An estimation method of Li-ion battery impedance using z-transform

An estimation method of circuit parameters of a Lithium-ion battery model is proposed in this paper. The model consists of a DC voltage source and an internal impedance. The proposed method gives the circuit parameters from a measured voltage and current waveforms during its operation. The internal voltage is a function of the State Of Charge (SOC). The internal impedance can be expressed by a resistor, which expresses the voltage drop in a high frequency region, and some RC parallel circuits, which express the transient characteristic of the battery. Their parameters are estimated from the impedance in z-domain. The proposed method gives satisfactory accurate parameters without initial values. The accuracy of the proposed method is confirmed by a comparison with the measured transient characteristic of a battery.

I. Introduction

Lithiumion battery has attracted much attention because of its high energy density and large charging/discharging current. It is used in a lot of equipment from a small mobile device to a large-scale energy storage system. The dynamic characteristic of the battery heavily depends on its State of Charge (SOC) and current, because the maximum battery current of a large capacity battery is limited by the terminal voltage, which depends on the SOC and the internal impedance of the battery. A numerical simulation plays an important role in the effective usage of the battery and a design of an energy storage system [1]–[8]. An accurate battery model is indispensable to the simulation. The model parameters have a close relation to its life time. A dynamic modeling during its actual operation is important for a development of a self-diagnosis circuit of an energy storage system. An estimation method of a Li-ion battery model, which expresses its transient characteristic, is proposed in this paper. The modeling is carried out using symbolic computation capabilities of Maple ™.