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Original question:

BQ27220: BQ27220

BQ27220: Generating correct profile parameters for bq27220

Lumina Gui
Genius 3610 points
Part Number: BQ27220

Hi TI Teams,

We encountered the following problem when using the bq27220 as a 1-cell LiCo battery to monitor the power level.

1) Gauge drift - multiple charging cycles cause charge percentage value shift from correct to the wrong one. 

By gauge drift we mean offset in coulomb counter data, for example gauge reporting 200mah left when battery completely drained. So both battery charge percentage and remaining capacity is incorrect.

2) Shutdown event generated too late - when voltage < 3.2V. We’ve calculated curves with 0.5/1A load and CellTermV 3.3V and it’s clear to us that this was wrong and we need to redo the whole thing.

System description as below:
● 1 cell LiCo battery 3.7V 2100mah with it’s own protection IC (DW01).
● Main board using bq27220yzfr fuel gauge
● Board current consumption: idle ~1ma, active ~300ma, peak 2A
● Target shutdown voltage at 3.2~3.3V

So we have the following question of how to generating the correct profile parameters for bq27220.

1)Which conditions/parameters should we use to generate data for the CEDV calculator?

2) What gauge parameters should we fine tune to get best performance? We’ve been using https://www.ti.com/tool/GPCCEDV to generate parameters for the gauge, and the original parameters and test conditions were not optimal.

Thanks,

Kind Regards

  • 0
    TI__Guru 60846 points

    The gauge uses FCC measured during a previous discharge from full to cell voltage = EDV2 together with coulomb count to calculate RM and SOC.

    For the gauge to be accurate, you have to:

    1. Make sure that charge termination is detected correctly. See the TRM www.ti.com/.../sluubd4a.pdf, 4.4.1 Detecting Charge Termination.

    2. Set EDV2/1/0 correctly for a typical discharge. The easiest way to do this is to log current and voltage during a discharge from full to empty, calculate passed charge for 7% (EDV2), 3% (EDV1) and 0% (EDV0) and then look up the corresponding voltages.

    3. If load and temperature change significantly, set EDV_CMP in CEDV Gauging Configuration to 1 and go through the steps from the user's guide from https://www.ti.com/tool/GPCCEDV. This tool returns coefficients that you can plug into the CEDV configuration which allows the gauge to auto-adjust (compensate) EDV2 and EDV1 based on load and temperature.

    Note that if your discharge use case changes significantly between discharges, the gauge will be more or less inaccurate depending on how much FCC changes because it takes the gauge at least one qualified discharge to update FCC (see TRM, 1.1.3 Capacity Learning (FCC Update) and Qualified Discharge).