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Why BQ78350-R1 calculates FCC(learned full charge capacity ) incorrectly for ncr18650B?

sai sumanth
Prodigy 110 points
Other Parts Discussed in Thread: BQ76930, BQ78350-R1, BQ78350, BQSTUDIO

I am using BQ78350EVM(bq76930 AFE) with 6 cell panasonic NCR18650. Configuration and  test setup is as follow

There were multiple chemistry files for ncr18650b, so I selected one with higher CHEMICAL ID(ID =2012).

Configured design capacity to 3350,Design voltage= 3600, EDV0 =2500, EDV1= 2800, EDV2= 3125.(Got those values form datasheet), EDV compensation enabled.

EMF,R0,R1,C0,C1,TC are configures according to the values given in BQ78350-R1 datasheet.

For complete configuration please find the .srec file in the attachment.

Setup:

battery pack +ve and pack -ve terminals are connected to i-charger output. Battery has two parallel cell voltage ports, one connected to BQ76930(AFE in BQ78350-R evm)  and second connected to balancing port of i-charger.

i-charger configured to charge and discharge cycle mode,charging at 3A and discharging at 1A.

Started BQ78350 EVM and started logging in bqstudio, after few cycles I observed that learned capacity is decreasing and pending EDV was aprox 3500.  EDV2 was configured to 3125, so pending EDV has to be 3125,but why is it taking 3500? Because of the incorrect pending EDV FCC kept decreasing. I thing there is a bug in the firmware.

PFA of log file and .srec file. (change file extension from bq78350srec.txt to bq78350srec.srec)

Kindly help me in figuring out the issue.

3704.bq78350srec.txt

ncr18650cycle2.txt

  • 0
    TI__Guru* 76230 points
    Hi Sai
    pls configure your gauge CEDV coefficients following the instructions in the link below:

    www.ti.com/.../GPCCEDV
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 110 points
    Thanks Oynx, that solved my issue.
    Now I am running those tests for 6s1p, but using the coefficients I want to further extend to other battery config like 8s4p.
    I am keeping battery operating temperature range and CURRENT PER CELL same.
    Let say for 6s1p following are the values(taken from example report)
    EMF 3983
    EDVC0 382
    EDVC1 0
    EDVR1 17
    EDVR0 1495
    EDVT0 4238
    EDVTC 7

    I make the following calculation for for 8s4p
    EMF =3983*8 //EDV*(no. of series cells)
    EDVC0 =382*4 //EDVC0* (no. of parallel cells)
    EDVC1 =0 // remeining capacity remains 0
    EDVR1 8*(17/4) //(no.series cells)*(EDVR1/(no of parallel cells))
    EDVR0 8*(1495/4) //(no.series cells)*(EDVR1/(no of parallel cells))
    EDVT0 4238 // temperature coefficient remains same
    EDVTC 7 // temperature coefficient remains same

    Can you please confirm on my calculation is correct?
  • 0 in reply to sai sumanth
    TI__Guru* 76230 points
    Hi Sai,
    I am not certain you can just make corrections to the coefficients that way. The preferred method would be to go through the process using the tool for the 8s4p pack

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 110 points
    Hi Onyx,
    I think if we can know the coefficients for 1s1p and battery model which is being used, then we can try to calculate the coefficients for any battery pack config. That will save a lot of time and effort.
    One more thing that can be done is , if its possible to get data of different battery config(with same cells) , we can try to see if there is any linear relation among the coefficients and find a solution. Do you have any such data?

    Thanks
    Sai
  • 0 in reply to sai sumanth
    TI__Guru* 76230 points
    it could be possible. I will have to ask our algorithm folks if that can be done. I haven't personally done that my self before. We don't have the data for a different battery config
    thanks
    Onyx