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BQ27410-G1 Wrong SOC and mWh calculation

srm
Prodigy 40 points
Other Parts Discussed in Thread: BQ27410-G1

Hi,

Currently i'm using BQ27410-G1 while life testing 5 800mAh Lithium Polymer batteries, by charging and discharge the batteries continuously until >500 cycles or more is reached. The batteries are charged to 4,1V and discharged to 25%. The cycle life is then evaluated.

My problem is that the Fuel Gauges BQ27410-G1 on each battery, in the beginning of the test gave the correct results. And after ~20 cycles two of the fuel gauges calculated the wrong max capacity and SOC level.

OK Fuel Gauge

NOT OK Fuel Gauge


Here is a screenshot from my test program - Output values:


All screenshots from BQ test software and my test program

6138.Fuel Gauge screenshots.zip

BQ test software settings for all fuel gauges

6036.Battery test.zip

What I'm I doing wrong? HELP

Kind regard,

Søren Mikkelsen


  • 0
    TI__Genius 10235 points

    This is likely related to not having OCV measurements, and thus Qmax updates. The issue that occurs is that the Impedance Track algorithm expects that a cell will occasionally relax to a low current condition for 30 to 120 minutes.  At that point an Open Circuit Voltage (OCV) measurement is taken and the algorithm uses that information to reset various charge accumulation registers.  As can be seen in some packs this is not really a stringent requirement as the gauge can run many cycles and still maintain accuracy.  However,  if this situation of no OCV measurements continues long enough, the charge accumulation registers will accumulate error.  This happens because there is no way to achieve absolute accuracy of calibration for the Coulomb Counter.  In fact, continuous cycling will exacerbate Coulomb Counter accuracy, because firmware also counts on low current conditions in order to remove offset error.

    So, to avoid this problem and get Qmax updates, simply be sure to allow a relaxation every 5 or 10 cycles. The relaxation should be 2 hours after charge, and 5 hours after discharge. For cycles in between the Qmax update cycles, smaller, or no relax periods can be used.

    Another recommendation is to discharge to 10% (instead of 25%), at least on the Qmax update cycles. The Qmax updates have a disqualification region (also called voltage flat region) where the voltage measurement error can introduce error in SOC. At 25% SOC, the battery is typically in this flat region.

    If you'd like to know more detail about this and the Impedance Track algorithm, please read SLUA450.

  • 0 in reply to Chase7071949597
    Prodigy 40 points

    Hi,

    Thanks works fine now :-)