Chol-Ho Kim

A Modularized Charge Equalizer for an HEV Lithium-Ion Battery String

Based on the fact that a hybrid electric vehicle (HEV) connects a high number of batteries in series to obtain more than approximately 300 V, this paper proposes a modularized charge equalizer for an HEV battery pack. In this paper, the overall battery string is modularized into M*N cells, where M is the number of modules in the string and N is the number of cells in each module. With this modularization, low voltage stress on the electronic devices can be achieved, which means that there is less chance of a failure on the charge equalizer. The power rating selection is one of the most important design issues for a charge equalizer because it is very closely related to equalization time. To solve this problem optimally, this paper presents a power rating design guide. In addition, this paper considers system-level design issues, such as cell voltage acquisition, equalizer control logic, and system-level grounding. The simulation and experimental results are presented to show the usefulness of the optimal power rating selection guide and the low voltage stressed charge equalization process.

Design of a Charge Equalizer Based on Battery Modularization

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 effectively 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.

A Modularized Charge Equalizer Using a Battery Monitoring IC for Series-Connected Li-Ion Battery Strings in Electric Vehicles

In the lithium-ion battery systems for electric vehicles (EVs), a battery management system (BMS) is essential for enhancing the batterys life cycle and safety. As a result, a BMS is required to realize both the effective cell monitoring and balancing. Moreover, individual cell balancing and monitoring circuit with a smaller size are required in a large number of battery cells. To meet these requirements, a modularized charge equalizer using the monitoring integrated circuit (IC) is proposed for EV battery strings. The proposed scheme exhibits efficient charge equalization with simple control of the monitoring IC. In the proposed equalizer, the battery string is modularized into a master module and multiple slave modules. A central equalization converter in the master module is shared by all of the battery cells through the module and cell switches in the slave module, instead of a dedicated charge equalizer for each of the cells. Individual charge equalization can be controlled by the cell monitoring IC in the slave module. With this configuration, the battery monitoring and balancing can be effectively merged into one controller. Moreover, the individual charge equalizer can be effectively implemented without affecting the size or cost based on the numbers of cells. In this paper, a prototype for 88 lithium-ion battery cells is optimally designed and implemented. Experimental results verify that the proposed method exhibits outstanding balancing performance with simple operation methods.

A Modularized Two-Stage Charge Equalizer With Cell Selection Switches for Series-Connected Lithium-Ion Battery String in an HEV

In lithium-ion battery system for hybrid electric vehicle, charge equalizer is essential to enhance the battery life cycle and safety. However, for a large number of battery cells, a conventional equalizer has the difficulty of individual cell balancing and the implementation size problem as well as the cost. Moreover, it shows high voltage stress of electrical elements in the equalization converter due to the high voltage of battery pack. To improve these drawbacks, this paper proposes a modularized two-stage charge equalizer with cell selection switches. The proposed circuit employs the two-stage dc-dc converter to reduce the voltage stress of equalization converter. Contrary to conventional method, the proposed equalizer can achieve the individual cell balancing only through the cell selection switches. With the two-stage converter and the cell selection switches, the proposed equalizer leads to the great size reduction with lower cost which brings advancement of individual cell balancing in a large number of battery cells. In this paper, a prototype for 88 lithium-ion battery cells is optimally designed and implemented. Experimental results are presented to verify that the proposed equalization method has a good cell balancing performance showing the low voltage stress and small size with the lower cost.

A modularized charge equalizer using battery monitoring IC for series connected Li-Ion battery strings in an electric vehicle

In the proposed equalizer, series-connected battery string is modularized into M modules. Each module has voltage monitoring IC and cell selection switch for K cells. The monitoring IC not only monitors the battery voltage, but also controls the selection switch for cell balancing. With this configuration, the battery monitoring and balancing can be effectively merged by the same controller. Moreover, by using a central equalization converter, great size reduction and outstanding balancing performance can be obtained for a large number of battery cells. In this paper, to demonstrate usefulness of the proposed work, a prototype for 88 lithium-ion battery cells is optimally designed and implemented. Experimental results verify that the proposed equalization method has good cell balancing performance.

Automatic charge equalization circuit based on regulated voltage source for series connected lithium-ion batteries

In the lithium-ion batteries for electric vehicle applications, the charge equalizer is required to enhance life time and guarantee safety. For efficient charge equalization, a cell voltage sensing module should be used. However, the cost of the sensing module is relatively high. Thus, to achieve higher equalization performance without the cell voltage sensing module, a new automatic charge equalizer based on regulated voltage source is proposed. A fast regulated voltage source with average voltage of the batteries is implemented by a bidirectional dc-dc converter. The charge equalization can be automatically achieved by periodic connections between battery cells and regulated voltage source without the sensing module. The high equalization performance can be obtained by proposed reference voltage modulation and advanced skip mode. The operational principles and design considerations of the proposed equalizer are presented and equalization performance is verified by the prototype with 7Ah lithium-ion batteries.

Individual cell voltage equalizer using selective two current paths for series connected li-ion battery strings

This digest proposes an individual cell voltage equalizer using selective two current paths for series connected lithium-ion battery strings. In the proposed equalizer, a central equalization converter is shared by every battery cells through the cell selection switch, instead of a dedicated charge equalizer for each cell. The central converter is utilized as a controllable current source by a modified rectifier. With this configuration, although the proposed equalizer has a central dc-dc converter, individual charge equalization can be effectively achieved for the each cell in the strings. Furthermore, great size reduction and low cost can be obtained for an industrial battery equalizer. In this digest, an optimal power rating design guide is also employed to obtain a minimal balancing size while satisfying equalization requirements. A prototype for eight lithium-ion battery cells is optimally designed and implemented. Experimental results verify that the proposed equalization method has good cell balancing performance.

A cell selective charge equalizer using multi-output converter with auxiliary transformer

In rechargeable batteries such as lithium-ion, the charge equalization circuit is necessary to enhance life time and guarantee safety. For fast charge equalization, the equalization current is increased as increase of battery capacity, and thus the burden of power semiconductors is increased. Therefore this paper proposes new selective charge equalization circuit with small number of power semiconductors. In the proposed charge equalizer, power diode of transformer secondary side is eliminated and power MOSFET of transformer secondary side is controlled by an auxiliary transformer. By using current unbalance caused by effective voltage mismatching, the charge equalization can be achieved without current blocking switches. In this paper, the operational principle of the proposed equalizer is presented and principle of current unbalance is analyzed. To confirm the validity of the proposed scheme, a prototype of 8 lithium-ion batteries is implemented and experimental results are presented.

Design Considerations of a Lithium Ion Battery Management System (BMS) for the STSAT-3 Satellite

This paper introduces a lithium ion battery management system (BMS) for the STSAT-3 satellite. The specifications of a lithium ion battery unit are proposed to supply power to the satellite and the overall electrical and mechanical designs for a lithium ion battery management system are presented. The structural simulation results will be shown to confirm the behavior of both the BMS and the cells.

Wide range ZVS phase shift full bridge converter with low conduction loss

The conventional phase shift full bridge (PSFB) converter is widely used for front-end DC/DC stage of distributed power system (DPS). However, hold-up time requirement limits its operating duty ratio in normal operation and it results in the increase of conduction loss in conventional PSFB converter. Therefore, wide range ZVS PSFB converter with low conduction loss is proposed in this paper. The proposed converter adopts the large resonant inductor and replaces clamp diodes in conventional PSFB converter with MOSFET switches. With this modification, the proposed converter can achieve the ZVS operation over wide load range as well as can reduce the conduction loss of switches caused by the circulating current. There advantages result in the improvement of whole load efficiency. The operational principle and analysis of proposed converter are presented and verified by the 1.2kW prototype.