2168-6777 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JESTPE.2019.2914706, IEEE Journal of Emerging and Selected Topics in Power Electronics IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS Abstract - A novel cell voltage equalizer using a series LC resonant converter is proposed for series connected energy storage devices, namely battery, or super (or ultra) capacitor cells. The proposed circuit is an active voltage equalization circuit for energy storage devices that is low cost, small in size and equalizes the voltages quickly. Compared to state of the art solutions, the proposed series LC resonant circuit eliminates the complexity of multi-winding transformers and it can balance series connected energy storage devices in a short time by transporting energy successively between the cells having highest voltage difference. The detailed circuit operation and theoretical analysis are provided. Simulation and experimental results are presented to demonstrate the voltage equalization process from an initial voltage difference of 527 mV to a final difference of 10 mV in 900 seconds for three series- connected super capacitor cells. Index Terms— Resonant converter, supercapacitor, voltage deviation, voltage equalization. I. INTRODUCTION In order to minimize the impact of electrical demand peaks, electrochemical energy storage devices, including batteries and super (or ultra) capacitors are increasingly being adopted for industrial and consumer applications, including electric vehicles (EVs). These energy storage devices are appealing because of their small size and manufacturability. Due to manufacturing and device limitations, the voltages of individual battery and super capacitor energy storage cells are inherently low, typically in the range from 2 to 4 V. Therefore, energy storage cells are usually configured in series in order to reach a sufficiently high system voltage and to minimize system supply currents [1]-[3]. Since series connected cells have the same current flowing through them, due to individual cell variations, the series current typically yields an imbalance in the individual cell voltages in a string [4]-[6]. As a result, during charging process, energy storage devices with greater capacity will not reach their nominal voltage and therefore not reach their full capacity when charging process stops [7]-[9]. Furthermore, lower capacity cells will reach their maximum voltage quickly and can be damaged due to overvoltage [10] if the charging process is not terminated at the maximum cell voltage. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Y. Yu, A. A. Khan and W. Eberle are with the School of Engineering, University of British Columbia, Okanagan, Canada (email: yanqi.yu@alumni.ubc.ca; khan.ashraf@ubc.ca; wilson.eberle@ubc.ca). R. Saasaa is with the Murata Power Solutions, ON, Canada (email: raed.saasaa@murata.com). Therefore, control and protection circuits may be required to regulate individual cell voltages in an effort to ensure uniform voltages in a string [11]. Voltage equalization circuits have been proposed to eliminate the voltage imbalance issue for series connected energy storage cells. Generally, these voltage equalization circuits can be divided into two groups, passive and active voltage equalization [12]-[14]. Passive voltage equalization methods use energy- consuming devices such as resistors to remove excess energy from energy storage cells with higher voltages. The excess energy is dissipated in the resistors as heat [15]. These circuits are relatively simple in the structure, but are inefficient, and the excess heat generated potentially poses a threat to normal operation of energy storage devices. Furthermore, these circuits can require bulky and expensive heat sinks. Active voltage equalization circuits remove excess charge from the energy storage cells at higher voltages and then transport the extra charge to the ones at lower voltages. In general, active voltage equalization circuits achieve energy recovery via optimal energy distribution. In the past two decades, many active cell voltage balancing circuits have been proposed to overcome the previously mentioned limitations of passive voltage equalization methods [16]. Generally, active voltage balancing circuits can be categorized into four groups: adjacent cell-to-cell circuits, direct cell-to-cell circuits, cell-to-pack circuits and multi-cells- to-multi-cells circuits. Adjacent cell-to-cell circuits (AC2C) transport energy between adjacent cells. The topologies using adjacent cell-to- cell circuits include switched capacitor topologies in [17] and [18], the bidirectional Cûk topology [19], the quasi-resonant topology [20], and the topology with multiple transformers [21]. Energy distribution among energy storage devices can be achieved through the control of multiple paralleled switches. Adjacent cell-to-cell circuits have the advantages of control simplicity and modular design. However, these circuits have long voltage balancing times since the extra energy is transported between adjacent cells through all cells in the string. In addition, the efficiency is low since energy has to pass through all the cells and balancing circuits in the topology. Direct cell-to-cell circuits (DC2C) use a shared voltage equalization circuit for all cells. DC2Cs include the flying capacitor topology presented in [22], and the single inductor topologies presented in [23] and [24] and the quazi-resonant topologies in [25] and [26]. The advantages include coupling effect elimination and high efficiency can be achieved. However, the switching devices suffer from high voltage stress, particularly when the number of cells in the string is high, which A Series Resonant Energy Storage Cell Voltage Balancing Circuit Yanqi Yu, Raed Saasaa, Ashraf Ali Khan, Member, IEEE, and Wilson Eberle, Member, IEEE