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BQ78350-R1A: Cell Balance on 12S3P LiFePO4 pack

Lon Schneider
Prodigy 190 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ78350

.  Cycling is showing cell balance starting to converge but in successive charge  cycles, balance diverges.

 

The application is a 12S3P pack using LiFePO4 cells. Charge is 42 V (3.5V/cell to limit its going to COV).   Charge current is 10A with Taper at 600 mA.

 

Balance resistors are 10 Ω, so balance current is 350 mA.

 

Attached is a summary of charge and discharge cycles, and two CHG/DSG logs .  .

 

Cell Balancing Config

Cell Balance Threshold

3400

mV

Cell Balancing Config

Cell Balance Window

20

mV

Cell Balancing Config

Cell Balance Min

20

mV

Cell Balancing Config

Cell Balance Interval

2

s

 

Because balance time is limited, the EE chose an unusually short Interval. However, the log file does not seem to indicate that being the problem.

 

We are not getting the behavior related to Cell Balance Interval that is indicated in  the graph in app note SLUUC76. It shows cell balancing only occurring during Cell Balance Interval. Our Log data (1 second scans) shows the CB Flag in some cases running much longer without interruption than the 2 second time that is programmed.  Perhaps the interruption is too short to capture.

 

The log file attached for Cycle 3 notes the CB Flag in Operation Status, cell voltages and imbalance,  Columns at the right note when any cell voltage is > Cell Bal Thld, whether imbalance is > MIN,  and whether all cells are > Thld.  

 GG_rev5_pfrom_pack_9.gg.csvSUMMARY OF CYCLE TESTS.xlsx2304009 Cycle(3) CHG_DSG COPY FOR ANALYSIS homew.xlsx2304009 Cycle(5) CHG_DSG.xlsx

  • 0
    TI__Guru 57747 points

    Hello Lon,

    Thank you for providing the summary. We will try to go through it and respond

  • 0
    TI__Mastermind 23081 points

    Hello Lon,

    Lon Schneider said:
    Cycling is showing cell balance starting to converge but in successive charge  cycles, balance diverges.

    Please explain what you mean by diverges. The message is unclear to me.

    Lon Schneider said:
    We are not getting the behavior related to Cell Balance Interval that is indicated in  the graph in app note SLUUC76

    You should not be looking at SLUUC76 for cell balancing algorithm. You should look at  SLVA155

    Please see below how cell balancing works



    The  Cell Balance Interval is too fast in your settings and this might be a little bit overkill as the BQ78350 checks for conditions 1 and 2 in the above picture. Change this to 20 s and see if it helps. Updating too fast increases the processing load and hence the battery-electronics power consumption without any performance improvement. 

    Note: The adjusted cell balance threshold will reset at the start of every charge cycle. Try auto-exporting your gg file while charging and see if the Cell Balance Threshold is ever adjusted.

    I did not see the CB flag set in the operation status A register in the Cycle 3 notes. 

    Please share your schematic to check the filter resistor values. These should be 1k Ohms.

    The SLVA155 I shared earlier also exhibits Balancing procedures and common troubleshooting. 

    From SLVA155, see below


    Hope all these info is beneficial.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 190 points

    Hello Jose,

    Many thanks for the quick reply.    

    I had read the app note and the description in the TRM. That didn't fully explain how to interpret the significance of the Interval setting, but becausethe initial value was much faster than the default value of 20 seconds we have been testing with 20 seconds for the last couple of days. So far, Cell Balance is working with the Interval changed to 20 seconds.  Since I was not sure that this change was the cause for it working, I was very reassured by your response,  

    Our filter resistors are 1K. 

    I'd like to run a few more cycles before checking the RESOLVED box, but it is looking good so far. 

    Thanks again,

    Lon