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bq27531 SOH is not changing

Bryan McLaughlin
Expert 1110 points
Other Parts Discussed in Thread: EV2400

Hello again,

During the past few months of bring up and testing I was reviewing my notes and noticed the SOH parameter hasn't changed since my initial learning cycle. 

I've been using the batteries down to about 20% SOC daily, sometimes down to 0%, then recharging. I'm doing this about twice daily, though not at regular intervals or with consistent rest periods.

The initial value after the learning cycle for three setups was 2995, 2981, and 2961 for a Design Capacity of 3000mAh. (Cell is 2900mAh min, 3000mAh typical). The SOH at that time was 93, 92, and 93% respectively.

The numbers are now 93, 92, 91% with FCC of 2517, 2559, and 2403. I should note that one board is using CCCV, one is using MLC, and one is using MLC w/SOH Compensation. 

Just hoping to understand this better before I commit to a GG file...

  • 0
    TI__Genius 14985 points

    Hi Bryan,

    What are Qmax and Ra values for each battery now as compared to their original golden values?

    SOH is just an FCC simulation at the fixed rate of SOH Load I and will depend on those learned values.  

    Also, if SOH Load I is small compared to your battery then SOH will decrease slowly.  It should be set to a typical value of your system current.  

  • 0 in reply to dMax
    Expert 1110 points

    Unfortunately I have only been sampling and logging the Standard Commands whenever it reaches 100% SOC, I'm not set up to read the Data Flash. I can jumper in the EV2400 and check though.

    I can easily monitor 0x29 SOH Status register; it's been reading 0x03 ("SOH value ready" according to TRM). How should the status number change as I charge and discharge? Any pattern I should look for that could help debug?

  • 0 in reply to Bryan McLaughlin
    Expert 1110 points

    dMax,

    I grabbed a snapshot of Registers and Data Flash from three setups, see attached. I tried to highlight registers of interest, hopefully it will save you some time.

    7268.GS-Logs.xlsx

  • 0 in reply to Bryan McLaughlin
    Expert 1110 points

    Just pinging for a response. Let me know if I can make the data log more digestible to save you time.

    I appreciate the help.

    Bryan

  • 0 in reply to Bryan McLaughlin
    TI__Genius 10235 points

    Bryan,

    Have you done any discharge cycles on the cells at the SOH Load I to determine what the actual capacity is? SOH is a parameter that isn't going to change very quickly as long as the cell isn't being abused. On some lifetime tests we are doing here, we are noticing 90% of the original cell capacity after about 300 cycles assuming C/2 charge and discharge rates. 80% is typically reached after 600+ cycles. This seems to be about on par with what you are seeing.