3216 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 58, NO. 7, SEPTEMBER 2009 Design of a Charge Equalizer Based on Battery Modularization Hong-Sun Park, Associate Member, IEEE, Chol-Ho Kim, Student Member, IEEE, Ki-Bum Park, Student Member, IEEE, Gun-Woo Moon, Member, IEEE, and Joong-Hui Lee, Member, IEEE 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. Numerous algorithms and circuits were developed to meet the foregoing demands, and some interesting results have been obtained. However, for a large number of cells, for example, 80 or more batteries, the previous approaches might not easily satisfy the foregoing requirement. 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, and then, an intramodule equalizer and an outer-module equalizer are designed. This battery modularization scheme ef- fectively reduces the number of cells that we consider in an equalizer design procedure; thus, the design of a charge equalizer becomes easier. Furthermore, by applying the previously verified charge equalizers to the intramodule and the outer module, we can make the equalizer design more flexible. Several examples and experimental results are presented to demonstrate the usefulness of the charge equalizer design method. Index Terms—Battery modularization, charge equalizer design, hybrid electric vehicle (HEV), lithium-ion battery. I. I NTRODUCTION S ERIES-CONNECTED battery strings have been used for many applications, such as electric vehicles, hybrid electric vehicles (HEVs) [1]–[6], electric scooters, and uninterruptible power supplies. Among them, an HEV battery is severely exposed to a charge and discharge environment, because an HEV can recover energy from the wheels during regenerative braking (an energy source that was wasted in the past) and reuse it to propel the vehicle at low speeds or provide extra power for high acceleration [1]–[3]. Furthermore, this repeated charge and discharge phenomenon causes a cell mismatch problem, because the batteries have inevitable differences in chemical and electrical characteristics from manufacturing; they also ex- Manuscript received March 31, 2008; revised September 29, 2008. First published February 18, 2009; current version published August 14, 2009. This work was supported in part by the Korea Ministry of Education, Science, and Technology under the STSAT-3 program of the Korea Aerospace Research Institute. The review of this paper was coordinated by Mr. D. Diallo. H.-S. Park, C.-H. Kim, K.-B. Park, and G.-W. Moon are with the Division of Electrical Engineering, School of Electrical Engineering and Computer Sci- ence, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail: hspark@angel.kaist.ac.kr; railroads@angel.kaist.ac.kr; parky@ powerlab.kaist.ac.kr; gwmoon@ee.kaist.ac.kr). J.-H. Lee is with the Corporate R&D Center, SK Energy Institute of Technology, Daejeon 305-712, Korea (e-mail: hui@skenergy.com). Digital Object Identifier 10.1109/TVT.2009.2015331 perience mismatched ambient temperatures when they are used and accelerate asymmetrical degradation with aging [7]–[12]. The problem is that when these imbalanced batteries are left in use without any control, such as cell equalization, the energy storage capacity severely decreases [10], and, in the worst case, there may be an explosion or fire [7]–[9]. The charge equalization for a series-connected battery string is, therefore, necessary to prevent these phenomena and extend the useful lifetime. Numerous charge-balancing algorithms and circuits have been developed [8]–[10], [14]–[28] and well summarized [11]– [13]. As shown in Fig. 1, they can be classified into two cate- gories: dissipative and nondissipative. The nondissipative method can be divided into three parts: a charge type, a dis- charge type, and a composite charge–discharge type. For a small number of cells, some interesting results have been achieved, for example, automatic or selective equalization based on a multiwinding transformer [15]–[19], intelligent cell balance with an individual dc–dc converter [20]–[22], bidirec- tional equalization with a bidirectional dc–dc converter [8], [9], [24] or a switched capacitor [10], [25]–[27], and a two-stage equalization method [28]. However, based on the fact that ap- proximately 80 or more batteries are stacked in series to obtain a dc source of more than 300 V in an automotive application [1], [10], [15], [17], the application of the foregoing approaches directly tends to produce difficult problems. Such problems include the difficulty of implementing a multiwinding trans- former, the prolonged equalization time caused by a cell-to-cell energy shift, the complexity of controlling a large number of bidirectional dc–dc converters, the bulky size and high imple- mentation cost of applying an individual dc–dc converter to each cell, and the high voltage stress caused by a step-up converter. To avoid these problems, we propose a charge equalizer design method based on a battery modularization technique. In this method, a long battery string is divided into several modules, and then, an intramodule equalizer and an outer- module equalizer are designed. This modularization technique effectively reduces the number of cells that we take into account when designing a charge equalizer. Thus, the design of the charge equalizer becomes much easier. By applying conven- tional charge equalizers, which have already been verified, to the intramodule and the outer module, we can make the design of a charge equalizer more flexible. Furthermore, a newly designed equalization circuit can show the advantages of the original circuit and overcome the disadvantages that the original circuit was unable to remove. Several examples and 0018-9545/$26.00 © 2009 IEEE