I buy it from the manufacturer? Do they come in a clean slate? Like for example, if I can't really use bqStudio what should I do? Is there a way to get this device up and running without bqStudio or the EV2400?

    -Carlos
  • 0 in reply to Carlos Ramos21
    TI__Expert 5300 points
    Hi Carlos,

    You should not try to bring this up without using bqStudio and ev2400. The ev2400 only connects to the I2C bus on the gauge, it bridges comms between USB from your pc over to the I2C on the gauge. bqStudio is a free download, so that is easy to install and use.

    Because the ev2400 only talks to the low voltage I2C bus on the gauge, it doesn't matter if your stack voltage is high or low, it never connects to any of the high voltage directly.

    You will have to configure the settings in the gauge for your specific case. For example, you can divide your 400V down to between 0V - 1V, but configure the gauge as if it was really seeing 40V. So then when the gauge reports voltage output, you will multiply the readings by 10 in your host, to get the actual voltage.

    I would recommend you start with an EVM, use a lower voltage that it can support (such as 40V), make sure you understand how to configure the gauge for that case. Then next you can do an experiment using voltage scaling - for example, apply 40V but configure the gauge as if it is only 10V, so then all voltage readings from the gauge will need to be scaled by 4X in your host to get actual voltage.

    You will also need to configure the chemistry ID for the cells you are planning to use - this is described in the TRM and other application notes on the website. I recommend you go through this first on the EVM, to make sure you understand it well, before moving onto your dedicated board.

    Once you are comfortable with this approach, then you can build your own board for the 400V system.

    Thanks,

    Terry