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Original question:

BQ27441-G1: BQ27441-G1

BQ27441-G1: Qmax drops too much

Achimo
Prodigy 30 points
Part Number: BQ27441-G1
Other Parts Discussed in Thread: GPCCHEM

In my 1S2P battery pack Qmax drops with each discharge cycle and the cell voltage for 0 % SoC rises in small steps from 2.7 V for new cells to 3.3 V after a few cycles. The full charge capacity and the runtime of the device powered by the battery pack also drops continuously.

My parameters are:

designVoltage 3700, terminateVoltage 2500, reserveCapacity 200, designCapacity_mAh 6700, designEnergy 24790, taperRate 240 (=280 mA), dsgCurrentTh 700 (=95 mA), chgCurrentTh 700 (=95 mA), quitCurrent 1000 (=67 mA), simRate 0x219 (280 - 3350 mA), loadMode 3 (constant current, average of 14s)

I charge with up to 2 A and discharge with 5 W. My microcontroller stops discharging at 0 % SoC. I wait 2 h when full and 2 h when empty so that Qmax is recalculated.

Why do I 'loose' battery capacity?

  • 0
    TI__Guru* 76230 points

    hi Achimo,

    This gauge has select battery chemistries that it can work with. if your cell chemistry is not a good fit, you could have all sorts of problems similar to what you are seeing.

    I would suggest you use our tool gpcchem to determine if your cell would work with the bq27441. If the tools report comes back indicating a  large error for this device, then I am  afraid you can't use it and will have to change devices. If it comes back with an acceptable error, then you will need to provide us lgg and log files of your tests for debug purposes.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Prodigy 30 points

    Hi Onyx,

    thank you, I will prepare the data for gpcchem.

    I use two NCR18650BF cells in parallel. I assume that these standard cells shouldn't be a problem. Do you agree?

    The only thing that is not perfect in my battery is the cell temperature measurement: I use the internal temperature sensor of the fuel gauge.The BQ27441 is placed on the back side of a PCB mounted on the cells. Do you think this is a problem? I will measure the temperature difference as a first step.

    Best regards,

    Achimo

  • 0 in reply to Achimo
    TI__Guru* 76230 points
    The temperature should be fine. It might not be the most accurate but it will work
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 30 points

    Hi Onyx,

    thank you.

    The temperature difference of the battery and the BQ27441 is up to 3 °C while discharging and up to 10 °C while charging due to the heat generated on my PCB. During relaxation, when Qmax is recalculated, the temperature difference is below 1 °C. So this really seems to be no problem.

    I sort of solved the problem with dropping Qmax by turning off the battery at 2.6 Volts instead of 0 % SoC. This way the BQ27441 learns a stable Qmax value and during discharge, the SoC nicely decreases to 0 % SoC. Then I wait some (5 to 15) minutes until the voltage reaches 2.6 V and stop discharging.

    This way my problem is solved, but I am a little bit disappointed that I have to check the cell voltage and cannot rely on the SoC alone. Therefore I will collect data for GPC. Which parameters are calculated by GPC for the BQ27441?

    Best regards

    Achimo

  • 0 in reply to Achimo
    TI__Guru* 76230 points
    it doesn't set parameters. it just determines if the gauge is a good fit. for your cell
    thanks
    Onyx