Cell Impedance Measurement by Laplace Transformation of Charge or Discharge Current–Voltage

In our previous study overpotential resistances of nickel/metal-hydride and lithium-ion batteries were measured to estimate the battery temperature rises during rapid charge and discharge cycles by using our battery thermal model. However, the cell impedance Z(ω) measured by ac impedance meter did not agree with those induced by charge/discharge characteristics. Therefore, Z(ω) values were again measured by the method of Takano et al. [J. Electrochem. Soc., 147, 922 (2000)], who measured Z(ω) of lithium-ion batteries by Laplace transformation of both signals of the voltage-step input and its current response. This method has been extended here to Laplace transformation of current-step or current-pulse input and its voltage response to measure Z(ω) for any charge/discharge current of nickel/metal-hydride or lithium-ion battery. Nearly the same Z(ω) was obtained by the three different methods (voltage-step, current-step, and current-pulse inputs), and the measured Z(w) did not depend on either the charge/discharge current or the state of charge/charge input. Moreover, the Z(ω) measured by the current-pulse method, which includes the Warburg impedance at low frequency, approaches the overpotential resistance that can provide a good estimate of the battery temoerature rise in our batterv thermal model.