Cost-Reduced Inductor-Based Voltage Equalizers With Design of Free-Wheeling Networks for Li-Ion Battery Pack

Inductor-based equalizers are characterized by high power, but current inductor-based equalizers rarely consider the cost of the isolated power supplies for driving switches and the design of free-wheeling networks for inductor current. To tackle these issues, two inductor-based voltage equalizers are proposed in this article. On the one hand, combining diodes and MOSFETs to form a switch array can achieve the common source electrode shared by MOSFETs on the same sides, reducing the required number of isolated power supplies and the circuit cost. On the other hand, free-wheeling networks with lower inductor current descent during the dead time are provided in the proposed equalizers; thus, the equalization speed and efficiency can be improved compared with the current inductor-based methods. Topological structures, operation principles, and mathematical modeling analysis are illustrated in detail first. Then, circuit parameters are selected based on the simulation results. Comparisons between the proposed and the existing methods over the equalization performance, cost, and size are also conducted. Finally, experiment platforms are established. The results verify the feasibility and high performance of the proposed equalizers.

I. Introduction

Due to the characteristics of high energy density, Li-ion batteries are widely used in electric vehicles. Li-ion battery cells are low in voltage, typically in 2.5–4.2 V. To meet the requirement of high voltage, cells need to be connected in series to form a battery pack. However, as there are differences in manufacturing processes and working environments, inconsistency among cells occurs, which can lead to a decrease in the available capacity of a charge-discharge cycle. What is worse, the aging process accelerates after numerous charge-discharge cycles when battery packs work under unbalanced conditions [1], further reducing the service life of a pack. In order to improve the consistency, voltage equalizers are needed for balancing battery packs.