Series-connected Li-ion Battery Research Articles

A Modularized Two-Stage Active Cell Balancing Topology with Reduced Balancing Time for Series Connected Li-Ion Battery String

A Modularized Two-Stage Active Cell Balancing Topology with Reduced Balancing Time for Series Connected Li-Ion Battery String

이차전지를 이용한 2kW 태양광전원용 전압보상장치의 운용 방안에 관한 연구

Recently, korean government has been promoting numerous policies and projects to increase the capacity of renewable energy source up to 63.8[GW] by 2030, which is 20[%] of the total power generation. However, the output of PV systems may vary greatly depending on the environmental conditions, moreover, there are problems that the PV system may be disconnected from inverters as the PV string voltage violates the operation range even if partial PV modules are shaded. Therefore, in order to prevent the PV systems being disconnected from the inverter due to partial shades on PV modules, this paper presents an operation method of voltage compensation device for PV modules, which can compensate the decreased PV string voltage by connecting Li-ion batteries in series. And also, this paper performs the modeling of voltage compensation device for PV module using PSCAD/EMTDC and implements a 2kW scaled test device for voltage compensation device, accordingly. From the evaluation results of operation characteristics of voltage compensation device based on the modeling and implemented test device, it is confirmed that the proposed voltage compensation device is a useful tool for improving operation efficiency of PV systems since the input power of PV inverter can be kept within the proper operation range by compensating the decreased voltage of PV modules using series-connected Li-ion batteries when partial shades are occurred.

Battery Management System Performance Enhancement using Single Sliding Mode Based Charge Equalization Controller

In this paper, the study aims to evolve a precision model of a Battery charge equalization controller (BCEC) that manage each cell of a lithium-ion (Li-ion) battery, monitoring a nd balancing by charging a nd discharging a series-connected Li-ion battery with several cells (n) in electric vehicle (E.V.) applications. An intelligent Sliding mode controller is evolved to activate bidirectional cell switches and regulate a chopper circuit’s direct current (DC-DC flyback converter circuit) with PWM generation. To implement a small-scale BCEC, the model consists of individual models of an E.V., cells of Li-ion battery, a fly-back converter, and a single sliding mode controller (SSMC) for charging and discharging are integrated with n series-connected cells of Li-ion-battery. The BCEC output shows that the cells are operated within a safe range, and the difference in the state of charge (SOC) is maintained to be 0.1%. Comparing the developed BCEC with the existing controllers, with the performance, control algorithm, efficiency, power loss, size, and cost.

Equalization scheme for series connected Li-ion battery pack based on drop combination strategy

Equalization scheme for series connected Li-ion battery pack based on drop combination strategy

Thermal and electrical performance evaluations of series connected Li-ion batteries in a pack with liquid cooling

Lithium-ion batteries are widely used in electric and hybrid electric vehicles due to their wide nominal range and high power densities. However, operating temperature, which has a pronounced effect on battery thermal performance and electrical performance, needs to be maintained within a specified range. In the present study, a Li-ion battery pack has been tested under constant current discharge rates (e.g. 1C, 2C, 3C, 4C) and for a real drive cycle with liquid cooling. The experiments are performed using cold plates at 10°C, 20°C, 30°C, and 40°C coolant temperatures to obtain thermal and electrical parameters. For this, three 20Ah LiFePO4 prismatic cells are connected in series and a battery thermal management system is designed and developed for liquid cooling. To measure the temperature variation, 18 thermocouples are installed on the principal surface of all three cells: i.e., six on each cell. The results show that the battery pack temperature can be maintained within the required range at all four selected discharge rates with the coolant at 30°C. The discharge capacity of the battery pack increases with increasing coolant temperature and is found to achieve a maximum of 19.11 Ah at a 1C discharge rate with the coolant at 40°C.

Charge Equalization Controller Algorithm for Series-Connected Lithium-Ion Battery Storage Systems: Modeling and Applications

This study aims to develop an accurate model of a charge equalization controller (CEC) that manages individual cell monitoring and equalizing by charging and discharging series-connected lithium-ion (Li-ion) battery cells. In this concept, an intelligent control algorithm is developed to activate bidirectional cell switches and control direct current (DC)–DC converter switches along with pulse width modulation (PWM) generation. Individual models of an electric vehicle (EV)-sustainable Li-ion battery, optimal power rating, a bidirectional flyback DC–DC converter, and charging and discharging controllers are integrated to develop a small-scale CEC model that can be implemented for 10 series-connected Li-ion battery cells. Results show that the charge equalization controller operates at 91% efficiency and performs well in equalizing both overdischarged and overcharged cells on time. Moreover, the outputs of the CEC model show that the desired balancing level occurs at 2% of state of charge difference and that all cells are operated within a normal range. The configuration, execution, control, power loss, cost, size, and efficiency of the developed CEC model are compared with those of existing controllers. The proposed model is proven suitable for high-tech storage systems toward the advancement of sustainable EV technologies and renewable source of applications.

Charge-equalisation for Li-ion batteries by direct charge-transfer circuit

ABSTRACTThis paper proposes a charge-equalisation circuit for series-connected battery pack based on direct charge transference. The battery cells inside the pack can mutually transfer their charges directly via the bidirectional flyback converter within this proposed charge-equalisation circuit. Since electric charge is transferred directly between the highest and lowest charged cells, the losses dissipated during the transfer processes can be minimised. Meanwhile, the equalisation time can be reduced. A battery-management IC is used to detect the cell voltages and drive the battery-selection switches. A battery pack that consists of four series-connected Li-ion battery cells is used in the experiment. The charge-equalisation circuit topology and its control scheme are verified by experiments. From the experimental results, the proposed circuit is able to shrink the maximum voltage difference between cells into 30 mV range after 170min. The results successfully verify the feasibility of the circuit topology and the control scheme.

Voltage equalization control algorithm for monitoring and balancing of series connected lithium-ion battery

Lithium-Ion (Li-Ion) batteries are commonly used as automobile energy storage systems for powering applications due to their lucrative features. However, a battery management system with individual cell monitoring and balancing of Li-Ion batteries for long use and casualties' protection are still major issues in electric vehicle applications. This paper deals with the development of a voltage equalization control algorithm for individual cell monitoring and balancing of series connected Li-Ion battery cells. The developed states and sequences of the control algorithm manage the whole processes of battery cell monitoring, charging, and discharging, respectively. A charge equalization model is implemented with series connected 10 Li-Ion battery cells utilizing the developed control algorithm. Results show that charging and discharging, and cell balancing performance of the control algorithm are capable of quickly responding to reach the state of charge difference of 2.5% among all cells, defending the existing anomaly, providing tolerable stress to components and operating at a higher efficiency of 84.9%.

A State-of-Charge and Capacity Estimation Algorithm for Lithium-ion Battery Pack Utilizing Filtered Terminal Voltage

In electric vehicle (EV) and hybrid electric vehicle (HEV) application, accurate information of state-ofcharge (SOC) and capacity of each cell are required for elaborate SOC/capacity estimation algorithm of the battery pack. However, the measurement of the states of all cells using sophisticated algorithms increases the computation time beyond practicality, because the computation time required for the SOC/capacity estimation of the battery pack is directly affected by the number of unit cells. In this work, a simple SOC and capacity estimation algorithm for Li-ion battery pack is newly proposed by using filtered terminal voltage. The SOC estimation algorithm using filtered terminal voltage extracts an estimated current information from the terminal voltage of the battery pack through equivalent-circuit model (ECM)-based filters without sensing the current. Consequently, it drastically reduces computational steps for the SOC estimation algorithm. With the fact that all the current flowing through the series-connected cells in pack are identical, the estimated current value of each cell should be identical. As a result, it can be known that this algorithm enables us to obtain the relative proportion of SOC/capacity information of each cells and battery pack with minimal complexity increase. To validate the performance of the proposed approach, a scaled-down HEV profile is used for a pack consists of twelve 18650 series-connected Li-ion batteries (12S1P). The experimental results verify the performance of the proposed battery pack SOC estimation algorithm.

Balancing Control Strategy for Li-Ion Batteries String Based on Dynamic Balanced Point

The Li-ion battery is becoming the optimal choice for the Electric Vehicle’s (EV) power supply. In order to protect the Li-ion battery from charging damage and to prolong the battery’s life, a special control strategy based on the dynamic balanced point along with a non-dissipative equalizer is presented. The main focus of the proposed control strategy is to insure that the individual cell of a battery pack will be rapidly, efficiently and simultaneously balanced, by adjusting equalizing current of each cell dynamically. In this paper, a model of a four series connected Li-ion batteries pack has been established to evaluate the proposed control strategy. Superior performance is demonstrated by the simulation and experiment.

“Time Shared Flyback Converter” Based Regenerative Cell Balancing Technique for Series Connected Li-Ion Battery Strings

A cell voltage equalizer circuit for future plug-in hybrid electric vehicles (PHEV) or renewable energy storage is proposed in this paper. This topology has fewer passive components compared to the conventional topologies found in the literature, and therefore, it could reduce implementation complexity. This circuit is based on a time shared flyback converter, and any number of series connected cells in a string could be used without any apparent issues ensuring good modular architecture. Each cell in a module shares a single converter during its allocated time slot allocated by a low-power microcontroller. In addition, dynamic allocation of time slots is possible to achieve a faster cell balancing, and the circuit dynamically distributes depleted charge among the cells in a regenerative fashion. The operating principles and design procedures of the proposed topology have been presented in the paper. The prototype of a four-cell lithium-ion battery balancer circuit with the proposed topology has been constructed, and the test results have been included.

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

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

Li-ion battery charger based on digitally controlled phase-shifted full-bridge converter

This study is aimed to design and implement a battery charger for 40 series-connected Li-ion batteries. The batteries together set up a rated 144 V DC bus for the latter stage. To obtain higher power output capability and achieve higher conversion efficiency, a phase-shift full-bridge converter is adopted as the power stage of the charger. The operation modes and design considerations are carefully studied and depicted. This charger is controlled by a digital signal processor controller which is responsible for steering the circuit to fulfil constant-current constant-voltage charging profile, accompanied by the overcharging protection mechanism of Li-ion batteries.

Fuzzy Controlled Individual Cell Equalizers for Lithium-Ion Batteries

A fuzzy logic control battery equalizing controller (FLCBEC) is adopted to control the cell voltage balancing process for a series connected Li-ion battery string. The proposed individual cell equalizer (ICE) is based on the bidirectional Cuk converter operated in the discontinuous capacitor voltage mode (DCVM) to reduce the switching loss and improve equalization efficiency. The ICE with the proposed FLCBEC can reduce the equalizing time, maintain safe operations during the charge/discharge state and increase the battery string capacity.