A Robust State of Charge Estimation Approach Based on Nonlinear Battery Cell Model for Lithium-Ion Batteries in Electric Vehicles

This study proposes a robust SOC estimation scheme for lithium-ion batteries in commercialized electric vehicles: pure electric vehicles and hybrid electric vehicles. Development of a nonlinear-model-based robust SOC estimation method and comprehensive performance comparison with the existing SOC estimation methods under the two kinds of operating scenarios are considered. The features of the results of this study are summarized as follows: a) a robust SOC estimator is designed to deal with the model uncertainty and parameter variation, b) an analysis of asymptotic stability of the proposed robust SOC estimator is provided and c) a comprehensive analysis was performed not only on the different aging levels of battery cells but also on the different operational conditions of the electrified vehicles. The effectiveness of the proposed SOC estimation method was experimentally verified using two 18 650 cylindrical battery cells with different degradation levels. The proposed SOC estimation scheme ensures considerable accuracy under both pure electric vehicle operating mode and hybrid vehicle operating mode even if the battery cell is aged.

I. Introduction

Towards zero-emitting transportation, electric vehicles (EVs), such as the pure electric vehicle (PEV) and hybrid electric vehicle (HEV) have been considered the most important part of the automotive industry. As a technical bottleneck of EVs, the importance of a battery system has been increased and a lot of study groups in many countries have launched their own projects to improve the performance of the battery systems [1], [2]. Because of the higher performance standards required by EVs, lithium-ion (Li-ion) batteries have been gaining general acceptance in the development of EVs for the reason that the Li-ion battery exhibits advantageous properties, such as a higher energy and power density, higher nominal voltage, and long lifespan compared with the other chemical types of battery [3]. In particular, the declining cost of the Li-ion battery has contributed to a continual enlargement of its share in the commercialized EV market [4], [5].