Adaptive Battery Equalization Algorithm for Capacitor-based Battery Management System

Lan-Rong Dung,Yu-Syuan Peng

doi:10.13189/ujeee.2016.040102

Abstract

In recent years, as energy-saving and environmental protection were widely concerned around the world, the lithium iron phosphate (LFP) battery has been widely used in hybrid electric vehicle (HEV) and electric vehicle (EV). Generally the battery pack for HEV is composition of the number of cells in series. In this case, imbalance among cells due to the difference of degradation and temperature will be accelerated by the cycles of charge and discharge without an appropriate battery equalization management system. However, the balancing system only intended to reduce the difference of cell voltage or Stage of Charge (SoC) before. Adaptive Battery Equalization Algorithm for Capacitor-based Battery Management System this thesis proposed re-explains the meaning of battery equalization. Instead of long-term using battery equalizer on standby, this thesis equalizes the battery when charging to save much spending time. It could not only adjust the balancing mechanism automatically in order to keep the available charge but also raise the average SoC to prevent the effect of battery aging and improve the cycle number of battery. Experiment results indicate that the available charge increases 1.5% and the battery capacity efficiently improves 7.3%.

Highlights

Lithium-ion batteries are widely used as an energy storage element and voltage source in many applications due to its high energy density, low self-discharge rate, and high voltage each cell
Battery should be used in appropriate region between the charging voltage limit (CVL) and the discharging voltage limit (DVL)
When one cell voltage is on CVL or DVL, the battery pack should stop charging/discharging immediately

Summary

Introduction

Lithium-ion batteries are widely used as an energy storage element and voltage source in many applications due to its high energy density, low self-discharge rate, and high voltage each cell. The little variance will cause significant differences of battery characteristics, such as charge capacity, internal resistance, self-discharge rate, and etc. When one cell voltage is on CVL or DVL, the battery pack should stop charging/discharging immediately. With active cell equalization, charge can be transferred between the cells in the battery pack which uses a storage element like inductor, transformer, or capacitor. Among these equalization circuits, the switched capacitor is the promising method due to no bulky and large magnetic components. This thesis uses switched capacitor equalizer which is a power saving method as storage element [11]. The proposed battery equalization algorithm can save much balancing time and adjust the equalization mechanism for different battery aging conditions to meet the valid and efficient results

Adaptive Equalization Algorithm

Experimental Results

Conclusions