Search

This paper introduces a novel approach for rapidly balancing lithium-ion batteries using a single DC–DC converter, enabling direct energy transfer between high- and low-voltage cells. Utilizing relays for cell pair selection ensures cost-effectiveness in the switch network. The control system integrates a battery-monitoring IC and an MCU to oversee cell voltage and ensure battery protection. A prototype circuit with twelve lithium-ion batteries demonstrates the method's efficacy, achieving a remarkable balancing time of 48 min during charging with a maximum efficiency of 89.85%. Comparative analysis with other methods underscores the superior performance of the proposed balancing circuit in terms of balancing time and implementation cost. Furthermore, this paper delves into hardware aspects of battery management systems (BMSs) for electric vehicles and stationary applications. It offers an overview of prevailing concepts in state-of-the-art systems, aiding readers in assessing considerations essential for BMS design in various applications. The discussion includes examples of battery packs sourced from commercially available electric vehicles. Subsequently, the manuscript addresses implementation aspects concerning the measurement of critical physical variables such as voltage, current, and temperature, alongside balancing strategies. [ABSTRACT FROM AUTHOR]

The battery pack is at the heart of electric vehicles, and lithium-ion cells are preferred because of their high power density, long life, high energy density, and viability for usage in relatively high and low temperatures. Lithium-ion batteries are negatively affected by overvoltage, undervoltage, thermal runaway, and cell voltage imbalance. The minimisation of cell imbalance is particularly important because it causes uneven power dissipation by each cell and, hence, temperature distribution that adversely impacts the battery lifetime. Several papers in the literature proposed advanced cell-balancing techniques to increase the effectiveness of basic cell-balancing approaches, reduce power losses, and reduce the number of components in balancing circuits. The new developments and optimisations over the last few years have been particularly intense due to the increased interest in battery technologies for several end-use applications. This paper reviews and discusses recent cell-balancing techniques or methods, covering their operating principles and the optimised utilisation of electrical components. [ABSTRACT FROM AUTHOR]

Lithium‐ion batteries have a very wide application range. They can power up small electronic devices such as smart watches to larger electric vehicles. Due to its varied range of applications, they come in different packaging and in such battery packs, even when individual cell voltage exceeds by a few milli‐volts above 4.2 V, it may result in thermal runaway and explode the cell. During discharge cycle, cell imbalances hinder the use of battery to its full capacity. This in turn decreases the battery lifetime. The individual battery cells should be equalized on a regular basis to keep the imbalances to a minimum and to have a good battery life. The process of balancing the individual cell charges by measuring the cell state of charge (SoC) and its voltage in a battery pack is known as cell balancing. This paper details an active cell balancing technique that uses a buck converter for balancing a series connected battery pack of lithium‐ion cells. A buck converter along with a pair of MOSFET switches for each cell, one turned on for charging the cell and the other one turned on while discharging the cell is used in this experiment. An algorithmic model suitable for reconfigurable battery systems that measures the individual cell voltages and is developed for balancing a pack of series connected Li‐ion battery cells. The developed model is simulated using MATLAB for verifying its performance. A state of charge of 25% is maintained across the cells and when SoC value drops below this even a difference of 0.02% is sensed by the algorithm to initiate balancing function. This balancing is found to take 275 ms to balance three 3.7 V batteries and thus the model is found to respond faster. The results show that this method can self‐adaptively attain satisfactory performance within a limited equalizing period.Lithium ion batteries have a very wide application range. They can power up small electronic devices such as smart watches to larger electric vehicles. Due to its varied range of applications, they come in different packaging and in such battery packs, even when individual cell voltage exceeds by a few milli‐volts above 4.2 V, it may result in thermal runaway and explode the cell. During discharge cycle, cell imbalances hinder the use of battery to its full capacity. This in turn decreases the battery lifetime. The individual battery cells should be equalized on a regular basis to keep the imbalances to a minimum and to have a good battery life. The process of balancing the individual cell charges by measuring the cell State of Charge (SoC) and its voltage in a battery pack is known as Cell Balancing. This paper details an active cell balancing technique that uses a buck converter for balancing a series connected battery pack of lithium‐ion cells. The buck converter is constructed from two MOSFET switches, one turned on for charging the cells and the other one is used for discharging the cells. An algorithmic model that measures the individual cell voltages based on charge equalization is developed for a pack of series connected Li‐ion battery cells. The developed model is simulated using MATLAB for verifying its performance. It is found that this algorithmic model produces a state of charge of 2.5% among the cells in a short period as compared to the conventional cell balancing technique. This parameter directly defines the cell equalization time and thus it is found that the system responds faster. The results show that this method can self‐adaptively attain satisfactory performance within a limited equalizing period. [ABSTRACT FROM AUTHOR]

In this paper, a high-efficiency and low-cost active cell-to-cell balancing circuit for the reuse of electric vehicle (EV) batteries is proposed. In the proposed method, a battery string is divided into two legs to transfer the charge from each cell in one leg to that in the other and a bidirectional CLLC resonant converter is used to transfer energy between the selected cells. Thanks to the proposed structure, the number of bidirectional switches and gate drivers can be reduced by half compared to the conventional direct cell-to-cell topologies, thereby achieving lower cost for the system. The CLLC converter is used to transfer the charge, and it is designed to work at resonant frequencies to achieve zero-voltage zero-current switching (ZVZCS) for all the switches and diodes. Consequently, the system's efficiency can be enhanced, and hence, the fuel economy of the system can also be improved significantly. To verify the performance of the proposed active cell-balancing system, a prototype is implemented for balancing the three EV battery modules that contain twelve lithium-ion batteries from xEV. The maximum efficiency achieved for the charge transfer is 89.4%, and the balancing efficiency is 96.3%. [ABSTRACT FROM AUTHOR]

In this paper, a new control strategy, along with an innovative switching pattern, is proposed for the transfer of charge from one cell to another, whether they are adjacent or nonadjacent, or even to multiple cells simultaneously. Therefore, this strategy improves the efficiency of state of charge balancing, which is critical for lithium polymer (LiPo) batteries. Unlike in previous studies, all these functions are realized with a simple structure that employs standard DC–DC buck‐boost converters, without any additional complexities. By implementing the proposed control strategy, the effects of inaccuracies in the gain of current sensors, as well as multiplicative measurement noises, are reduced. The proposed method is applied to a set of five series LiPo cells, and a comprehensive investigation is conducted to assess both its balancing algorithm and operation. The simulation results illustrate the effective balancing performance and the achievement of the desired charge transfer through the proposed approach. An experimental setup, which includes five LiPo cells with a capacity of 1530 milliampere‐hours (mAh), is used to verify the proposed method. The outcomes demonstrate that the balancing of all cells is accurately performed under different initial conditions, within an acceptable time frame. [ABSTRACT FROM AUTHOR]

Recently, as the perception of eco-friendliness has changed, the demand for energy storage devices has been rapidly increasing due to the growth of the electric vehicle industry and smart grid facilities, which are emerging as an alternative to next-generation electricity supply and demand. Therefore, the importance of battery management technology is growing, and various voltage balancing techniques between battery cells are being studied in order to maintain high efficiency and continuous performance of batteries. This paper proposes a voltage balancing topology using a single input-multiple output (SIMO) two-switch flyback converter in a series battery configuration to resolve voltage imbalance between batteries. The characteristic of the proposed topology is that each cell on the secondary side of the two-switch flyback converter is connected to one high-frequency transformer to share the magnetic flux, and voltage balancing is performed according to the switch operation of the converter. At this time, the accumulated excess energy of the converter is refluxed to the power supply side through the freewheeling diode and converted into reactive power. The verification of the usefulness of the theoretical analysis in this paper was based on the analysis of the dynamic characteristics and steady state of the circuit through PSIM and experiments, and was conducted for one module composed of four cells. [ABSTRACT FROM AUTHOR]

Thermal and state-of-charge (SOC) imbalances are well known to cause nonuniform aging in batteries. This paper presents the electrothermal control of a multilevel converter-based modular battery to address this issue. The modular battery provides a large redundancy in synthesizing terminal voltage, which gives extra degrees of freedom in control on cell level. There are multiple tightly coupled control objectives including the simultaneous thermal and SOC balancing as well as battery terminal voltage control. The main purpose of this paper is to devise an electrothermal control scheme for cases where full future driving information is not accessible. The control scheme is based on decomposition of controller into two orthogonal components, one for voltage control and the other for balancing control. This problem decomposition enables the application of constrained linear quadratic model predictive control scheme to solve the balancing problem elegantly. The control scheme is thoroughly evaluated through simulations of a four cell modular battery. The results show that a rather short prediction horizon is sufficient to achieve robust control performance. [ABSTRACT FROM AUTHOR]

Li‐ion batteries are influenced by numerous features such as over‐voltage, undervoltage, overcharge and discharge current, thermal runaway, and cell voltage imbalance. One of the most significant factors is cell imbalance which varies each cell voltage in the battery pack overtime and hence decreases battery capacity rapidly. To increase the lifetime of the battery pack, the battery cells should be frequently equalized to keeps up the difference between the cells as small as possible. There are different techniques of cell balancing have been presented for the battery pack. It is classified as passive and active cell balancing methods based on cell voltage and state of charge (SOC). The passive cell balancing technique equalizing the SOC of the cells by the dissipation of energy from higher SOC cells and formulates all the cells with similar SOC equivalent to the lowest level cell SOC. The active cell balancing transferring the energy from higher SOC cell to lower SOC cell, hence the SOC of the cells will be equal. This review article introduces an overview of different proposed cell balancing methods for Li‐ion battery can be used in energy storage and automobile applications. [ABSTRACT FROM AUTHOR]

This paper introduces a novel approach for rapidly balancing lithium-ion batteries using a single DC–DC converter, enabling direct energy transfer between high- and low-voltage cells. Utilizing relays for cell pair selection ensures cost-effectiveness in the switch network. The control system integrates a battery-monitoring IC and an MCU to oversee cell voltage and ensure battery protection. A prototype circuit with twelve lithium-ion batteries demonstrates the method’s efficacy, achieving a remarkable balancing time of 48 min during charging with a maximum efficiency of 89.85%. Comparative analysis with other methods underscores the superior performance of the proposed balancing circuit in terms of balancing time and implementation cost. Furthermore, this paper delves into hardware aspects of battery management systems (BMSs) for electric vehicles and stationary applications. It offers an overview of prevailing concepts in state-of-the-art systems, aiding readers in assessing considerations essential for BMS design in various applications. The discussion includes examples of battery packs sourced from commercially available electric vehicles. Subsequently, the manuscript addresses implementation aspects concerning the measurement of critical physical variables such as voltage, current, and temperature, alongside balancing strategies.

The cell-to-cell variability of batteries is a well-known problem particularly when it comes to the assembly of large battery packs. Different battery cells exhibit substantial variability due to manufacturing tolerances, which should be assessed and managed carefully. Such variability has been approached mostly from the point of view of the chemical and physical phenomena, but these solutions are normally too complicated for the system-level design of electric applications. This paper proposes a combined cell-to-cell variability model of the capacity and internal resistance of a Li-ion battery that accounts for the variability effects in the cell manufacturing process. The proposed model allows to verify some known properties, such as the correlation between the capacity and internal resistance, to be verified qualitatively and the amount of variability and its impact on the design of battery packs to be assessed quantitatively. Using this model, the issue of how to consider the variability when constructing battery packs was also addressed. Modern battery packs normally incorporate some cell balancing circuitry, which is meant to balance cell voltages during charging at the expense of a bypassed (unstored) charge. For discharge, the cell-to-cell variability hides a part of the usable capacity of the battery pack. This paper proposes the use of variability information to assemble battery packs with minimal intracolumn variance of capacity. A weight-based variance minimization method, based on the correlation between cell capacity and weight is proposed to avoid resorting to direct battery capacity measurements, which is time-consuming and requires costly measurement equipment. The simulation result shows that the proposed weight-based approach allows an acceptable management of the cell-to-cell variability without the discharging experiment. [ABSTRACT FROM PUBLISHER]

The second-generation hybrid and Electric Vehicles are currently leading the paradigm shift in the automobile industry, replacing conventional diesel and gasoline-powered vehicles. The Battery Management System is crucial in these electric vehicles and also essential for renewable energy storage systems. This review paper focuses on batteries and addresses concerns, difficulties, and solutions associated with them. It explores key technologies of Battery Management System, including battery modeling, state estimation, and battery charging. A thorough analysis of numerous battery models, including electric, thermal, and electro-thermal models, is provided in the article. Additionally, it surveys battery state estimations for a charge and health. Furthermore, the different battery charging approaches and optimization methods are discussed. The Battery Management System performs a wide range of tasks, including as monitoring voltage and current, estimating charge and discharge, equalizing and protecting the battery, managing temperature conditions, and managing battery data. It also looks at various cell balancing circuit types, current and voltage stressors, control reliability, power loss, efficiency, as well as their advantages and disadvantages. The paper also discusses research gaps in battery management systems.

This paper presents an energy sharing state-of-charge (SOC) balancing control scheme based on a distributed battery energy storage system architecture where the cell balancing system and the dc bus voltage regulation system are combined into a single system. The battery cells are decoupled from one another by connecting each cell with a small lower power dc–dc power converter. The small power converters are utilized to achieve both SOC balancing between the battery cells and dc bus voltage regulation at the same time. The battery cells' SOC imbalance issue is addressed from the root by using the energy sharing concept to automatically adjust the discharge/charge rate of each cell while maintaining a regulated dc bus voltage. Consequently, there is no need to transfer the excess energy between the cells for SOC balancing. The theoretical basis and experimental prototype results are provided to illustrate and validate the proposed energy sharing controller. [ABSTRACT FROM PUBLISHER]

The battery pack is at the heart of electric vehicles, and lithium-ion cells are preferred because of their high power density, long life, high energy density, and viability for usage in relatively high and low temperatures. Lithium-ion batteries are negatively affected by overvoltage, undervoltage, thermal runaway, and cell voltage imbalance. The minimisation of cell imbalance is particularly important because it causes uneven power dissipation by each cell and, hence, temperature distribution that adversely impacts the battery lifetime. Several papers in the literature proposed advanced cell-balancing techniques to increase the effectiveness of basic cell-balancing approaches, reduce power losses, and reduce the number of components in balancing circuits. The new developments and optimisations over the last few years have been particularly intense due to the increased interest in battery technologies for several end-use applications. This paper reviews and discusses recent cell-balancing techniques or methods, covering their operating principles and the optimised utilisation of electrical components.

Among various active cell balancing circuits, a switched capacitor circuit is promising because it can be implemented with low cost and small size. However, when the switched capacitor is applied in the lithium-ion battery, cell balancing speed is generally slow when the number of batteries is high. Therefore, this paper proposes the chain structure of the switched capacitor to increase balancing speed, particularly among outer cells. In this paper, the cell balancing principle of the conventional switched capacitor is explained, and the reason why slow cell balancing of the switched capacitor is shown in the lithium-ion battery is analyzed. To improve cell balancing speed, two circuits with chain structure are proposed. The balancing performance of the proposed circuits is confirmed by computer simulation, and a comparison between conventional and proposed circuits is presented. The theoretical analysis on the cell balancing speed of conventional structures and the proposed chain structure is also shown in this paper. Experimental tests were carried out to verify the validity of the proposed structures, and the experimental results show an improved balancing performance of the proposed circuit. [ABSTRACT FROM PUBLISHER]

The economic value of high-capacity battery systems, being used in a wide variety of automotive and energy storage applications, is strongly affected by the duration of their service lifetime. Because many battery systems now feature a very large number of individual cells, it is necessary to understand how cell-to-cell interactions can affect durability, and how to best replace poorly performing cells to extend the lifetime of the entire battery pack. This paper first examines the baseline results of aging individual cells, then aging of cells in a representative 3S3P battery pack, and compares them to the results of repaired packs. The baseline results indicate nearly the same rate of capacity fade for single cells and those aged in a pack; however, the capacity variation due to a few degrees changes in room temperature (±3 °C) is significant (±1.5% of capacity of new cell) compared to the percent change of capacity over the battery life cycle in primary applications ('20-30%). The cell replacement strategies investigation considers two scenarios: early life failure, where one cell in a pack fails prematurely, and building a pack from used cells for less demanding applications. Early life failure replacement found that, despite mismatches in impedance and capacity, a new cell can perform adequately within a pack of moderately aged cells. The second scenario for reuse of lithium ion battery packs examines the problem of assembling a pack for less-demanding applications from a set of aged cells, which exhibit more variation in capacity and impedance than their new counterparts. The cells used in the aging comparison part of the study were deeply discharged, recovered, assembled in a new pack, and cycled. We discuss the criteria for selecting the aged cells for building a secondary pack and compare the performance and coulombic efficiency of the secondary pack to the pack built from new cells and the repaired pack. The pack that employed aged cells performed well, but its efficiency was reduced. [ABSTRACT FROM AUTHOR]

Among the various cell balancing circuits for series-connected batteries, the bidirectional cell-to-cell balancing structure using two switches per cell shows good balancing ability. However, an increase in the number of switches leads to an increase in both cost and size. To reduce the number of switches, this paper proposes a new cell-to-cell balancing circuit with a center-cell concentration structure. The proposed circuit collects the charges of an overcharged cell into the center cell, and then the collected charges are redistributed to the other cells. The proposed circuit has a unidirectional balancing structure with single switch per cell. However, it can achieve competitive balancing ability when compared with the bidirectional structure. In this paper, the operational principles of the proposed circuit are analyzed, and a comparison between the conventional and proposed circuits is shown. To verify the validity of the proposed circuit, experiments with 15.5-Ah batteries are carried out, and the experimental results demonstrate the improved cell balancing ability although a single switch is used per cell. [ABSTRACT FROM AUTHOR]

This paper presents the development of a hardware-in-the-loop (HIL) simulation system that facilitates the experimental testing of cell balancing circuits by emulating energy storage components. The system is developed with four cell simulators, each of which can emulate the battery voltage with the capabilities of sourcing and sinking current for the cell balancing circuit being tested. The cell simulator can also operate in battery testing mode for characterizing the behavior of a battery. One Li-polymer cell has been characterized using the cell simulator and a battery model has been built by the extracted parameters from the measurements. An active cell balancing experiment was conducted with a real 4-cell Li-polymer module followed by the same experiment using the HIL simulation system to emulate the Li-polymer cells based upon the method described in this paper. With the real battery module having a 9.4% SOC deviation initially, the final SOC deviation is reduced to 1.0% and the balancing time is 51.4 min. The HIL simulation gives very close results where the final SOC deviation and the balancing time are 1.2% and 52.0 min, respectively. The results demonstrate that the system can provide representative results for testing cell-balancing circuits with minimum resources and time. [ABSTRACT FROM AUTHOR]

An active charge equalization technique based on a dc/dc converter topology is proposed in this paper. The technique achieves cell balancing of batteries in a stack in terms of both voltage and charge as the pack is being charged/discharged and in idle periods to maximize the energy and reliability of stack operation. A set of MOSFET switches controlled by a voltage monitoring circuit ensures that each battery module has the same output voltage by transferring charge from an individual battery module with the highest voltage to a weak module. An overvoltage and overdischarge protection circuit is presented to reduce the degradation of battery life and to operate each battery within the voltage limits. [ABSTRACT FROM PUBLISHER]

This paper presents a hierarchical balancing algorithm for use in large-scale reconfigurable ac battery packs. Using a decentralized battery management system, the pack is organized in control layers with each layer controller responsible for a small number of slave controllers, thereby significantly reducing communication and centralized processing requirements. The hierarchical balancing algorithm uses all available voltage steps in the pack ensuring a high-quality sinusoidal output voltage while balancing the objects of different layers. Balancing priority can be swapped dynamically between layers according to the maximum state of charge (SoC) variation between the objects of each layer while also ensuring that the output voltage reference is always met even when a cell failure occurs, as long as there are an adequate number of cells available in the pack. An experimental demonstration of the proposed balancing algorithm is presented using an ac battery pack comprising 144 20-Ah lithium–titanate cells. Cell balancing to within 2 mV of the cell open-circuit voltage is achieved between cells with 20% initial SoC variation and 5% capacity difference. [ABSTRACT FROM PUBLISHER]

To overcome the problem that the balancing performance of existing switched-capacitor (SC) cell balancing systems drops along with the increase in the number of series-connected battery cells, a novel SC cell balancing circuit is presented in this paper. The same as other SC balancing systems, only a pair of complementary square-wave signals is required to control the proposed circuit. With resonant SC design, all switches employed in the proposed balancing circuit operate under zero-current switching. The equivalent model is derived to reveal the balancing performance of the proposed balancing circuit. The system feasibility and theoretical analysis are verified by both of simulation and experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Among the wide diversity of existing technologically mature batteries, lithium-ion (Li-ion) batteries have become popular because of their longevity, high energy density, high efficiency and lack of memory effect. Differential charging of cells with age has turned balancing management systems into an important research subject. This paper proposes a new battery management system (BMS) to improve the capacity usage and lifespan of large Li-ion battery packs and a new charging algorithm based on the traditional multistage method. The main advantages of the proposed system are its versatility and ability to implement different charging and balancing methods in a very accessible way. The combination of charging methods with balancing methods represents an evolution when compared with other works in the literature. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a new control strategy, along with an innovative switching pattern, is proposed for the transfer of charge from one cell to another, whether they are adjacent or nonadjacent, or even to multiple cells simultaneously. Therefore, this strategy improves the efficiency of state of charge balancing, which is critical for lithium polymer (LiPo) batteries. Unlike in previous studies, all these functions are realized with a simple structure that employs standard DC–DC buck‐boost converters, without any additional complexities. By implementing the proposed control strategy, the effects of inaccuracies in the gain of current sensors, as well as multiplicative measurement noises, are reduced. The proposed method is applied to a set of five series LiPo cells, and a comprehensive investigation is conducted to assess both its balancing algorithm and operation. The simulation results illustrate the effective balancing performance and the achievement of the desired charge transfer through the proposed approach. An experimental setup, which includes five LiPo cells with a capacity of 1530 milliampere‐hours (mAh), is used to verify the proposed method. The outcomes demonstrate that the balancing of all cells is accurately performed under different initial conditions, within an acceptable time frame.

Battery management systems are important for the safe and efficient operation of electric vehicles. Although high hardware performance and effective configurations of batteries have been realized, a management algorithm is required for ensuring optimal system performance. This review focuses on optimal controllers for charging, thermal control, and cell balancing of electric vehicles. A potential approach for practical applications is the direct optimal control method, particularly model predictive control (MPC). The objective function, prediction model types, and manipulated variables are summarized, along with the computational performance. Typical nonlinear MPC, linear MPC, explicit MPC, and hierarchical MPC are the main formulations for the optimal control of EVs. The AI‐based approach learns the optimal control law as a function from the optimal control result data. Although few studies have applied the reinforcement approach to battery systems, additional safety considerations for constraints must be considered for real applications. Cell variations, aging factors, and uncertainty considerations have been analyzed for improving the controller design. Addressing the computational issue with a reliable optimizer is critical to the implementation of an optimal controller for EVs. [ABSTRACT FROM AUTHOR]

Recently, as the perception of eco-friendliness has changed, the demand for energy storage devices has been rapidly increasing due to the growth of the electric vehicle industry and smart grid facilities, which are emerging as an alternative to next-generation electricity supply and demand. Therefore, the importance of battery management technology is growing, and various voltage balancing techniques between battery cells are being studied in order to maintain high efficiency and continuous performance of batteries. This paper proposes a voltage balancing topology using a single input-multiple output (SIMO) two-switch flyback converter in a series battery configuration to resolve voltage imbalance between batteries. The characteristic of the proposed topology is that each cell on the secondary side of the two-switch flyback converter is connected to one high-frequency transformer to share the magnetic flux, and voltage balancing is performed according to the switch operation of the converter. At this time, the accumulated excess energy of the converter is refluxed to the power supply side through the freewheeling diode and converted into reactive power. The verification of the usefulness of the theoretical analysis in this paper was based on the analysis of the dynamic characteristics and steady state of the circuit through PSIM and experiments, and was conducted for one module composed of four cells.

This paper presents a method for achieving individual electrochemical cell balancing by using a cascaded full-bridge multilevel converter where a single electrochemical cell is connected to each converter module. As a result, balancing at cell level is possible without additional circuitry, making this topology ideal for long-service-life grid storage and applications using second-life cells where the cells are inherently poorly matched. In order to estimate the relative state of charge between cells, the control flexibility of the multilevel converter is used to remove each cell from the current path without interrupting the operation of the system. This process eliminates the effect of the internal cell resistance and fast transient electrochemical phenomena, and therefore, the measured voltage serves as a high-quality “pseudo-open-circuit” voltage measurement. The proposed balancing strategy is validated using a 25-level cascaded full-bridge multilevel converter prototype for the individual balancing of 12 lithium polymer cells, during consecutive charging and discharging cycles. Successful balancing to within 5 mV of open-circuit voltage is observed between cells with 45% difference in nominal capacity and 55% initial state-of-charge variation. [ABSTRACT FROM PUBLISHER]

A charge equalizer with a combination of asymmetrical pulse width modulation and pulse frequency modulation control based on a modified half-bridge converter for electric vehicles is proposed in this paper. The reliability of the charge equalizer can be improved because the split capacitors are not required and the soft switching of power switches can be achieved by the proposed control. In addition, with the proposed rectification, the number of bridge rectifiers at the secondary side of the transformer can be reduced by half, which also cuts the cost and conduction loss by half. The circuit topology, operation analysis, and control strategy of the proposed charge equalizer are described in detail. Finally, the experimental results are provided to verify the feasibility and performance of the proposed charge equalizer. [ABSTRACT FROM PUBLISHER]

Abstract Lithium‐ion batteries have a very wide application range. They can power up small electronic devices such as smart watches to larger electric vehicles. Due to its varied range of applications, they come in different packaging and in such battery packs, even when individual cell voltage exceeds by a few milli‐volts above 4.2 V, it may result in thermal runaway and explode the cell. During discharge cycle, cell imbalances hinder the use of battery to its full capacity. This in turn decreases the battery lifetime. The individual battery cells should be equalized on a regular basis to keep the imbalances to a minimum and to have a good battery life. The process of balancing the individual cell charges by measuring the cell state of charge (SoC) and its voltage in a battery pack is known as cell balancing. This paper details an active cell balancing technique that uses a buck converter for balancing a series connected battery pack of lithium‐ion cells. A buck converter along with a pair of MOSFET switches for each cell, one turned on for charging the cell and the other one turned on while discharging the cell is used in this experiment. An algorithmic model suitable for reconfigurable battery systems that measures the individual cell voltages and is developed for balancing a pack of series connected Li‐ion battery cells. The developed model is simulated using MATLAB for verifying its performance. A state of charge of 25% is maintained across the cells and when SoC value drops below this even a difference of 0.02% is sensed by the algorithm to initiate balancing function. This balancing is found to take 275 ms to balance three 3.7 V batteries and thus the model is found to respond faster. The results show that this method can self‐adaptively attain satisfactory performance within a limited equalizing period.

The consistency of lithium-ion battery packs is extremely important to prolong battery life, maximize battery capacity and ensure safety operation in electric vehicles. In this paper, a model predictive control (MPC) method with a fast-balancing strategy is proposed to address the inconsistency issue of individual cell in lithium-ion battery packs. Firstly, an optimal energy transfer direction is investigated to improve equalization efficiency and reduce energy loss. Then, a MPC-based equalization algorithm is developed to obtain the optimal constant equalization current by directly minimizing equalization time of battery's SOC. Moreover, a fast-solving strategy for MPC is designed to reduce the computational burden of cells' equalization. Finally, the performance of proposed MPC algorithm has been compared with other MPC-based equalization methods in three different equalization topologies (cell-to-cell, cell-to-pack and module-based equalization topology), the results indicate that the proposed algorithm achieves faster equalization speed and less energy loss in three equalization topologies. Importantly, the proposed algorithm avoids the repeated charging and discharging of intermediate batteries effectively, and ensures the single-point convergence of cells' SOC. Furthermore, the effectiveness and accuracy of proposed fast-solving strategy for MPC algorithm is verified by comparison with common solving strategies, the results show the proposed method takes less computational time to obtain the accurate optimal balancing current, indicating that the proposed fast-solving strategy can improve computation speed and reduce computational burden. • A MPC-based algorithm is developed for three different equalization topologies. • Optimal energy transfer direction is investigated to improve equalization efficiency. • Optimal constant current is obtained by minimizing equalization time of cells' SOC. • Fast-solving strategy is designed to reduce computation cost of cells' equalization. [ABSTRACT FROM AUTHOR]

To meet the load voltage and power requirements for various specific needs, a typical lithium–ion battery (LIB) pack consists of different parallel and series combinations of individual cells in modules, which can go as high as tens of series and parallel connections in each module, reaching hundreds and even thousands of cells at high voltage (HV) levels. The inhomogeneity among the cells and modules results in voltage imbalances during operation and reduces the overall system efficiency. In this work, a robust and flexible active balancing topology is presented. It can not only mitigate the charge imbalance within a module, i.e., intramodular equalization, but also help to balance the state of charge (SoC) level of the modules in a high voltage pack, i.e., intermodular equalization, which is an often-overlooked topic. The proposed concept was proven by experimental verification on parallel and series configurations of cells in realistically sized modules and practical battery management system (BMS) hardware, when the LIB was both idle and under load. [ABSTRACT FROM AUTHOR]

In this paper, a high-efficiency and low-cost active cell-to-cell balancing circuit for the reuse of electric vehicle (EV) batteries is proposed. In the proposed method, a battery string is divided into two legs to transfer the charge from each cell in one leg to that in the other and a bidirectional CLLC resonant converter is used to transfer energy between the selected cells. Thanks to the proposed structure, the number of bidirectional switches and gate drivers can be reduced by half compared to the conventional direct cell-to-cell topologies, thereby achieving lower cost for the system. The CLLC converter is used to transfer the charge, and it is designed to work at resonant frequencies to achieve zero-voltage zero-current switching (ZVZCS) for all the switches and diodes. Consequently, the system’s efficiency can be enhanced, and hence, the fuel economy of the system can also be improved significantly. To verify the performance of the proposed active cell-balancing system, a prototype is implemented for balancing the three EV battery modules that contain twelve lithium-ion batteries from xEV. The maximum efficiency achieved for the charge transfer is 89.4%, and the balancing efficiency is 96.3%.

This paper presents small-signal modeling, analysis and closed-loop controller design guidelines for a distributed battery energy storage system with energy sharing controller which has recently been presented in the literature in order to achieve cell balancing with high cell balancing speed and energy efficiency. The derived small signal models provide deeper insight into the dynamics of the energy sharing controlled battery system under different operating modes, including discharge mode, constant current charging mode and constant voltage charging mode. Based on the derived small signal models, closed-loop controller design guidelines are provided based on rule-of-thumb frequency-domain design criteria. The small signal models and designed controllers are validated by MATLAB®/SIMULINK simulation and experimental prototype results. [ABSTRACT FROM AUTHOR]

Flexible, manageable, and more efficient energy storage solutions have increased the demand for electric vehicles. A powerful battery pack would power the driving motor of electric vehicles. The battery power density, longevity, adaptable electrochemical behavior, and temperature tolerance must be understood. Battery management systems are essential in electric vehicles and renewable energy storage systems. This article addresses concerns, difficulties, and solutions related to batteries. The battery management system covers voltage and current monitoring; charge and discharge estimation, protection, and equalization; thermal management; and battery data actuation and storage. Furthermore, this study characterized the various cell balancing circuit types, their components, current and voltage stresses, control reliability, power loss, efficiency, size and cost, and their benefits and drawbacks. Secondly, we review concerns and challenges in battery management systems. Furthermore, we identify problems and obstacles that need additional attention for optimal and sustainable battery management systems for electric vehicles and renewable energy storage systems. Our last topic will be on issues for further research. [ABSTRACT FROM AUTHOR]

Summary: The slow balancing speed of switched‐capacitor (SC)‐based equalizers makes this structure difficult to apply in series‐connected battery strings. In order to reduce the number of energy conversion processes and achieve leapfrog transmission of energy, a hybrid‐structured voltage equalizer (HSVE) is developed in this work for battery strings to achieve high‐speed any‐unit‐to‐any‐unit (AU2AU) equalization, in which each unit can also achieve internal balance in any arbitrary imbalance status. Compared to the conventional equalizers using complex monitoring and control strategy, the proposed equalizer does not need any cell monitoring circuits—all MOSFETs are triggered by a pair of complementary pulse signals with a fixed switching frequency and constant duty ratio, and electricity can automatically and directly exchange among all battery cells. Hardware experiments on super‐capacitors and Li‐ion batteries are conducted in this study, which prove the feasibility of the proposed voltage equalizer. The proposed battery management strategy will have wide applications in modern battery‐related industries such as electric vehicles (EVs). [ABSTRACT FROM AUTHOR]

Lithium-Ion battery packs are an essential component for electric vehicles (EVs). These packs are configured from hundreds of series and parallel connected cells to provide the necessary power and energy for the vehicle. An accurate, adaptable battery management system (BMS) is essential to monitor and control such a large number of cells. Series and parallel connected cells also experience different production and operational conditions, which makes it challenging for the BMS to ensure the safe operation of each individual cell. The main functions of the BMS include battery state estimation, cell balancing, thermal management, and fault diagnosis. Robust estimation of the state of charge (SOC) is crucial for providing the driver with an accurate indication of the remaining range. This paper presents the state of art of battery pack SOC estimation methods along with the impact of cell inconsistency on pack performance and SOC estimation. Cell balancing methods, which are necessary due to cell inconsistencies, are discussed as well. Four categories of pack SOC estimation methods are presented, including individual cell, lumped cell, reference cell, and mean cell and difference estimation methods. The SOC estimation methods are compared in terms of algorithm type, computational load, and engineering effort to help practitioners decide which method best fits their application.