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BQ78350-R1: Design voltage for learning cycle

Marius GROS
Prodigy 20 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, , , BQSTUDIO

Hello all,

I'm currently working on the bq78350 with a 6s8p battery pack.

I'm trying to start a learning cycle but according to this document :

https://www.tij.co.jp/jp/lit/an/slua903/slua903.pdf?ts=1623425773497&ref_url=https%253A%252F%252Fwww.google.com%252F

I need to set the good design voltage that is number of series cell * average cell voltage.

But the maximum value for the design voltage is 5000mV when the good value is 21600 for my battery pack.

I guess that the design voltage parameter on the bq78350 is for one cell so i can't refer to the document i've seen for a learning cycle ?

Thanks for your help.

Marius GROS,

Regards

  • 0
    TI__Guru* 81034 points

    Hi Marius,

    Yes, the Design Voltage parameter should be set based on one cell for the BQ78350. This learning cycle document is for gauges that use the Impedance Track algorithm. The BQ78350 uses a CEDV gauging algorithm which is different. Instead, you should use this document (refer to the Gas Gauging section): https://www.ti.com/lit/an/slua924/slua924.pdf

    I also recommend using the BQ78350-R1A. It is functionally identical to the BQ78350-R1, but it comes pre-loaded with the latest version of the firmware. The only difference is improved robustness to the Shutdown routine.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hello Matt,

    Thanks for your help.

    Another question, I think I didn't understand the "FCC learn down/up". I thought it was the amount of capacity that FCC can increase or decrease after a qualified discharge so I set it to 20000mAh (knowing that my pack is 28000mAh).

    But after a qualified discharge, my fcc updated at 48000mAh. I might not understand the real meaning of FCC learn up/down.

    Regards,

    Marius

  • 0 in reply to Marius GROS
    TI__Guru* 81034 points

    Hi Marius,

    You are correct, it is the amount that the FCC is allowed to change by. It does not mean it will change by that much though, so something else is wrong. I would recommend setting the FCC Learn down/up to smaller values because the capacity of a battery will change slowly as the battery ages - it should go down over time but may learn up at the beginning since the capacity will vary from cell to cell.

    More importantly, you will need to figure out why the FCC learned such a large value. You can click on the 'Start Log' button in BQStudio as you discharge the battery to record all of the registers when this happens.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hi Matt,

    I've done another discharge of the pack with a FCC learn up/down set at 15000mAh and the FCC updated at 63000mAh which means that he just added 15000 to the old value (48000mAh).

    I think that's a normal behavior because we have a capacity of 28000mAh and the gauge knows it but he add this to the older value instead of reset to 0 and then update with the new capacity.

    Here is the log of the discharge and the gg file of the bq.

    Test 7 - 15A Discharge.xlsx

    18062021.gg.csv

    Thanks for your help !

    Regards,

    Marius

  • 0 in reply to Marius GROS
    TI__Guru* 81034 points

    Hi Marius,

    The log is very abnormal. The SOC is reporting 100% no matter how much you discharge. I see some problems in the data flash settings that may be causing this behavior:

    • The Learned Full Charge Capacity is programmed to -2536. This should initially be set to the same value as the Design Capacity.
    • The Dod at EDV2 parameter is set to zero. It should be set to 15232.
    • I recommend setting FCC Learn Up and FCC Learn Down to much smaller values. Maybe use the default values.

    Can you try updating these values and then collecting a discharge log again?

    Best regards,

    Matt

  • +1 in reply to Matt Sunna
    Prodigy 20 points

    Hi Matt,

    Sorry for this late answer I was not available :)

    The battery pack is currently at the costumer workshop for some tests.

    I'll come back to you as soon as possible for the next updates about the log discharge !

    Best regards,

    Marius

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hello Matt,

    Just for you to know that the problem hasn't been resolved but I just start a learning cycle on a new battery pack (same specifications) with the new value for Learned FCC, DOD at EDV2 and FCC Learn Up/Down and it works well.

    So I'll export the gg file of this pack on the older one with the problem.

    I just have a new problem with the balacing and I'm working on it.

    Thanks for your help !

    Best regards,

    Marius