Prediction of state of charge of Lithium-ion rechargeable battery with electrochemical impedance spectroscopy theory

For the purpose of predicting the state of charge of Lithium-ion rechargeable battery accurately, we introduced the method of electrochemical impedance spectroscopy to solve the problem. The experiment data of impedance spectroscopy is comprised of an inductive arc in the high-frequency region and two capacitive arcs in the low-frequency region, and by which the reasonable equivalent circuit of battery was established. The component parameters obtained at several state of charge values of the battery had been analyzed by a non-linear least-squares fitting procedure and some electrochemical knowledge. Through researching the changing regulation of parameters with the different States of charge, the frequency of maximum of the semicircle (fmax), the phase angle ø, the equivalent series capacitance (Cs) had been substantiated to be the suitable parameters for analyzing and predicting the state of charge values of the lithium-ion battery.

I. Introduction

State of charge (SOC), The State of charge of a battery is its available capacity expressed as a percentage of its rated capacity. The users can estimate the life of the battery and know when the batteries need to be recharged or changed through the data of SOC. It is one of the important parameters during the use of the battery. Therefore, prediction of the state of charge of battery has becomes a pivotal part of battery management. At present, several methods of estimating the state of charge of a battery have been used. Some are specific to particular cell chemistries. Most depend on measuring some convenient parameter which varies with the state of charge. Such as: open circuit voltage law, Ah measurement, internal resistance method and so on. And some intelligent forecasting methods which are combined with smart arithmetic, but there are certain shortcomings.