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BQ78350-R1: Valid Charge Termination - Which are exactly the conditions to met it ?

Pietro Vignali
Intellectual 925 points
Part Number: BQ78350-R1

Hello,

I'm quite familiar with other older CEDV gauges, and the  capacity synchronization to FCC (with RSOC=100%) at charge termination, by reaching a voltage and current threshold during charge current tapering down.

This feature is handy and always present the battery at 100% to the user.

On studying a new 7S pack based on bq78350-r2 fw 2.01 build 25, I have difficulty to reach the same sync effect and RSOC does not reach 100%.

All the battery settings are correctly set:
CEDV Gauging Configuration:
CCT=1
CSYNC=1
SC=1
Advanced Charging Algorithm - Termination Config:
Charge Term Taper Current: 40mA (meaning 400mA, because I have IPSCALE0 =1)
Charge Term Voltage = 100mV

I noted during several test that the gauge never sync when the current taper down and reach those thresholds.

On doing other tests with different settings I noted instead this:
Setting Charge Term Taper Current: 200mA (meaning 2000mA because of IPSCALE0=1), the Valid Charge  Termination is met after 40s the Average Capacity is below ~116mA (meaning 1160mA)
Setting Charge Term Taper Current: 100mA (meaning 1000mA because of IPSCALE0=1), the Valid Charge  Termination is met after 40s the Average Capacity is below ~60mA (meaning 600mA)
Setting Charge Term Taper Current: 80mA (meaning 800mA because of IPSCALE0=1), the Valid Charge  Termination is met after 40s the Average Capacity is below ~20mA (meaning 200mA)

Please note the attached charts taken during my tests (the Remaining capacity was artificially dropped by hand before each run, in order to simplify the test).

Question:

what are exactly the conditions to met the valid charge temriation ?
The voltage threshold is ok, all cells are > 4200-100 = 4100mV
How is the current interpreted ?
Does the IPSCALE setting interfere with the calculations ?

Thanks in advance

Pietro

  • 0
    TI__Guru* 81034 points

    Hi Pietro,

    I notice your question is very similar to your thread from early this month: https://e2e.ti.com/support/power_management/battery_management/f/180/t/721301

    Just to clarify, are all questions still open?

    Can you tell me how you have this configured in hardware? Are you using the EVM and do you have the individual cell voltages connected? Have you gone through the calibration process for the device to make sure the gauge is accurately measuring voltage and current?

    Below is the valid charge termination conditions that must be met.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 925 points
    Hello Matt,
    the previous question was posted by me, but at that time I had a different fw in the gauge. In fact Batt inspected the SREC and found out the very old version, and suggested me to upgrade to R2 build 25, and so I did.
    After having installed the newer firmware and re-applied the previous calibrating settings that I saved, I made further tests, and instead continuing on that thread I made a new one, sorry for the confusion (I will close that thread).

    As you can see I noticed that the fuel gauge detect the valid charge termination sooner or later, but I still don't understand when exactly this happens.

    I have gone through the Valid Charge Termination conditions several time, and I can confirm that:
    - the batery is charging , DSG=0
    - Average Current: you see from the chart
    - Max (CellVoltage 1-7) + 100mV > Charging Voltage/7, in fact all cells > 4100mV and ChargingVoltage/7=4200mV, so this condition is met
    - the accumulated charge capacity > 0.25mAh. This is hard to confirm from my side. I know that the pack was charging during all the time so I presume it has accumulated correctly. Since I have IPSCALE0=1, I expect that this calculation is scaled accordingly.

    I expect that the two 40-s period to occur one after each after the conditions are first met, but from the charts above it does not seems like that, and this confuse me a bit.

    Having to release battery in the market soon, It is important for me to understand this process because I need the user to read RSOC=100% at the end of charge.

    I can provide srec and GG if needed.
    Thanks in advance.
    Pietro
  • 0 in reply to Matt Sunna
    Intellectual 925 points

    Matt,

    just to be sure I recalibrated the gauge and made further test.

    I notice that the gauge always met a Valid Charge Termination (with RemCap synced to FCC), but this happnes much later than expected, around ~250-300sec after the Average Current has dropped below the threshold set in the DF. Please see the charts below.

    Is this the correct behaviour ?

    Can you please explain exactly why the temrination is detected after this delay ?

    Thanks in advance

    Pietro

  • 0 in reply to Pietro Vignali
    Intellectual 925 points
    Matt,
    I forget to tell that this setup consists of a Li-Ion 7S2P pack with a cusomt designed 7S BMS PCB based on bq78350-R1 upgraded to R2 v2.01.
    Pietro
  • 0 in reply to Pietro Vignali
    TI__Guru* 81034 points
    Hi Pietro,

    I sent your data to our firmware engineer today to see if he can explain the longer than expected time for RSOC to adjust. I would expect this to be about 80 seconds based on the documentation. Hopefully I will have an explanation tomorrow. Sorry for the delay.

    Matt
  • 0 in reply to Matt Sunna
    TI__Guru* 81034 points
    Hi Pietro,

    I learned that the condition for Average_Current < Taper Current is actually slightly different in the implementation. In reality average current is not directly used. Instead charge accumulated during the 40s window is converted into current and that is used to check against charge term taper current. Most of the time it does not make any difference but sometime depending on charge accumulation it may take more or less time to satisfy this condition.

    I'm not sure if this explains the delays you are observing but it may be something to look at. Can you go ahead and send me your .gg file?

    Thanks,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 925 points

    Hi Matt,

    thanks for the explanation, even if it is a bit complex to uderstand exactly...

    I will try to make other tests with different battery RSOC and Current Taper Threshold to try to understand the behaviour.

    Please find attached the GG file.

    7S2P_valid_charge_term_evaluation.gg.zip

    Thanks in advance

    Pietro

  • 0 in reply to Pietro Vignali
    TI__Guru* 81034 points
    Thanks for sending your file Pietro. I don't see anything unusual in your settings that would explain additional delay. Let me know if the new explanation of the behavior lines up with what you see from your tests.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 925 points

    Matt,

    thanks for exhamining the GG file.
    Playing a bit more with the Charge Term Current Threashold and the SOC of the battery at the time of entering in the Current Taper phase did not change so much. Practically, I noticed that the delay in detecting a Valid Charge Termination happens with a similar delat of ~300msec.

    Based on this tests, I decided to set a threshold at 100 (=1000mA) because it will cause a Valid Charge Term at about 400mA, that is my goal.

    I also noted that if, a battery almost full is placed in charge, with the current tapering down almost immediately after having reached an initial peak of ~400mA or so, it cause a Valid ChargeTermination more quickly, after about ~160sec..

    At the end, I can't determine when and why this happens exactly (a bit frustrating), but at least it "happens" and so I can expect that the batteries in the field  will always  sync to RSOC = 100% at the end of each charge, that is my initial desire.

    In case you have more complete explanations to the Valid Charge Termination process, please let me know.

    Thanls and Regards
    Pietro