A single transformer for active cell equalization method of lithium-ion batteries with two times fewer secondaries than cells

In this paper, the concept of active cell-balancing technique, by using a multiple-outputs double-forward converter for lithium-ion (Li-ion) batteries, is investigated. It controls two times more cells than secondaries, and it equalizes eight cells in a series. In this method, four secondaries can reasonably be wound with the same back electromotive force (EMF). This means a low pin count on the transformer and a low bill of materials (BOM). The bridge uses four N-channel MOSFETs as switches, which means two times fewer transistors than cells, resulting in fewer switching losses. This scheme is applied for controlling the minimum voltage among the cells of the lithium-ion battery. It uses a multi-winding transformer based on a forward double converter structure. Conventional schemes using a multi-winding transformer for electric vehicles (EVs) require an equal number of secondaries per cell. This scheme requires one secondary for two adjacent cells, thus the number of secondaries is reduced by a factor of two. Also, the redistribution of charge from a high cell to a low cell does not require many switching components and little intelligence to determine low cell voltage detection. The basic principle of this method is to use the overall battery pack voltage as a reference to supply individual cells, using a forward converter containing a transformer with a well-chosen winding ratio. The experimental and simulation results are performed to verify the feasibility of the proposed system

Forward converter current fed equalizer for lithium based batteries in ultralight electrical vehicles

In this paper, the concept of a forward balancing technique fed by a buck converter for lithium-based batteries in Electrical Vehicle (EV) applications is investigated. The proposed active topology equalizes eight cells in a series in a battery pack, by using a forward converter for each battery pack and the whole battery packs, using a buck converter. The battery bank consists of four battery packs, which are in series. Therefore, the proposed system will equalize 32 cells in series. In this paper, the proposed circuit employs a single transistor used in a Zero Voltage Switch (ZVS) for the forward converter. In practice, this means a capacitor in parallel with the switch at the same time a demagnetizing of the transformer is obtained. The circuit realizes a low Electromagnetic Interference (EMI) and reduces ringing. To overcome the problem of many pins on a coil former, the transformer secondary windings are made by using hairpin winding, on a ring core. It permits, e.g., having eight secondaries and uniform output voltages. Each secondary winding is made by two hairpin turns using two zero-Ohm resistors in series. The proposed topology has less components and circuitry, and it can equalize multiple battery packs by using a single buck converter and several forward converters for each battery pack. Experimental and simulation results are performed to verify the viability of the proposed topology

Online state of charge estimation for the aerial lithium-ion battery packs based on the improved extended Kalman filter method.

An effective method to estimate the integrated state of charge (SOC) value for the lithium-ion battery (LIB) pack is proposed, because of its capacity state estimation needs in the high-power energy supply applications, which is calculated by using the improved extended Kalman filter (EKF) method together with the one order equivalent circuit model (ECM) to evaluate its remaining available power state. It is realized by the comprehensive estimation together with the discharging and charging maintenance (DCM) process, implying an accurate remaining power estimation with low computational calculation demand. The battery maintenance and test system (BMTS) equipment for the aerial LIB pack is developed, which is based on the proposed SOC estimation method. Experimental results show that, it can estimate SOC value of the LIB pack effectively. The BMTS equipment has the advantages of high detection accuracy and stability and can guarantee its power-supply reliability. The SOC estimation method is realized on it, the results of which are compared with the conventional SOC estimation method. The estimation has been done with an accuracy rate of 95% and has an absolute root mean square error (RMSE) of 1.33% and an absolute maximum error of 4.95%. This novel method can provide reliable technical support for the LIB power supply application, which plays a core role in promoting its power supply applications

Development of Hybrid Fuel Cell / Li-ion Battery Systems

Electrochemical power systems are needed to de-carbonise the transport industry. Fuel cells and battery systems alone may not be able to meet the diverse set of requirements, but when hybridised, their applicability to this sector is vastly increased. This raises questions around the specific nature of hybridisation. This thesis aims to expand our understanding of fuel cell and lithium-ion battery hybridisation for automotive applications, through a combined experimental and computational approach. Prior to undertaking such research, an understanding of each individual system is required. This is perused along the themes of current heterogeneity, and applied to parallel battery cells in two common electrical configurations and across the active area of a 100 cm2 polymer exchange fuel cell. First, it is shown the electrical configuration of the parallel string has significant impact on the current distribution, impacting the charge throughput of each cell and the usable capacity of the module. Degradation modelling showed the lifetime of the module is reduced by 4.5% in the less optimal configuration. Secondly, the current and thermal distribution within a fuel cell is investigated for a range of operating conditions such as flooding, drying and cold start. Electrochemical impedance spectroscopy is used to understand the conditions of the membrane and reactant time constants in-situ. Results indicate how the design of fuel cells can be refined to improve performance in challenging operating conditions. Finally, the investigation of electrical and thermal hybridisation is conducted on a passenger sized vehicle. A common modelling framework is developed, using the models developed in the fuel cell and battery chapters, to assess electrical energy management systems. A novel fuzzy logic controller is developed which mutates the output membership functions based on the ‘state-of-degradation’, a parameter derived from an interconnected electrochemical surface area loss and system state model. The controller is able to extend the lifetime of the fuel cell by 32.8% in its presented configuration. The common framework is then developed to include dynamic thermal models of the fuel cell, battery pack, radiator and auxiliaries to investigate whether combining the battery pack and fuel cell stack onto a single coolant loop is feasible. The system is tested against a range of operating conditions and its performance is discussed. These findings are expected to aid the transport industry in the transition to a zero emission future

State of Charge Estimation of Parallel Connected Battery Cells via Descriptor System Theory

This manuscript presents an algorithm for individual Lithium-ion (Li-ion) battery cell state of charge (SOC) estimation when multiple cells are connected in parallel, using only terminal voltage and total current measurements. For battery packs consisting of thousands of cells, it is desirable to estimate individual SOCs by only monitoring the total current in order to reduce sensing cost. Mathematically, series connected cells yield dynamics given by ordinary differential equations under classical full voltage sensing. In contrast, parallel connected cells are evidently more challenging because the dynamics are governed by a nonlinear descriptor system, including differential equations and algebraic equations arising from voltage and current balance across cells. An observer with linear output error injection is formulated, where the individual cell SOCs and local currents are locally observable from the total current and voltage measurements. The asymptotic convergence of differential and algebraic states is established by considering local Lipschitz continuity property of system nonlinearities. Simulation results on LiNiMnCoO$_2$/Graphite (NMC) cells illustrate convergence for SOCs, local currents, and terminal voltage.Comment: 7 pages, 4 figures, 1 table, accepted by 2020 American Control Conferenc

A smart high-voltage cell detecting and equalizing circuit for LiFePO4 batteries in electric vehicles

A battery management system (BMS) plays an important role in electric vehicles (EVs) in order to achieve a reasonable-lasting lifetime. An equalizing method is essential in order to obtain the best performance. A monitoring system is required to check if any cell voltage is high or low. In this paper, an equalizing and monitoring system for an ultra-light electric vehicle is proposed. The monitoring system detects if one cell is fully charged or all cells are fully charged and the equalizing system tops each cell at the desired voltage. To solve this issue, a light-emitting diode (LED) band gap is used as a voltage reference to inform the user if any cell is at its high voltage. A smart monitoring displays on the liquid crystal display (LCD), if one cell is high or all cells are high. This detection also provides a signal to the microcontroller to turn on/off the charger if all cells are high. Also, a Bluetooth module was designed to command the microcontroller the charger to turn on/off via voice/text message by using a smartphone. Additionally, a new smart monitoring system based on the Bluetooth model (HC05) and mobile app has been made in order to monitor individual cell voltage. A major feature of the system is to draw a very-low current, so that the system does not contribute significantly to the self-discharge of the battery and the circuit does not need sophisticated control. Manufacturers of large electric vehicles may have more intelligent systems that may require a permanent connection to the grid and allow high standby losses, where more state of charge (SOC) may be lost per day. The paper is rather focused on reducing the standby losses, and to activate the equalizer only when charging and/or driving. The experimental results are performed in order to verify the feasibility of the proposed circuit

A Modular Multi-level Converter for Energy Management of Hybrid Energy-Storage Systems in Electric Vehicles

Electric vehicles (EVs) are substantial applications of clean energy. Their effectiveness for mainstream transportation is predicated on the efficient use of stored energy within the vehicles’ power pack. Among rechargeable storage solutions, lithium-ion (Li-ion) battery cells have high energy density making them suitable to supply the EVs’ average power. However, the peak power requirements of the vehicles exert stress on the Li-ion cells due to their low pulsating power capabilities. Ultracapacitors can be used instead as the power-pulsating storage elements given their superior power density. Incorporating the two cell types for energy storage signifies a hybrid configuration that leads to challenging tasks in managing the energy between cells due to varying cell dynamics. Therefore, this study investigated the design of an end-to-end hybrid energy-storage and management system. The limitations of existing power electronics and control schemes were identified based on comparative analysis, both on a cell level and on a system level. Subsequently, an energy system was developed that utilized modular multi-level converters to manage the energy between the different cell types. The formulated control strategy accounted for various power modes and added immense flexibility in charge sharing through diverse switching states. Furthermore, the proposed configuration eliminated the conventional need for a system level drive inverter feeding the EV motor. Electro-mechanical modeling results and physical design merits verified the proposed configuration’s effectiveness in improving EV efficiency

A comprehensive overview of electric vehicle charging using renewable energy

The integration of PV with the electric vehicle (EV) charging system has been on the rise due to several factors, namely continuous reduction in the price of PV modules, rapid growth in EV and concern over the effects of greenhouse gases. Over the years, numerous papers have been published on EV charging using the standard utility (grid) electrical supply; however, there seems to be an absence of a comprehensive overview using PV as one of the components for the charger. With the growing interest in this topic, it is timely to review, summarize and update all the related works on PV charging, and to present it as a single reference. For the benefit of a wider audience, the paper also includes the bries description on EV charging stations, background of EV, as well as a brief description of PV systems. Some of the main features of battery management system (BMS) for EV battery are also presented. It is envisaged that the information gathered in this paper will be a valuable one–stop source of information for researchers working in this topic

Modeling and control of stand-alone AC microgrids: centralized and distributed storage, energy management and distributed photovoltaic and wind generation

El aumento de la penetración de energías renovables en la red eléctrica es necesario para el desarrollo de un sistema sostenible. Para hacerlo posible técnicamente, se ha planteado el uso de microrredes, definidas como una combinación de cargas, generadores distribuidos y elementos de almacenamiento controlados gracias a una estrategia global de gestión energética. Además, las microrredes aumentan la fiabilidad del sistema puesto que pueden funcionar en modo aislado en caso de fallo de red. Esta tesis se centra en el desarrollo de microrredes AC en funcionamiento aislado. El objetivo principal es el diseño y la implementación de estrategias de gestión energéticas sin utilizar cables de comunicación entre los distintos elementos, lo que permite reducir los costes del sistema y aumentar su fiabilidad. Para ello, se abordan los siguientes aspectos: • Gestión energética de una microrred AC con generador diesel, almacenamiento centralizado y generación renovable distribuida • Diseño de técnicas de control “droop” para repartir la corriente entre inversores conectados en paralelo • Gestión energética de una microrred AC con almacenamiento distribuido y generación renovable distribuida • Control de la etapa DC/DC de inversores fotovoltaicos con pequeño condensador de entrada en el seno de una microrred • Control de extracción de máxima potencia sin sensores mecánicos para sistemas minieólicos en el seno de una microrred.The introduction of distributed renewable generators into the electrical grid is required for a sustainable system. In order to increase the penetration of renewable energies, microgrids are usually proposed as one of the most promising technologies. A microgrid is a combination of loads, distributed generators and storage elements which behaves as a single controllable unit for the grid operator. Furthermore, microgrids make it possible to improve the system reliability because they are capable of standalone operation in case of grid failure. This thesis is focused on the development of AC microgrids under stand-alone operation. Its main objective is to design and implement overall control strategies which do not require the use of communication cables, thereby reducing costs and improving reliability. For this purpose, the following aspects are tackled: • Energy management of an AC microgrid with diesel generator, centralized storage and distributed renewable generation • Design of droop methods so that the current is shared among parallel-connected inverters • Energy management of an AC microgrid with distributed storage and distributed renewable generation • Control of the DC/DC stage in photovoltaic inverters with small input capacitors within a microgrid • Sensorless MPPT control for small wind turbines within a microgrid.Programa Oficial de Doctorado en Energías Renovables (RD 1393/2007)Energia Berriztagarrietako Doktoretza Programa Ofiziala (ED 1393/2007