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Capacity estimation at low temperatures

Mats55098
Prodigy 120 points
Other Parts Discussed in Thread: BQ20Z65

Hi

I have designed a Battery Pack with bq20z65 and 4 lithium ion cells in a 2p2 configuration.

Capacity estimation seems to be very correct at room temperature, RemCap gets a good value immediately after reset and tracks the situation very well during charge and discharge.

- After a full charge at room temperature does the EVSW show RemCap/FCC = 2400/2400 mAh (which is correct).
- I lower the temperature to -20 C, EVSW still shows RemCap/FCC= 2400/2400 mAh   (The EVSW reads the temperature correctly ~-20C)
- I discharge with constant current = 1200 mA, during the first 75 minutes do RemCap decrease to 900 mAh but FCC is still 2400, when suddenly it jumps to RemCap/FCC = 0/1500, and the battery is empty.

It seems to me that only "coloumb counting" is used for continuos update of RemCap and FCC update after discharge, and no corrections are based on temperature or Impedance.    (The circuit did measure the temperature, and the cell voltages mesured at 1200 mA, -20 C are significantly lower than at room temperature so the basis for a correct estimation seems to be there)

Do you have any idea of what I am doing wrong or have misunderstood? 

Best Regards

/Mats

 

  • 0
    TI__Expert 3865 points

    Hi Mats,

    Please check if:

    1. Did you program the correct chemistry to the device?

    2. Did you allow relaxation after charging and discharging?

    At which % did RSOC drop to 0% suddenly? Can you provide a datalog when the problem occurred for further investigation?

  • 0 in reply to Jackie Hui
    Prodigy 120 points

    Hi Jackie thanks for your reply!

    1. Yes I think so, my cells are Sanyo UF653450S, and the manufacturer access command 0008 returns 0107.

    2. I think so, I did not monitor relaxation mode, but before starting discharge the unit was at rest for at least 30 minutes.    Also during one of the runs I switched off the load a few times during the discharge which can be seen in the attached log.   (Quit current=10 mA, DSGRelaxTime=1 s)

    RSOC is not in the log but dividing RemCap with FCC gives 34%  (which sudden drops to 0 % from sample 618 to 619)

    Please take a look att the attached log.
    (Is relaxation time needed for updating FCC???   In this case I used a constant current load which I manually switched on and off a few times.
    In our real application will typically the battery rest for some time after charging (over night) so that relaxation mode can be entered.    BUT after the application has been turned on it will usually stay on consuming current until the battery is close to empty.   Large drops in temperature like +20 to -20 C may occur, which affects the capacity significantly)

    Regards

    /Mats

     

      

     

     

     

  • 0 in reply to Mats55098
    TI__Expert 3865 points

    Mats,

    We will need the complete datalog (logging all the flags) in order to analyze the problem. Also, it will be helpful if you provide the room temp datalog as well.

    Another thing you can try is to run a 2nd cold temp cycle to check if the accuracy improves. This means you charge the battery at room, rest at room, sock for at least 30min at cold, and start discharge at cold.

  • 0 in reply to Jackie Hui
    Prodigy 30 points

    Hi Mats,

    Hi everyone,


    Could you (or anyone else) please let me know how you managed to solve this problem? I am experiencing almost the same problems: At low temperature (-15 to -20 °C), the Full charge capacity, the remaining capacity and the absolute state of charge drop to zero. The relative state of charge goes suddenly to 100 % (even if the battery is empty)

    I am using the BQ20Z45R1 and Lithium-Ion batteries (2 Cells in series). I have done the learning cycle with required relaxation periods (using bqEASY).

     

    Thanks in advance and best regards,

    Adamou

  • 0 in reply to Adamou Bouba
    TI__Genius 10235 points

    Can you provide a log file and a before and after GG file of the situation? Also, what chem ID are you using in the gauge? Do you know if it's the correct ID for your cell? This is a fairly common problem for low temps and high discharge loads. There might be some things we can tweak to make the gauge work better.