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SOC MEASUREMENT FOR 27 CELLS

Camilo Suarez
Prodigy 20 points
Other Parts Discussed in Thread: BQ34Z100, BQ78PL114, BQ76930, BQ76PL536, BQ76940, BQ78350

Hi, I need to implement active balancing for a battery bank having 27 Li-ion cells with a capacity of 15600mAh and a total voltage of 100V. I read about bq78pl114 but it is just for 12 cells and it is not recommended for Li-PO batteries which I could use in a future. Reading another forum it was recommended to use EMB1428 and EMB1499 as a solution for active balancing, but it is needed to implement the balancing algorithm, I would like to use one algorithm based on individual SOC of each cell, which would be the better solution? I think there must be an smarter way than using bq34z100 for each cell could you help me please?

Another question, why the battery capacitty determines the right IC gauge?, my application is for EV but there a lot of IC with SOC estimation for batteries having 16000 mAh using impedance track technology but they are intended to be used in portable electronics, but impendance track is not found for IC intended for batteries with a capacity above 32000mAh.

Thanks a lot

  • 0
    TI__Intellectual 2795 points

    A balancing algorithm for active balancing is pretty straight forward to be developed on your own.

    Depending on your requirements, it would be possible to use the bq34z100 as a gauge solution. As active balancing can keep cells balanced at all points in the charge/discharge cycle, the bq34z100 individual cell SoC could be assumed for all cells. An MCU would be needed to implement the active cell balancing algorithm integrating SoC data from the bq34z100. Depending on your current load, you may need to scale the bq34z100 results.

  • 0 in reply to Stephen Holland
    Prodigy 20 points

    thnaks for you cooperation, let me know if I understood, I would need to use one bq34z100 for every one of the cells, 27 bq34z100 for 27 cells, is it rigth?.  For protecting the cells I am thinking to use the bq76930 IC, it was projected for 10 cell, is it possible to use three of them  in a stacked array for covering the 27 cells?.

  • 0 in reply to Camilo Suarez
    TI__Guru 54715 points

    If using the bq34z100, you would use one for the pack, it will gauge based on the average cell voltage.  Refer to the data on that part.   For that gauge to work properly the cells must maintain balance.  You will need a monitor function such as the bq76PL536 to measure cell voltages.  A device designed to stack such as the bq76PL536 will be easier to implement than a non-stacking device such as the bq76930. If using the bq76930 to measure you will need level shifters to bring communication to a common voltage for the MCU which will operate the balancing and perform any protection functions.  If you want to use the hardware protection features from the bq76930s you will need to level shift the CHG and DSG outputs of each to a common point, use logic to combine them and drive the pack FETs or switching device.

    Once you have your MCU reading cell voltages, implementing any desired protection and controlling your balancing algorithm, you may decide if you want to implement your own state of charge algorithm also or interface to the bq34z100 separately.  You could use more than one bq34z100 if it has a cell count limit, each part would need to be level shifted back to the common interface reference and its status combined.  Questions on the bq34z100 would be best considered in the Gas Gauge Forum.

    Be sure to consider isolation requirements in your system.

  • 0 in reply to WM5295
    TI__Genius 12215 points
    Camilo/Willy/Stephen

    For 27s application, how about using 2*(bq76940+bq78350), then use a MCU to read SOC from each bq78350? As bq78350 can be configured to compute the SOC based on the lowest votlage cell or average voltage of all cells it monitors? A MCU can access the information from 2 bq78350 with an I2C isolator?
  • 0 in reply to Steven Yao
    TI__Guru 54715 points
    Yes, another possible architecture.
    Also fixed a part number typo above.