Method and apparatus for measuring pure resistance of in-vehicle battery

An apparatus for measuring a pure resistance of an in-vehicle battery which supplies electric power to a load of the vehicle comprising:

• means for periodically measuring a terminal voltage and a discharging current of the battery when the discharging current, which increases monotonously to exceed a prescribed value and decreases monotonously from the maximum value to the prescribed value or lower, flows;
• means for acquiring a first approximate equation of a voltage-current characteristic curve, indicative of correlation between the terminal voltage and the discharging current, for the increasing discharging current and a second approximate equation of the voltage-current characteristic for the decreasing discharging current;
• and
• means for setting a first point on the voltage-current characteristic curve represented by said first approximate equation and a second point on the voltage-current characteristic curve represent by said second approximate equation, assuming a first assumed point and a second assumed point on the voltage-current curves represented by the first approximate equation and the second approximate equation, the first assumed point providing the same resistance as a second synthesized resistance composed of a pure resistance of the battery and a second polarization resistance component, which produces a second voltage drop when a second discharging current corresponding to said second point flows, and the second assumed point providing the same resistance as a first synthesized resistance composed of the pure resistance of the battery and a first polarization resistance component, which produces a first voltage drop when a first discharging current corresponding to said first point flows, acquiring a first corrected gradient exclusive of a voltage drop due to the second polarization resistance component and a second corrected gradient exclusive of a voltage drop due to the first polarization resistance component, said first corrected gradient being acquired by correcting a first gradient of a first line connecting the second point and the first assumed point is corrected by a difference between the voltage drops due to the second polarization resistance component, which are produced by the second discharging current and the discharging current at the first assumed point, and said second corrected gradient being acquired by correcting a second gradient of a second line connecting the first point and the second assumed point by a difference between the voltage drops due to the first polarization resistance component, which are produced by the first discharging current and the discharging current at the second assumed point, and acquiring an average gradient of said first corrected gradient and said second corrected gradient by averaging a sum of them so that the average gradient is measured as the pure resistance of the battery.