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Robotics application that uses off-the-shelf LifePo4 battery packs

Jonathan Navor
Genius 9475 points
Other Parts Discussed in Thread: BQ30Z50, BQ40Z50, BQ34110, BQ34Z100-G1

Hi Guys,

Good day. I am posting this in behalf of the customer and we need your help.

Here is the full inquiry below:

I am designing a robotics application that uses off-the-shelf LifePo4 battery packs. I am not permitted to rebuild the battery packs but I would like to incorporate some type of "fuel gauging" to monitor the batteries using a microcontroller. I am aware this is not the typical use-case, and I am also aware that external monitoring may not be as accurate, though I suspect it would be much more accurate than simply using a timer (wall clock) approach since there is no way to predict upfront what the current draw would be for any particular run. (For reference, I am using the Bioenno 12.8v 9Ah, and 12.8v 3Ah battery packs).I am considering two potential cases, both external to the battery pack. The first is simply monitoring the batteries at the terminals. The other is the possible ability to modify the battery packs enough to slip a temperature probe inside. I'm not sure how much difference that would ultimately make, however. I have contacted Bioenno several times about the possibility to purchasing a pre-made battery with communication, or custom-refitting batteries for me to include communication. They are not able to assist in either capacity. My number one goal is to get a reasonable idea of how much battery capacity is left based on the charge/discharge characteristics (coulomb-counting). Secondarily, it would be nice to also know the voltage and present current flow from the battery. But primarily I'm interested in knowing when the robot must return to its charger based on its workload and battery power profile.  Can you please assist me with this? 

I read your app sheet(https://www.ti.com/lit/an/slyt528/slyt528.pdf) and it's a reasonable suggestion. My concern is the cost of setup software, and whether or not my proposed concept would work okay. (I'm familiar with the 'bq30z50/55' chips, from other battery packs- though can’t use on this project due to size).

I would like to *EXTERNALLY* monitor a set of NON-SMART LifePo4 packs that I have w/o rebuilding them, though I would dedicate a chip to each one.  Is this something you can help me with? Also, it would be nice if there was some pre-built board I could get that wouldn't break the bank.

Thank you for your time and support. 

Best regards,

Jonathan

  • 0
    TI__Guru* 81034 points

    Hi Jonathan,

    Unfortunately, you would need to modify the packs to incorporate any type of gauging. Battery gauge ICs must measure cell voltages, current, and cell temperatures - accurate values for these are critical to gauging. For older lead acid batteries, the discharge curve is more linear, so voltage alone could be used to estimate SOC, but for lithium batteries, it is a much more difficult problem. LifePO4 batteries are particularly challenging because the discharge curve has a very long flat region where the voltage changes at a very slow rate. The cell voltage will vary with temperature, load current, and age of the battery - so voltage alone will be a very poor way to measure state-of-charge.

    I realize this may be disappointing news, but it is important to know what you are getting into. If you have access to cell voltages, pack current, and cell temperature, then incorporating a gauging IC would be possible. The BQ40Z50 is the most commonly used gauge for 4S packs. There is a good video on going through all of the steps for the BQ40Z50 here: https://training.ti.com/bq40z50-setup-and-going-production

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Genius 9475 points

    Hi Matt,

    Thank you for your response. Here is the follow up question from our customer. 

    Customer's response below:

    How is monitoring a pack is overly different from monitoring individual batteries inside the pack, particularly since what I'm looking for is fuel-gauging, and there's already a BMS inside the pack for managing charge and so forth. Just no way of communicating remaining charger, and voltage / current information back to a controller. I know that it's typically done as an all-in-one type arrangement, and typically they incorporate battery temperature into the equation-- but if willing to accept some minor loss of fidelity due to that-- otherwise I'm not quite understanding the difference between doing it externally vs internally.

    I hadn't thought about it until you said it... but I actually do have access to temperature and voltage, on the BMS board that is currently built-into the battery. The BMS that's already in the pack has to deal with the same stuff, even though it doesn't communicate any of the details I'm interested in to an external controller. It *might* be possible for me to find out more about the onboard BMS from Bioenno. Additionally, I guess I'd be okay in swapping out the BMS's for something better, if I knew what it was / where to obtain it. And then there would still presumably be the issue of monitoring / setup software to do the initial parameter setup..?

    Thank you and regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Guru* 81034 points

    Hi Jonathan,

    Here is a video you can share with your customer that covers the fundamentals of battery gauging: https://training.ti.com/fundamentals-battery-gauging-algorithm

    I understand they are willing to accept a minor loss of fidelity, but it is not that simple. Battery gauge ICs are very complex and do not have options to give up one of the variables. They need to be used as designed to function. And yes, there is a complex setup process for gauges - they are not really plug-and-play. The video I shared in my first response illustrates this process for the BQ40Z50. 

    The BQ34110 and BQ34Z100-G1 are gauges that do not require connections to each individual cell, but they still require the ability to measure the pack current and cell temperature and they do require significant work for parameter setttings.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Genius 9475 points

    Hi Matt,

    Our customer wanted to thank you for your support and they appreciate it a lot. Moving on with their inquiries. 

    If they are willing to refit the battery pack, Please share an off-the-shelf BMS board with communication (don't care what kind as long as it's *WIRED* [*not* wireless / bluetooth]) that you would recommend?

    The current BMS (inside the pack) will already have the temperature sensors, and access to individual batteries. Again, they don't mind refitting a BMS board, but if you have any recommendation to make it easier please share us. 

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Guru* 81034 points

    Hi Jonathan,

    Unfortunately I am not very familiar with the BMS boards available on the market. We are experts on the TI ICs.

    I would search for a 4S BMS with gauge looking for one that uses the BQ40Z50. They will need to find one that meets their specific parameters.

    Best regards,

    Matt