Yunlong Shang

A Cell-to-Cell Battery Equalizer With Zero-Current Switching and Zero-Voltage Gap Based on Quasi-Resonant LC Converter and Boost Converter

In conventional equalizers, the facts of bulky size and high cost are widespread. Particularly, the zero-switching loss and zero-voltage gap (ZVG) between cells are difficult to implement due to the high-frequency hard switching and the voltage drop across power devices. To overcome these difficulties, a direct cell-to-cell battery equalizer based on quasi-resonant LC converter (QRLCC) and boost dc-dc converter (BDDC) is proposed. The QRLCC is employed to gain zero-current switching, leading to a reduction of power losses. The BDDC is employed to enhance the equalization voltage gap for large balancing current and ZVG between cells. Moreover, through controlling the duty cycle of the BDDC, the topology can online adaptively regulate the equalization current according to the voltage difference, which not only effectively prevents overequalization but also abridges the overall balancing time. Instead of a dedicated equalizer for each cell, only one balancing converter is employed and shared by all cells, reducing the size and implementation cost. Simulation and experimental results show the proposed scheme exhibits outstanding balancing performance, and the energy conversion efficiency is higher than 98%. The validity of the proposed equalizer is further verified by a quantitative and systematic comparison with the existing active balancing methods.

A Switched-Coupling-Capacitor Equalizer for Series-Connected Battery Strings

Due to the low cost, small size, and easy control, the switched-capacitor (SC) equalizer is promising among all types of active cell balancing methods. However, the balancing speed is generally slow and the balancing efficiency is seriously low when the SC equalizer is applied into a long battery string. Therefore, an automatic switched-coupling-capacitor equalizer (SCCE) is proposed, which can realize the any-cells-to-any-cells equalization for a battery string. Only two switches and one capacitor are required for each battery cell. All mosfets are controlled by one pair of complementary pulse width modulation signals, and energy can be automatically and directly delivered from any higher voltage cells to any lower voltage ones without the need of cell monitoring circuits, leading to a high balancing efficiency and speed independent of the cell number and the initial cell voltages. Contrary to the conventional equalizers using additional components for the equalization among modules, the proposed equalizer shares a single converter for the equalization among cells and modules, resulting in smaller size and lower cost. A prototype for four lithium battery cells is implemented, and an experimental comparison between the proposed SCCE and the conventional SC equalizer is presented. Experimental results show the proposed topology exhibits a substantially improved balancing performance, and the measured peak efficiency is 92.7%.

An Automatic Equalizer Based on Forward–Flyback Converter for Series-Connected Battery Strings

This paper proposes an automatic any-cells-to-any-cells battery equalizer, which merges the forward and flyback converters through a common multiwinding transformer. The windings of the transformer are divided into two groups, which have opposite polarities. The principles of the proposed equalizer are that the equalization in one group is achieved based on forward conversion and the balancing between the two different groups is based on flyback conversion, by which the magnetic energy stored in the transformer can be automatically reset without using additional demagnetizing circuits. Moreover, only one MOSFET and one primary winding are required for each cell, resulting in smaller size and lower cost. One pair of complementary control signals is employed for all MOSFETs, and energy can be automatically and directly delivered from any high-voltage cells to any low-voltage cells without the requirement of cell monitoring circuits, thereby leading to a high balancing efficiency and speed. The proposed topology can achieve the global equalization for a long battery string through connecting the secondary sides of transformers without the need of additional components for the equalization among modules, which also overcomes the mismatching problem of multiple windings. The validity of the proposed equalizer is verified through experiments, and the balancing efficiency can reach up to 89.4% over a wide range of conditions.

A switched-coupling-capacitor equalizer for series-connected battery strings

An automatic switched-coupling-capacitor equalizer (SCCE) is proposed, which can realize the any-cells-to-any-cells equalization for a battery string. Only two switches and one capacitor are needed for each battery cell. All MOSFETs are controlled by a pair of complementary PWM signals, and energy can be automatically and directly delivered from any higher voltage cells to any lower voltage ones without the need of monitoring circuits. The inherent advantages of the proposed system are the simple control, high efficiency, and easy modularization. A prototype for four lithium-ion battery cells is implemented, and a comparison between the proposed circuit and the conventional one is presented. Experimental results show the proposed circuit exhibits a substantially improved balancing performance, and the measured peak efficiency is about 92.7%.

Modularized charge equalizer using multiwinding transformers for Lithium-ion battery system

In the Lithium-ion battery system of electric vehicles, the battery imbalance will lead to the decline of the safety and life cycle of the battery pack. To solve this problem, a modularized battery equalizer scheme using multipled transformers which have two secondary windings is proposed in this paper. With these secondary windings, modularization and energy balancing are achieved. Multiple equalization modules can operate at the same time and the energy can be transformed from the higher module to the lower one efficiently by the proposed equalizer.

An Interleaved Equalization Architecture with Self-Learning Fuzzy Logic Control for Series-Connected Battery Strings

This paper proposes an interleaved equalization architecture for series-connected lithium-ion battery strings, which can deliver energy from a battery module to the cells in the next adjacent battery module, resulting in an enhancement of the equalization current and efficiency. Particularly, the global equalization is achieved without the need of additional stage circuits to balance among modules, leading to a small size and low cost. A two-module equalizer based on resonant LC converter and buck converter with soft switching is implemented to verify the validity of the proposed interleaved architecture. Furthermore, a self-learning fuzzy logic control (SLFLC) algorithm is employed to online regulate the equalization period based on the voltage difference among cells and the cell voltage, not only greatly abbreviating the balancing time but also effectively preventing overequalization. The SLFLC has the outstanding advantages of high balancing precision, easy implementation, and strong robustness. Experimental results demonstrate that the proposed equalizer has good balancing performances with soft switching and high efficiency, and achieves zero-voltage gap among cells. Moreover, the proposed SLFLC algorithm abridges the total equalization time by about 27%, and reduces the equalization cycles by about 59% compared with the traditional control algorithm.

Relevance between fractional-order hybrid model and unified equivalent circuit model of electric vehicle power battery

Lithium-ion battery has gradually become the main power battery for electric vehicles. Battery models play an important role in battery design and safe operation. However, an accurate, intuitive, and efficient model is not available easily, because it is highly complex, nonlinear and uncertain. Battery models are divided into electrochemical model, thermal model, stochastic model, neural network model, and equivalent circuit model (ECiBaM) [1]. ECiBaMs use different physical components, such as voltage source, resister and capacitor to simulate the battery I-V characteristics. Due to the intuitive simulation and electrical design, ECiBaMs have been widely used, such as Thevenin, PNGV, and n-order RC. However, the traditional ECiBaMs did not take into account the battery nonlinear capacity. Thus, it is difficult to describe the battery run time accurately [2].

An automatic battery equalizer based on forward and flyback conversion for series-connected battery strings

This paper proposes an equalizer using multi-winding transformers based on forward and flyback conversion. The voltage equalization can be achieved based on forward conversion in one group and flyback conversion among different groups. Contrary to the conventional equalizers using multi-winding transformers, the proposed equalizer only requires one MOSFET for each cell and does not need additional demagnetizing circuits to reset the magnetic energy stored in the multi-winding transformers, resulting in a smaller size and lower cost. Moreover, only one pair of complementary control signals are employed for all MOSFETs, and energy can be automatically and directly delivered from any cells with high voltages to any cells with low voltages. The validity of the proposed automatic equalizer is verified through experimental results, and the balancing efficiency among cells can reach up to 89.4% over a wide range of balancing conditions.

A battery equalizer with zero-current switching and zero-voltage gap among cells based on three-resonant-state LC converters

For the conventional switched capacitor converter (SCC) based equalizers, it is difficult to achieve the full equalization among cells due to the inevitable voltage fall across MOSFET switches. Particularly, when the voltage gap among cells is larger, the balancing efficiency is lower, but the balancing speed gets slower as the voltage gap gets smaller. In order to soften these downsides, this paper proposes a battery equalization topology with zero-current switching (ZCS) and zero-voltage gap (ZVG) among cells based on three-resonant-state SCCs. An additional resonant path is built to release the charge of the capacitor into the inductor in each cycle, which lays the foundations to obtain ZVG among cells, improves the balancing efficiency at a large voltage gap, and increases the balancing speed at a small voltage gap. A four-lithium-ion-cell prototype is applied to validate the theoretical analysis. Experiment results show the proposed topology demonstrates good balancing performance with ZCS and ZVG among cells.