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BQ34Z100-G1: How does the gauge learn new battery SOC after production assembly of product?

Peter Money
Prodigy 140 points
Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQSTUDIO

I am using a 5000mAh 2s Li-Po battery and have created the golden file image which is programmed into the gas gauge during production. The golden has Design Capacity of = 5000mAh and the learnt cell capacity Qmax Cell 0 = 5675mAh

When the battery is installed in the product we do not know how much charge is in the cells, the battery manufacturer quotes between 10-30% or 40-60%.

On initial charge of the product, the state of charge value reaches 100% a significant time before the cells are fully charged. The battery charger operates independently to the gas gauge, so after the gas gauge reaches 100% SOC the charger may continue to charge for a further 1Ah or more. The gas gauge is reporting the battery current and voltage correctly.

Consequently the gas gauge Qmax cell 0 has increased up to > 6000mAh which is incorrect. When we test the system by discharging the cells into a calibrated load, the gas gauge SOC remains at 100% until we have taken around 1Ah from the cells, from which point it starts to decrease as expected. The problem is that the cells will be fully discharged before the SOC approaches 0%.

My question is:

How does the gas gauge learn the product cell SOC?. Do we have to perform several charge/discharge cycles to get the gas gauge synchronised?

Thank you

Peter

  • 0
    TI__Expert 4640 points
    Hi Peter,

    Did you run the learning cycle with the cells attached prior to creating the golden image? Are the cells always attached to the gauge inside a single pack, or what do you mean by "when the battery installed in the product we do not know how much charge is in the cells"?

    Thanks

    Miguel
  • 0 in reply to Miguel Angel Rios
    Prodigy 140 points

    Hi  Miguel,

    I have run the learning cycle and created the golden file. The gas gauge is system side, the cells are not user replaceable so once the product is assembled the cells are always attached to the gauge. The cells are designed to be 5000mAh but are actually higher and the golden file has Qmax cell 0 learned value as 5670mAh. The golden file has been tested on around 50 units and it has been giving reasonable results.

    However I tested a unit as detailed below and got unexpected SOC results:

    1. Re-programmed the gas gauge with the golden file

    2. Charged the product fully, as determined by the charger current tapering to 0mA. Initial battery charge state was unknown.

    3. Attached a load to the system side of the product and discharged the cells at 0.2C whilst monitoring SOC, cell current, and voltage

    The SOC is not fall as expected. The SOC stayed at 100% until approximately 1000mAh had been drawn from the cells, it then started to fall as expected.

    My question is regarding the initial accuracy of the gas gauge on new product. How does the gas gauge determine the initial SOC for the cell? Does the gas gauge adapt to the cell over the first few product charge cycles?

    If I want to check gas gauge SOC accuracy, is it recommended to perform a number of charge cycles on the product first? If so then how many?

    Thank you

    Peter

  • 0 in reply to Peter Money
    TI__Mastermind 25955 points
    Hi Peter,

    Thank you for the detailed response.

    With respect to the SOC staying at 100% and not decreasing until 100 mAh has been drawn from the cells, please do not overcharge the cells. If possible, use the FC bit to determine when charging should cease. If FC bit cannot be used to stop charging, we will need to explore more deeply - please send me the maximum charging voltage and the chemID that is being utilized in the gauge.

    With respect to checking SOC accuracy:

    1. Fully charge at room temperature. Allow the cell to rest and fully relax ( > 2-5 hours )
    2. Change to desired test temperature. Allow cell to soak for ~ 1 hour or until cell has reached thermal equilibrium
    3. Begin logging with bqStudio (all columns)
    4. Discharge to terminate voltage with desired load. Allow the cell to rest and fully relax ( > 2-5 hours )

    Check the SOC accuracy by inspection. If the device is going to be loaded at different rates, to improve accuracy ensure the prior run was at the rate the next run will be at and that the prior run is a qualified discharge ( at that rate in excess of 530 seconds).

    Sincerely,
    Bryan Kahler