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BQ78350: BQ78350 - Encountered several situations

GaryC
Expert 3295 points
Part Number: BQ78350
Other Parts Discussed in Thread: BQSTUDIO

Our model BQ78350 has encountered several situations in 6S1P application, Can you help me analyze it?

Before leaving the factory, we will put the battery power below 30%, Some customers report that the battery cannot be charged or discharged.

1. One part is normal charging and unable to discharge. Reading the Gauge status is SOV, but the actual battery voltage is not high. After unlocking (PF Clear), it can be used normally. Is it caused by float voltage? Is there no anti-blocking mechanism inside the Gauge?

2. Part of the battery is low and cannot be charged (the actual voltage is about 20.8V). The gauge status is SOV, VIMR, but the actual battery voltage is not high or unbalanced. After unlocking (PF Clear), it can be used normally . What is the possibility to trigger SOV and VIMR under low voltage conditions?

3. One part is that the battery is high (about 80%) and cannot be charged (the actual voltage is about 20.8V). The gauge status is SOV, VIMR, but the actual battery voltage is not high or unbalanced. After unlocking (PF Clear), the gauge returns high battery and cannot be used normally. After re-burning the Gauge FW, the gauge reports low battery (about 8%) and can be used normally.

  • In the absence of PF, what mechanism does the Gauge have to make charging invalid?
  • Isn’t the capacity according to an algorithm?
  • Why is there such a large difference in capacity?

BR, Gary

  • 0
    TI__Guru* 81034 points

    Hi Gary,

    Can you share the data flash settings for the BQ78350? It would be good to check the settings related to SOV and VIMR. 

    It would be best if you can export the data flash for one of the failing packs. If you can connect to BQStudio, you can click 'Export' from the Data Memory screen to export the .gg.csv file.

    The SOC reported by the gauge is based on cell voltage only at power-up, so when you re-burn the firmware it will estimate the SOC based on the voltage using the data for the ChemID that is selected. After this point the gauge will use coulomb counting (counting the charge into and out of the battery). When it reaches full charge, the SOC will sync to 100% (if the CSYNC bit is set in the gauging parameters) and when the battery discharges down to around 7% SOC, the EDV2 voltage will be detected and the Full Charge Capacity of the battery will be updated in data flash. This is all described in this document: https://www.ti.com/lit/an/slua924/slua924.pdf

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Expert 3295 points

    Hi Matt,

    Thanks for your support.

    Please help confirm whether there is a problem with the parameter.
    Chemistry ID: SDI INR18650-20S3

    Attached .gg.csv file

    gauge.gg.csv

  • 0 in reply to GaryC
    TI__Guru* 81034 points

    Hi Gary,

    I looked through the data flash settings. It is hard to tell what might cause the failures you have seen without a log of the event. Are you able to repeat the failing sequence on any of these batteries? If so, it would be good to collect a log in BQStudio (click Start Log on the Register screen) to see exactly what is happening.

    A few things I observe:

    - The AFE SOV threshold is set to 4350 mV which is only slightly higher than the COV threshold of 4300 mV. The AFE SOV cannot be disabled since it is a hardware protection, but it might be good to set the threshold slightly higher. I am not sure why the cells would be hitting this threshold though.

    - The Cell Balancing start voltage and VIMR start voltage are both set much lower than the default. They are both set to 2900 mV. The VIMR Delta Threshold is set low than default (200mV vs. 500mV). I wonder if there is measurement variation during balancing that might be affecting the readings which might cause a false trigger of SOV and VIMR. What values are you using for the cell input resistors and capacitors - this can affect the variation observed during balancing.

    - The device is configured to automatically Shutdown after 2 hours. When the device powers back up, it loses the Remaining Capacity it has been tracking and the gauge has to estimate SOC based on voltage only. SOC estimates will be less accurate coming out of Shutdown.

    Best regards,

    Matt 

  • 0 in reply to Matt Sunna
    TI__Expert 3295 points

    Hi Matt,

    Thanks for your help.

    We had got the two fail report for you reference.

    22.gg.csv

    1037.gg.csv

    BR, Gary

  • 0 in reply to GaryC
    TI__Guru* 81034 points

    Hi Gary,

    Thanks for sending these. In both files, if you look at the PF Status data, you can see the cell voltages had a large spread when the permanent failure occurred. Do you know what might be causing the cell voltages to diverge so much? It looks like you have cell balancing enabled - do you have enough balancing current in the hardware design (are you using external balancing FETs)? 

    Regards,

    Matt