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BQ40Z80: Impedance Tracking Algorithm - Theory

Leo Rodriguez
Intellectual 945 points
Part Number: BQ40Z80

Hi BGP Team,

I have a couple of questions regarding the theory behind QMAX updates with IT algorithm. From Subsection 7.4.2 of the BQ40Z80 Technical Reference Manual:

A QMax update is disqualified under the following conditions:

Temperature — If Temperature() is outside of the range 10°C to 40°C.

Delta Capacity — If the capacity change between suitable battery rest periods is less than 37%.

 


Can you clarify how do we arrive at these conditions (from a theoretical standpoint) and what happens if we deviate? 

Thanks,

Leo

  • 0
    TI__Guru** 105760 points

    Hello Leo,

    When there are extreme temperatures on the batteries many of their characteristics change so the gauge waits for a battery cycle within 10DegC to 40DegC to get an accurate Qmax update to calculate the batteries other parameters like SOC.

    Usually extreme cold is environmental, and hot conditions occur sometimes when the batteries are being charged in a box with low ventilation. If you stay at extreme temperatures that prevent a Qmax update the gauging will degrade over time because it is not learning the new Qmax of the battery.

    The capacity must change more than 37% because the gauge uses set data points on the OCV curve to determine DOD which is the basis of the gauging calculations. If you pass too few data points it could lead to inaccurate calculation on Qmax.

    The same will occur if you only cycle batteries less than 37%, over time the gauge will become less accurate because it never updates the Qmax as the batteries degrade.

    You don't need to have a Qmax every cycle, as long as it get periodically updated the gauge should work well.

    Sincerely,

    Wyatt Keller