Charge equalization converter for lithium-ion batteries in hybrid electric vehicles

Abstract

The charge equalizer design for a series connected battery string is very challenging because it needs to satisfy many requirements such as implementation possibility, equalization speed, equalization efficiency, controller simplicity, size and cost issues, voltage and current stress, and so on. For a small number of cells, numerous algorithms and circuits were developed to meet the above demands and some interesting results have been obtained. However, for a large number of cells, such as a hybrid electric vehicle (HEV) traction battery application, the previous approaches might not satisfy the above requirement easily. To overcome these difficulties, we propose a charge equalizer design method based on a battery modularization technique. In this method, a very long battery string is divided into several modules, which is called module batteries. Then, an intramodule equalizer and an outer-module equalizer are designed, respectively. This battery modularization scheme effectively reduces the number of cells that we consider in an equalizer design procedure; thus, the design of a charge equalizer becomes much easier. By applying the previously verified charge equalizers to the intramodule and the outer-module, we can make the equalizer design more flexible. In addition, a newly designed charge equalizer shows the good points of the conventional battery equalizer. To demonstrate the usefulness of this charge equalizer design method, several examples and comparative experimental results are presented. In the current thesis, a current-fed charge equalizer based on the technique of battery modularization is proposed. Based on the fact that an HEV stacks a high number of batteries in series to obtain more than approximately 300 V, the HEV battery string is modularized into a set of groups. In detail, the overall battery string is modularized into M*N cells; in other words, M modules in the string and N cells in each module. For this modularized battery structu...