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BQ34Z100-G1: WRONG SoC STIMATE AFTER PERIOD AT HIGH SoC

Gorka Lorea
Expert 1620 points
Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQSTUDIO

Hi,

I am using a BQ34z100-g1 with an 8s2p LiFePO4 batteries. The nominal capacity of the battery is 3200mAh.

I have obtained the golden image of the battery and load it in the bq34z100-g1 and now I am doing cycles to the system simulating the real application. 

The FCC after the learning cycle was 3195mAh. After that, the system was working in standard operating during several months and the battery did 35 cycles This FCC rised to 3230mAh.

However, in the last 40 days the system has changed its operating. The current consumption has raised a lot and the SoC of the battery is always higher than 80-85%. Now the FCC has risen to 1255mAh (however the battery is in good condition).

1. How is it possible such a FCC drop? Is it due to the battery staying above 80-85% SoC for a long period?

2. Being the FCC in 1255mAh (nominal capacity of the battery is 3200mAh and FCC was 3230mAh) the SoH is 99%. I do not understand this either.

It is interesting to remark that after that if I do a complete discharge of the battery the FCC rise again until 2890mAh. 

Best regards, 

  • 0
    TI__Guru 52115 points

    Could you able to replicate this issue and provide a bqStudio log?  Also, provide the source file before you do the testing.

     
    Andy
  • 0 in reply to Andy Liu (BMS)
    Expert 1620 points

    Hi Andy,

    The issue has ocurred after 40 days operation. Obviously, I can not repeat it while  logging it. 

    I do not want an asnwer explaining the changes in the registers of the BQ24z100-g1. What I want to know is if with this operation (SoC always above 80-85%) during long period of time (more than 40 days) this FCC reduction can be explained.

    What do you think?  

    Best regards, 

  • 0 in reply to Gorka Lorea
    TI__Guru 61006 points

    #1: FCC is a function of load (among others). So if the load changed substantially, FCC will drop. Besides load, the drop is also heavily dependent on cell resistance, which in turn increases when temperature drops below room temperature. So depending on the use case, this isn't necessarily unexpected or incorrect. To know for sure, you'd have to analyze how the battery actually responds to the increased load (and other conditions) - for example logging time, current, voltage, temperature, remaining capacity, full charge capacity once a second for a complete discharge from full to empty with the high load use case.

    #2: SOH does not use actual FCC. It uses a virtual FCC that is calculated with a discharge simulation from full to terminate voltage using the SOH load current so if your actual FCC drops, SOH can still be unchanged (it will drop as chemical capacity decreases and cell resistance increases as the cell ages).