Co-estimation of state of charge and state of power for lithium-ion batteries based on fractional variable-order model

Abstract

This paper proposes a co-estimation scheme of the state of charge (SOC) and the state of power (SOP) for lithium-ion batteries in electric vehicles based on a fractional-order model (FOM). First, a series of FOMs and integer-order models (IOMs) is constructed using fractional- and integer-order calculus. The model parameters are then identified using particle swarm optimization over the whole SOC range, and the complexity and accuracy of the resulting models are evaluated. Second, a fractional-order extended Kalman filter-based SOC estimator is developed. Third, the co-estimation of SOC and SOP is formulated, and an SOP estimation method based on three restrictions and a correction method from constant-current to constant-power are proposed. Finally, the proposed model and method are verified by experiments. The main results are as follows: (1) The FOMs achieve better accuracy than IOMs over the whole SOC range (especially in the low SOC range), and the FOM with a pair of resistance-constant phase elements and one Warburg element (FO-2RCW) producing the best performance. (2) The SOC estimation accuracy based on the FO-2RCW with variable parameters and orders is less than 2% over the whole SOC range. (3) The proposed co-estimation method is validated to be effective under the dynamic operating conditions and shows high accuracy.