Battery models play important roles in battery manufacturing, design, safe operation, and many of its applications, such as in providing power sources of electric vehicles due to the high energy density, long life, and high cell voltage. Unlike the detection of impedance spectrum in a short linear interval, the I-V characteristic of lithium-ion batteries presents strongly nonlinearity such as the serious polarization in both ends of charging /discharging. To modeling the nonlinear characteristic of real battery accurately, this paper proposes a variable-order fractional equivalent circuit model that provides a novel and effective trade-off of model accuracy and simplicity. The proposed method is verified under the working conditions of pulse and constant current charging /discharging, where the maximum model error is less than 1.8%.
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