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BQ34Z100-G1: BQ34z100-g1. State of health

Gorka Lorea
Expert 1620 points
Part Number: BQ34Z100-G1

Hi,

I am using BQ34z100-g1 with an 8s2p LiFePO4 battery.

I have charge the battery to full at 23ºC and the the FCC has raised from 6400 (=designCapacity) to 6613mAh. 

After that I have introduce the battery in a climate room and set -20ºC. The FCC has dropped to 6075mAh. Then I have started a discontinuous dicharge at C/3. 

Now, the battery is at 75% SoC with the next parameters 

- FCC=6075 mAh

- RC = 4572 mAh

- TRC = 3852 mAh

- TFCC = 5233 mAh

- SoH = 100%

I have some doubts about them:

1. Why is SoH = 100%. I have read SoH is FCC/designCapacity() but in this case this is not correct. 

2. What does the parameter SOH_DISP do? Is it necessary to read a real SoH?

3. If TRC is 3852 mAh (and is lower than TFCC), how can be RC 4572 mAh?

Thank you very much in advance, 

  • 0
    TI__Guru* 76230 points
    HI GOrka
    1. SOH is calculated at room temp. It is FCC at room temp /design cap.
    2. Data sheet explains this parameter.
    3. if you have smoothing enabled which i believe you do, the gauge manipulates the true RM and FCC to ensure the reported SOC doesn't experience jumps due to load current and temperature. You should be reading the true FCC and rem cap if you need those values.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Expert 1620 points

    Hi Onyx,

    Thanks again for your answers. 

    However I still have some doubts about the SoH calculation. After some cycles in one of my batteries the SoH has dropped to 86%. The FCC is now 5185mAh and the design capacityt is 6400mAh. The temperature is always between 20 and 25ºC

    1. If I calculate SoH as FCC at room tem/desigCap = 81%. That is not the expected 86%.

    2. I can see that Qmax Cell 0 is 5505mAh and 5505/6400 happens to be 86%. 

    3. Is the SoH calculated as Qmax Cell 0/DesignCap? or is this just a coincidence? if so, why FCC at rom temp/DesignCap does not match with 86%?

    Thank you very much in advance, 

  • 0 in reply to Gorka Lorea
    TI__Guru* 76230 points
    Hi Gorka

    If you look at the DS, it stated there that SOH is calculated using FCC at room temp (25degC and SOH current rate. It isn't the FCC that is reported that is used in calculating SOH. It is the FCC that is calculated based on the SOH current rate.

    thanks
    Onyx