State-of-health Estimation for Lithium Iron Phosphate Batteries Based on Constant-voltage Charge Data Using a Resistor-inductor Network Based Equivalent Circuit Model

Abstract

State-of-health (SoH) is one of the critical battery states that must be estimated and monitored by the on-board battery management system in electric vehicles. In this paper, the capacity degradation for the lithium iron phosphate (LiFePO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> ) battery is detected based on the fast-dynamic behavior of the charge current during the constant-voltage (CV) period. Firstly, the time constant of the fast-dynamic CV charge current, i.e., Teq,1 of the simplified $2^{\mathrm{n}\mathrm{d}}-$order resistor-inductor (RL) network based equivalent circuit model (ECM) developed by the authors, is selected as a characteristic parameter correlating with the battery capacity fade, and the reference regression function between Teq,1 and the battery SoH is established. Furthermore, for the incomplete CV charge process, the reference regression function can be estimated by constructing the linear prediction model and the subsequent extrapolation. At last, the aging data from four LiFePO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> batteries are employed to evaluate the performance of the proposed method. The results show that the proposed method yields a satisfactory SoH estimation accuracy, and is robust to the incomplete CV charge process.