A Comparative Study on Open Circuit Voltage Models for Lithium-ion Batteries

Quan-Qing Yu,Rui Xiong,Le-Yi Wang,Cheng Lin

doi:10.1186/s10033-018-0268-8

Abstract

The current research of state of charge (SoC) online estimation of lithium-ion battery (LiB) in electric vehicles (EVs) mainly focuses on adopting or improving of battery models and estimation filters. However, little attention has been paid to the accuracy of various open circuit voltage (OCV) models for correcting the SoC with aid of the ampere-hour counting method. This paper presents a comprehensive comparison study on eighteen OCV models which cover the majority of models used in literature. The low-current OCV tests are conducted on the typical commercial LiFePO4/graphite (LFP) and LiNiMnCoO2/graphite (NMC) cells to obtain the experimental OCV-SoC curves at different ambient temperature and aging stages. With selected OCV and SoC points from experimental OCV-SoC curves, the parameters of each OCV model are determined by curve fitting toolbox of MATLAB 2013. Then the fitting OCV-SoC curves based on diversified OCV models are also obtained. The indicator of root-mean-square error (RMSE) between the experimental data and fitted data is selected to evaluate the adaptabilities of these OCV models for their main features, advantages, and limitations. The sensitivities of OCV models to ambient temperatures, aging stages, numbers of data points, and SoC regions are studied for both NMC and LFP cells. Furthermore, the influences of these models on SoC estimation are discussed. Through a comprehensive comparison and analysis on OCV models, some recommendations in selecting OCV models for both NMC and LFP cells are given.

Highlights

Due to the global energy crisis and environmental deterioration, electric vehicles (EVs) have had an unprecedented development opportunity in recent years [1]
Due to the open circuit voltage (OCV) is independent of ambient temperatures and aging stages, the OCV tests are performed under three temperatures (i.e., 10 °C, 25 °C and 40 °C) and two aging stages in this study
The OCV-state of charge (SoC) curves of NMC cells shown in Figure 1(a) change dramatically as the SoCs drop to 0% and gradually increase between 10% and 100% SoC regions, but for LiFePO4/ graphite (LFP) cells shown in Figure 1(b), the OCVSoC curves change dramatically as the SoCs drop to 0% and rise to 100%, and there are wide flat OCV plateau in the middle SoC regions

Summary

Introduction

Due to the global energy crisis and environmental deterioration, electric vehicles (EVs) have had an unprecedented development opportunity in recent years [1]. Polynomial functions with different orders are developed to fit OCV data points obtained from offline OCV tests [19,20,21]. Its adaptability to other battery types needs to be further investigated Sensitivities of these OCV models to ambient temperatures, aging stages, and numbers of data points remain largely an open issue. The key contribution of this paper is using an innovative approach to give a systematic comparison on the practicality of diversified OCV models Adaptability of these OCV models to different battery types is investigated.

Experiments

Findings

Conclusions