Lithium-ion Battery String Research Articles

An Improved Bi-Switch Flyback Converter with Loss Analysis for Active Cell Balancing of the Lithium-Ion Battery String

This paper focuses on the active cell balancing of lithium-ion battery packs. An improved single-input, multioutput, bi-switch flyback converter was proposed to achieve effective balancing. The proposed topology simplifies the control logic by utilising a single MOSFET switch for energy transfer and two complementary pulses to control the cell-selecting switches. The proposed topology can decrease the number of switching devices as well as the size and cost of the system. The bi-switch flyback converter eliminates the need for a separate buffer circuit to minimise leakage and electromagnetic inductance. Losses and energy efficiency were analysed at each end of the proposed topology. The appropriate MATLAB simulations investigated the balancing characteristics of various state of charge (SOC) imbalances. A comparison is made between the balancing speed and energy transfer efficiency of the proposed topology and a conventional topology that employs a multi-input and multi-output flyback converter in a static mode. The results of the MATLAB simulation were validated by the OPAL-RT (OP5700) real-time simulator. The balancing data of the proposed topology were compared using MATLAB simulation and real-time simulation. This work may reduce the time required to assemble and commission the hardware for the proposed topology’s real-time implementation.

Active balancing circuit for advanced lithium-ion batteries used in photovoltaic application

An approach to the analysis and design of a bidirectional DC power converter for the cell voltage balancing control of a series connected lithium-ion battery string is presented in this paper. The proposed Cell Balancing Circuit (CBC) is designed to transfer the energy from the fully charged battery cell to the weakest one using a switch mode power converter operation. This operation maintains cell batteries at the same State-Of-Charge (SOC) and voltage range. Unlike previous battery balancing circuits, the balancing method uses only one magnetic component, resulting small size system. Simulation and experimental results show that the proposed cell balancing method can not only enhance the bidirectional battery equalization performance, but can also reduce the switching loss during the equalization period. Experiment results are provided to verify the operating principle of the proposed balancing method. Specific conditions of experiments are used to reproduce photovoltaic operations

Redundant Thermal Fault Diagnosis and Localization for Lithium-ion Battery Strings

Redundant Thermal Fault Diagnosis and Localization for Lithium-ion Battery Strings

A minor-fault diagnosis approach based on modified variance for lithium-ion battery strings

In order to prevent battery accidents of electric vehicles (EVs), it is significant to quickly diagnose and recognize faults of lithium-ion battery strings. Nevertheless, the diagnosis of initial minor faults is an intractable problem because the minor fault voltages often do not trigger the cut-off voltage and are hidden in the normal voltage sequences which pose a great threat to the safe operation of electric vehicles. Therefore, this paper proposes a simple real-time minor fault diagnosis approach based on modified variance. By calculating the modified variance of the battery voltage sequences in a sliding-window, the proposed diagnosis approach is able to effectively discern the type and the time of minor battery abnormities, which comprises open circuit and short circuit faults. The purpose of the sliding-window is to take a sliding reading of the voltage sequence in a window with appropriate length in order to improve the accuracy of battery fault diagnosis. Then, the experimental results and the contrast with the previously existing approaches verify the feasibility of the proposed method with low computational cost, easy realization, and without the battery model. Particularly, it can be used to effectively diagnose the minor faults of the battery under different ambient temperatures because of strong robustness and sensitivity. In conclusion, the proposed minor fault diagnosis method has practicable applications in electric vehicles.

An any-cell(s)-to-cell(s) equalization method with a single magnetic component for Lithium-ion battery pack

This paper develops a bi-directional active equalization method using an inductor with multiple balancing paths. The new method allows the energy to transfer directly from any cell(s) to any cell(s) which can increase the equalization speed, reduce the energy conversion times and efficiently prevent the batteries from overcharging or undercharging. We adopt the bi-directional switches of low conduction loss for high equalization efficiency. To further improve the battery equalization system performance, the impact factors of the average balancing current which is the main measurement index of the balancing speed are first specifically discussed and the efficiency is also evaluated. The experiments with a four-20Ah Lithium-ion battery string are conducted and the results demonstrate that the developed approach has achieved a good overall performance of equalization.

An Integrated Cascade Structure-Based Isolated Bidirectional DC–DC Converter for Battery Charge Equalization

In this article, an isolated bidirectional dc-dc converter with an integrated cascade structure is proposed for a centralized charge equalization system. Compared with existing architectures, the proposed converter structure is equivalent to the integration of polarity switches and a bidirectional dc-dc converter. Equalization of cells with different voltage polarities can be realized by controlling the working state of the converter, which further improves the reliability and integration of the equalization system. The integrated cascade structure can not only increase the equalization current and make it work in a continuous state but also achieve a high voltage conversion ratio to improve the efficiency of the transformer. Theoretical derivations of the converter and the conditions of soft-switch implementation are analyzed in detail. Finally, equalization experiments on a 13-cell lithium-ion battery string are carried out, and a quantitative and comprehensive comparison of different centralized equalizer is conducted. The experimental and comparison results validate the superiorities of the proposed equalizer and demonstrate obvious performance enhancement over the conventional method.

A Low Cost and Fast Cell-to-Cell Balancing Circuit for Lithium-Ion Battery Strings

This paper proposes a fast cell-to-cell balancing circuit for lithium-ion battery strings. The proposed method uses only one push-pull converter to transfer energy between high- and low-voltage cells directly for a fast balancing speed. The switch network for selecting a certain pair of cells is implemented using relays to achieve a low cost. The control circuit is composed of a battery-monitoring IC and a digital signal processor (DSP) to monitor the cell voltage and to protect the batteries. In order to prove the validity of the proposed method, a prototype circuit is built with twelve lithium-ion batteries in a string. The experimental results show that it takes only 50 min to balance twelve lithium-ion batteries during the charge with 89.5% maximum efficiency. The outstanding performance of the proposed cell balancing circuit is verified through its comparison with other methods in terms of several factors, such as the balancing time and the implementation cost.

A multi-fault diagnosis method based on modified Sample Entropy for lithium-ion battery strings

The conventional fault-diagnosis methods are difficult to detect the battery faults in the early stages without obvious battery abnormality because lithium-ion batteries are complex nonlinear time-varying systems with absolute cell inconsistency. Therefore, this paper proposes a real-time multi-fault diagnosis method for the early battery failure based on modified Sample Entropy. By detecting the modified Sample Entropy of the cell-voltage sequences in a moving window, the proposed diagnosis method can diagnose and predict different early battery faults, including short-circuit and open-circuit faults, and can also predict the time of the faults occurring. The experimental results and the comparison with the conventional methods verify the validity of the proposed solution with strong robustness, high reliability and low computational cost, and without the need of a precise model. In summary, the proposed multi-fault diagnosis approach is feasible and promising in real electric vehicle applications.

An Optimized Mesh-Structured Switched-Capacitor Equalizer for Lithium-Ion Battery Strings

The existing switched-capacitor (SC)-based equalizers either have slow equalizing speed and low efficiency or have bulky volume and low reliability. Therefore, an optimized mesh-structured SC equalizer (MSSCE) is introduced to achieve a tradeoff between the equalization performances and size as well as cost. The proposed MSSCE achieves all the shortest balancing paths from one cell to any cell in a battery string, significantly improving the equalization speed and efficiency. The switching impedance of the proposed MSSCE is derived and analyzed, which provides design guidance for the switching frequency and the capacitance. A four-cell prototype was set up. Experimental results confirm that the proposed MSSCE enhances the balancing speed with a high efficiency of 91.7%, moreover which is robust to the imbalanced cell voltages.

A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles

In this paper, a cell balancing topology for a series-connected Lithium-Ion battery string (SCBS) in electric vehicles is proposed and experimentally verified. In particular, this balancing topology based on the modular balancer consists of an intra-module balancer based on a multi-winding transformer circuit and an outer-module balancer based on a switched capacitor converter, both offering the potential advantages and over conventional balancing methods, including short equalization time, simple control scheme, elimination of voltage sensors. In addition, a number of cells in the SCBS can be easily extended in this circuit. Furthermore, a system structure and an operating principle of the proposed topology are analyzed and experimentally verified for three different cases. The voltages of all cells in the SCBS reached the balanced state regardless of the various arrangement of the initial voltage, where the energy efficiency of the circuit reached 83.31%. Our experimental realization of the proposed balancing topology shows that such a technique could be employed in electric vehicles.

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.

A Multi-winding Transformer Cell-to-Cell Active Equalization Method for Lithium-Ion Batteries with Reduced Number of Driving Circuits

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 Modularized Equalization Method Based on Magnetizing Energy for a Series-Connected Lithium-Ion Battery String

A single charge equalizer using the multi-winding transformer (SCEMT) is useful for the precise and fast equalization. Since this type of equalizer requires voltage sensing circuits for each battery cell, the size and cost of the overall equalizer system increase. In this paper, a novel switching method, which does not require voltage sensing circuits, is proposed for the SCEMT. However, due to the problems related to the implementation of multiwinding in a single transformer, the SCEMT is definitely difficult to apply to long series-connected lithium-ion battery string. In order to overcome these drawbacks, a novel module charge equalizer based on the configuration of the SCEMT is developed. Unlike the conventional module charge equalizer using additional components, the proposed novel module charge equalizer utilizes the magnetizing energy of the multiwinding transformer for the equalization among modules. Therefore, proposed module charge equalizer does not suffer from the size, cost, and loss related to the modularization. The validity of the proposed module charge equalizer is verified through experimental results.

State-of-charge estimation and uncertainty for lithium-ion battery strings

The state-of-charge (SOC) estimation is of extreme importance for the reliability and safety of battery operation. How to estimate SOC and, to some degree, the SOC convention itself, is still a subject of great interest. Here a viable SOC convention valid for single cells and multi-cell strings is proposed and validated. Using a 3S1P string as an illustration in this work, the direct inference from a correct open circuit voltage versus SOC (OCV=f(SOC)) correspondence based on the proposed SOC convention is the best method for accurate SOC estimation among several possible approaches for strings. The thermodynamic aspect on this SOC convention is explained. Uncertainties in actual applications are also discussed. The understanding on this accurate SOC estimation approach shall facilitate reliable battery control and management.

다권선 변압기의 자화 에너지를 이용한 모듈화 전하 균일 장치

The modularized equalizers normally use additional components among the modules in the long series-connected lithium-ion battery string. In these approaches, the overall systems are heavy, bulky, and high-priced. Furthermore, the losses related to additional components decrease the system efficiency. To avoid these problems, a modularized equalizer, which has no additional components among the modules, is required. This paper proposes a novel control scheme using the magnetizing energy of the multi-winding transformer for the module equalization. In this scheme, the high duty cycle is applied to the module where the voltage is higher than the reference voltage and the low duty cycle is applied to the module where the voltage is lower than the reference voltage. Due to the different duty cycle, more electric charges are transferred from high voltage module to the low voltage module during the turn-off switching interval. Using the proposed control scheme, the equalizer system does not suffer from the size, cost, and loss related to the modularization. The experimental results are provided to verify the effectiveness of the proposed modularized equalizer.

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.

Study on Power Generation and Energy Storage System of a Solar Powered Autonomous Underwater Vehicle(SAUV)

This paper focuses on the solar cell in the application of AUV in order to solve the energy restriction to distance sailing. Analysis of the solar cell maximum power tracking control algorithm and battery equalization charging method for series connected Lithium-Ion battery strings. Then design the maximum power tracking controller and battery equalization charging circuit and introduced the realization method.

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.

A Merged Fuzzy Neural Network and Its Applications in Battery State-of-Charge Estimation

To solve learning problems with vast number of inputs, this paper proposes a novel learning structure merging a number of small fuzzy neural networks (FNNs) into a hierarchical learning structure called a merged-FNN. In this paper, the merged-FNN is proved to be a universal approximator. This computing approach uses a fusion of FNNs using B-spline membership functions (BMFs) with a reduced-form genetic algorithm (RGA). RGA is employed to tune all free parameters of the merged-FNN, including both the control points of the BMFs and the weights of the small FNNs. The merged-FNN can approximate a continuous nonlinear function to any desired degree of accuracy. For a practical application, a battery state-of-charge (BSOC) estimator, which is a twelve input, one output system, in a lithium-ion battery string is proposed to verify the effectiveness of the merged-FNN. From experimental results, the learning ability of the newly proposed merged-FNN with RGA is superior to that of the traditional neural networks with back-propagation learning.

Individual Cell Equalization for Series Connected Lithium-Ion Batteries

A systematic approach to the analysis and design of a bi-directional Cuk converter for the cell voltage balancing control of a series-connected lithium-ion battery string is presented in this paper. The proposed individual cell equalizers (ICE) are designed to operate at discontinuous-capacitor-voltage mode (DCVM) to achieve the zero-voltage switching (ZVS) for reducing the switching loss of the bi-directional DC/DC converters. Simulation and experimental results show that the proposed battery equalization scheme can not only enhance the bi-directional battery equalization performance, but also can reduce the switching loss during the equalization period. Two designed examples are demonstrated, the switch power losses are significantly reduced by 52.8% from the MOSFETs and the equalization efficiency can be improved by 68-86.9% using the proposed DCVM ZVS battery equalizer under the specified cell equalization process. The charged/discharged capacity of the lithium-ion battery string is increased by using the proposed ICEs equipped in the battery string.