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Original question:

BQ78350: SOC jumps from 43% to 7% after calibration.

BQ78350: SOC when setting the EDV_CMP bit

Jarni V
Prodigy 20 points
Part Number: BQ78350

Hi,

I have a question regarding the EDV_CMP bit.

If EDV_CMP is set to 1 and a full discharge is performed until reaching CUV:Threshold, when can we expect the capacity state of charge to sync to 0%? (e.g. prior to reaching cuv soc was 2% and then it hits cuv, will it update the remaining capacity and soc to 0?)

Additionally, is there any correlation between the chemistry ID and the parameters EDV0, 1, 2?

We have a bms with chem_id set to 2250 and one to 2591, both having EDV_CMP = 0. The one on 2250 smoothly goes down from 100% -> 0%, the one with 2591 goes down smoothly until reaching 35V (3.5V/cell) and jumps to 7% at that point, which seems to be CEDV2. Is this somehow related to above mentioned questions?

Best regards,
Jarni




  • 0
    Prodigy 20 points

    Here are the Data Flash settings of both of the batteries.

    Working correctly Chem ID 2250,
    SOC not working correctly Chem ID 2591

    BQ78350_CONFIG_CHEMID_2591.20240424.gg.csv
    BQ78350_CONFIG_CHEMID_2250.gg.csv

  • 0 in reply to Jarni V
    TI__Guru 61246 points

    The jump to 7% happens because cell voltage drops below EDV2. This is by design. EDV2 defines the 7% SOC point. The gauge relies on previously measured FCC. If FCC is correct, then SOC will be (close to) 7% when cell voltage drops below EDV2 (and there will be no jump). If you get a jump from a higher SOC value, FCC was too high (for example, the previous qualifying discharge for FCC updates happened at a significantly lower average current and/or temperature was significantly higher.

    CUV is a protection threshold. The gauge doesn't use this for SOC calculations. The gauge uses EDV0 as the 0% SOC reference point. EDV0 and CUV are separate parameters and while they may be correlated in your application, they are independent for gauging and protection.

  • 0 in reply to Dominik Hartl11
    Prodigy 20 points

    Thank you for your response,

    What does the programming of the chemistry actually do? Does it have an influence on how the state of charge will be calculated?

    Also could you please explain what the DOD at EDV2 (Fuel guaging, state) does? The default is 15232 but what would happen if it would be set to 10474? 

  • 0 in reply to Jarni V
    TI__Mastermind 37761 points

    Hey Jarni,

    Dominik is currently out of office, he should get back to you later this week.

    Regards,

    Nick Richards

  • 0 in reply to Jarni V
    TI__Guru 61246 points

    The gauge uses the ChemID to correlate open circuit voltage with depth of discharge and therefore SOC.

    Depth Of Discharge at EDV2 does directly what the name says, it assigns the DOD for a voltage of EDV2. This means that the gauge will force SOC to reflect DOD at EDV2 when voltage drops to EDV2.

    If you set DOD at EDV2 to 10474, DOD = 10474 / 16384 = 0.64. SOC is roughly 100% * (1 - 0.64) = 36%. This means that the gauge will force SOC to 36% when cell voltage drops to EDV2. Note that the gauge uses this threshold (which is related to Bat low %) to calculate FCC. If you set this to fall into the flat zone of the loaded voltage curve, you may get a rather large error.

  • 0 in reply to Dominik Hartl11
    Prodigy 20 points

    Thank you,

    Currently when we discharge with 9A or 12A we do not have the jump. When we have a lower current like 2A the error (/jump) in SOC still occurs.
    But we are doing the logging of different currents and temperatures for the gauge parameter calculator to see if we can resolve the problem on our side with the new configurations.

    Is there also something to set the soc back to 100% based on the voltage, like EDV2 does for the 7%?

  • 0 in reply to Jarni V
    TI__Guru 61246 points

    This gauge relies on FCC measurements from previous discharges. If your system discharges with 9A, the gauge learns FCC for 9A. If you then discharge with 2A, the gauge uses the FCC that it measured previously and there will be a discontinuity in SOC.

    Do you get SOC jumps for repeat discharges with 2A (does the gauge update FCC eventually avoiding SOC jumps)?

    A valid charge termination detection will set SOC to 100%. This is based on voltage and current during charging (close to end of charge). See 5.3 Valid Charge Termination, www.ti.com/.../sluubd3e.pdf