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BQ78350-R1: bq78350-R1 SOC and Chem ID

Larry Norman
Intellectual 855 points
Part Number:

I have a pack model with 15S3P LiFePO4. The cells are K226650UE01, and the GPC recommended Chem ID 0457.

With the pack charged to full:

  • Anytime I do a re-set with Chem ID 0457 the SOC is 18%.
  • I tried multiple different LiFePO4 IDs, and it is always 18% after a re-set.
  • I have tried multiple different Li-Ion Chem IDs, and the SOC is 0% after a re-set (this is with the cell voltages at ~3530mV).
  • I have even tried several of the Lead and NiMH IDs, and the SOC is always 18% with these after a re-set.

I would not expect the SOC estimation to be exact, but I would think that I should get something other than 18% among a dozen or so different Chem ID's.

At 15S, 10x scaling is used - could this be a factor? I think I have all the flags correctly set per the TRM. I am not using VAUX at all (tied to VSS).

Any help would be appreciated.

Thanks,

Larry

  • 0
    TI__Guru 54715 points
    Hi Larry,
    The SOC estimation at reset is based on the voltage and chemistry. If the voltage does not change the value should be the same, but changing chemistries I would agree, it seems it should change. It will take a few days to look at this.
    How are you loading the different chemistries?
  • 0 in reply to WM5295
    Intellectual 855 points
    I am using the chem ID tab of bqStudio.
  • 0 in reply to Larry Norman
    Intellectual 855 points

    So.

    I figured it out; DOD at EDV2 is a component of the Initial Capacity upon a Reset. The value in this field was 8800 (not sure where that came from), so I reset it to the default of 15232.

    However, the calculation method for the DOD at EDV2 value is expressed incorrectly:

    " DOD at EDV2 should be initialized to (1 – Battery Low%) × 16384, where Battery Low% = Battery_Low% ÷ 2.56 "

    This calculates to be nowhere near 15232 at a Battery_Low% of 7%.

    In the TRM (page 62), Battery_Low% is said to be expressed units of ".01%", and the default value is 700. However, in BqStudio, 700 cannot be entered, it must be entered as 7.00 as the units are actual %.

    In the calculation for DOD at EDV2, to get a value close to the default, the value used for Battery_Low% must be .07, and not the value of Battery_Low% from the TRM (700) or Battery_Low% from the bqStudio entry (7.00).

    Using .07 in the equation yeilds 15936. The default value of 15232 calculates out to a Battery_Low% of .18, or 18%.

  • 0 in reply to Larry Norman
    TI__Guru 54715 points
    Hi Larry,
    The chem ID tab in bqStudio is the way to load chemistry.
    Glad you found the problem.
    DOD at EDV2 does update. You may have reached an inappropriate EDV2 during your evaluation and changing chemistries and learned a very low number. Learned FCC is limited by the FCC Learn Up/Down limits, not sure if DOD at EDV2 is limited.
    The technical reference manual was updated last month for many accumulated issues, www.ti.com/.../sluubd3, please see the updated document. The DOD at EDV2 description was corrected in that update to remove the incorrect 2.56 factor. 93% of 16384 is 15237, close to default.
  • 0 in reply to WM5295
    Intellectual 855 points

    Thanks for the update.

    Is there a place to subscribe to a list that gives notifications of updates like this?

  • 0 in reply to Larry Norman
    TI__Guru 54715 points

    Hi Larry,

    If you have a My TI account you can sign up for "alerts" with the "Alert me" button next to the product name in the product folder on the web.  The document update produced a mention in an end of week email like this:

    Product Part Number: BQ78350-R1

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