Battery systems are affected by many factors, the most important one is the cells unbalancing. Without the balancing system, the individual cell voltages will differ over time, battery pack capacity will decrease quickly. That will result in the fail of the total battery system. Thus cell balancing acts an important role on the battery life preserving. Different cell balancing methodologies have been proposed for battery pack. This paper presents a review and comparisons between the different proposed balancing topologies for battery string based on MATLAB/Simulink® simulation. The comparison carried out according to circuit design, balancing simulation, practical implementations, application, balancing speed, complexity, cost, size, balancing system efficiency, voltage/current stress … etc.

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Passive and active battery balancing comparison based on MATLAB simulation

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.

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

The development of electric vehicle (EV) technologies, its applications, energy managements and storage systems are the most important sectors to the automotive industries on their environmental and global economic issues. The electrochemical batteries have a great market in EVs for their long-run and short-run energy storage profiles. Thus, to enhance the battery lifecycle and its performance over the charge and discharge periods, the perfect charge equalization of the long string battery pack is compulsory. The development of new charge equalization controller (CEC) and intensifying the features of existing CECs are now great deal in the field of high-tech storage systems towards the advancement of the sustainable EV technologies. This paper presents the EV technologies with their drive train architectures in different configurations and designs. A study on batteries regarding their formation, properties, energy management systems, advantages and disadvantages are also conducted in the review. A comprehensive review of the different type of CECs for EV applications is highlighted. From the rigorous review, it is concluded that a good equalization controller should have high equalization speed, high efficiency, small volume, simple wiring and execution, low cost and good extensibility. It is observed that the existing CECs have a good contribution to run the EV systems safely and efficiently with their balancing capabilities. However, they still have some problems to achieve all properties for efficient equalization towards the enhancement of battery performance and life. Consequently, a comparison on the salient feature characteristics among the CECs are explained on their topologies, types and execution times, difficulties in control, efficiencies, cost, components, merits and demerits to develop sustainable battery energy storage systems. All the highlighted insights of this review will hopefully lead to increasing efforts towards the development of the advanced CEC for the future high-tech battery energy storage systems in the vehicle applications.

Battery charge equalization controller in electric vehicle applications: A review

The bidirectional cell-to-cell equalization methods perform well to prevent the series-connected batteries from overcharging and undercharging. However, it remains a challenge to achieve fast equalization and high equalization efficiency with low complexity. To address the issue, we propose a new bidirectional cell-to-cell active equalization method using a multiwinding transformer. The new method allows the energy to transfer directly from the highest voltage cell to the lowest one by fly-back operation or forward operation which gives a short balancing path and guarantees a fast equalization speed. We adopt the bidirectional switches of low conduction loss for high equalization efficiency. Since the bidirectional switches are turned on or off simultaneously, a common driving circuit for the bidirectional switch has been designed to reduce the number of the driving circuits by half. The experiments with a six 50-Ah Lithium-ion battery strings are conducted and the results have demonstrated that our new approach has achieved a good overall performance of equalization in terms of speed, efficiency, and complexity of the circuit.

A MultiWinding Transformer Cell-to-Cell Active Equalization Method for Lithium-Ion Batteries With Reduced Number of Driving Circuits

A series of switched-capacitor (SC) cell balancing circuits is proposed for rechargeable energy storage devices like battery and supercapacitor strings in this paper. Taking a basic SC-based cell balancing unit as an equivalent resistor, the behavioral models of the proposed cell balancing circuits are developed to evaluate their balancing performance. Comparing with existing SC-based cell balancing circuits, the main advantage of the proposed circuits is that their balancing speed is independent of both of the number of battery cells and initial mismatch distribution of cell voltages. In order to improve the operation performance of SC-based cell balancing circuits in the respect of minimizing the equivalent resistance, optimizing methodologies of circuit parameters are introduced by referring the concepts of slow switching limit and fast switching limit as well as inductive switching limit of SC power converters. Simulation and experimental results are provided to verify the feasibility of the proposed cell balancing circuits.

Topology, Modeling, and Design of Switched-Capacitor-Based Cell Balancing Systems and Their Balancing Exploration

Voltage balancing between series-connected cells of rechargeable lithium-ion batteries is important for battery life, autonomy, and safety. Active balancing is the designated choice for applications that are sensitive to energy losses and maximized autonomy. First, a well-known next-to-next balancing technique is presented and its design and operating limitations are analyzed. Then, the paper focuses on an evolution of the converter's architecture offering better performances while being more compact, requiring fewer and smaller filtering needs and still, being simple to implement. The experimental results of the balancing operation of a pack of eight cells connected in series show demonstrative and satisfactory results.

An Optimized Topology for Next-to-Next Balancing of Series-Connected Lithium-ion Cells

In this paper, an innovative any cell(s) to any cell(s) active balancing converter for lithium battery stack management is presented. Based on multiphase converter legs connected to each lithium battery potential, it is able to transfer energy from any cell(s) to any cell(s). First, a basic “natural” balancing control strategy is presented. Then, based on the perspective of a high level of integration, an interleaved topology is proposed as an evolution of the converter for the downsizing of the passive components. It is explained how the large increase in the number of components is compensated by the high level of integration obtained for the given converter topology. Simulation and experimental results are provided to demonstrate the interest of the converter for active balancing of lithium-based battery stacks.

Multiphase Interleaved Converter for Lithium Battery Active Balancing

Voltage balancing between series-connected cells of rechargeable lithium-ion batteries is important for battery life, autonomy and security. Active balancing is the designated choice for applications that are sensitive to energy losses. A well-known next-to-next balancing technique is presented and its design and operating limitations are analyzed. This paper then presents a new technique for next-to-next balancing that improves on the conventional technique by offering better performances while being simpler to implement. Experimental results show an important reduction of the size of the magnetic coupler while maintaining efficiency at a level above 90%.

Optimized structure for next-to-next balancing of series-connected lithium-ion cells

In this paper, a simple and flexible active balancing converter for lithium battery stack management is presented. Based on multiphase inverter legs connected at each lithium based battery potential, it is able to transfer energy from any cell(s) to any cell(s). A simple "natural" balancing control strategy is presented. Then, based on the high level of integration, an interleaved evolution of the converter is proposed for the downsizing of the passive components. Simulation and experimental results are provided to demonstrate the interest of the converter for active balancing of lithium based battery stacks.

Multiphase interleaved converter for lithium battery active balancing

This paper presents a hardware-in-the-loop battery simulation platform capable of emulating individual cells of a series connected string. The platform enables an efficient and reproducible way to benchmark battery management systems. The focus is put on battery modeling, embedded system architecture and software development. A practical implementation of a lithium-ion cell emulator based on a linear amplifier is described.

Alexandre Collet

Papers

An Optimized Topology for Next-to-Next Balancing of Series-Connected Lithium-ion Cells

Multiphase Interleaved Converter for Lithium Battery Active Balancing

Optimized structure for next-to-next balancing of series-connected lithium-ion cells

Multiphase interleaved converter for lithium battery active balancing

Multi-cell battery emulator for advanced battery management system benchmarking