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Compared with the conventional hard switching battery equalizers, hierarchical battery equalizer based on the bipolar continuous-conduction mode buck–boost units could effectively reduce the switching loss and improve the conversion efficiency. This paper mainly addresses the key technical challenges associated with the optimal design of this battery equalizer. The inductor current is controlled to enter into the negative region. Thus, the mosfet body diode provides a freewheeling path for the inductor current during the dead band. This ensures zero-voltage switching turn-on of both power mosfets in each buck–boost unit without auxiliary snubbers. Therefore, the switching losses are significantly reduced. Meanwhile, the integral of the inductor negative current is precisely controlled to minimize the circulating current. This guarantees both minimized conduction losses and suitable equalization speed. Furthermore, an equalization algorithm, which compensates the battery internal resistance induced state-of-charge error, is proposed to improve the equalization accuracy. The operating principles and design considerations are analyzed in detail. An experimental prototype to balance four series-connected lithium-ion battery cells is implemented. The experimental results validate the design of the equalizer and demonstrate obvious efficiency enhancement over the conventional method. [ABSTRACT FROM AUTHOR]

Voltages of series-connected batteries gradually become imbalanced due to nonuniformity in terms of not only battery characteristics but also self-discharge rate that is significantly dependent on temperature. In large-scale energy storage systems, a large temperature gradient is very likely because of its huge geometry, and therefore, equalizers capable of relatively large equalization currents would be necessary to eliminate voltage imbalance originating from uneven temperature distribution. A two-switch string-to-battery voltage equalizer using a half-bridge converter with multistacked current doublers (MSCDs) is proposed for series-connected batteries in this paper. The proposed equalizer is capable of providing relatively large equalization currents without increasing ripple currents thanks to the interleaved operation of the MSCDs. Fundamental operational analysis for a discontinuous conduction mode, in which currents in the equalizer can be limited to desired levels without feedback control, is performed. The experimental results of equalization tests performed for four supercapacitor modules and lithium-ion batteries connected in series demonstrated the equalization performance of the proposed equalizer. [ABSTRACT FROM AUTHOR]

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. [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]

Cell voltage equalization is inevitable to ensure years of safe operation of series-connected energy storage cells, such as lithium-ion batteries and supercapacitors (SCs). Although various kinds of cell voltage equalizers have been proposed, most equalizer topologies require multiple switches and/or a multiwinding transformer, resulting in complex circuitry and poor modularity. In this paper, a single-switch single-transformer equalizer using a forward–flyback resonant inverter (FFRI) with a voltage multiplier is proposed. The required switch count of the proposed equalizer is the minimum without impairing modularity due to the single-switch circuitry with no need for a multiwinding transformer. An experimental equalization test performed for eight SCs connected in series successfully demonstrated the equalization performance of the proposed equalizer. The FFRI can be extended as a “resonant input cell,” and by combining one of resonant input cells and a voltage multiplier, a single-switch resonant equalization charger that is basically a charger with an equalization function is also derived. An experimental charging test using the resonant equalization charger was also performed, and its equalization-charging performance was demonstrated. [ABSTRACT FROM AUTHOR]

Voltage equalization is essential for series-connected energy storage cells, such as lithium-ion cells and supercapacitors (SCs), because it ensures years of service life and maximizes the available energies of cells. Conventional equalizers and multioutput chargers require many switches and/or a multiwinding transformer, which are not desirable in terms of circuit simplicity and extendibility. In this paper, single-switch multioutput chargers based on a combination of an input cell (which generates a square voltage wave) and a voltage multiplier (which produces multiple uniform output voltages) are proposed. A fundamental operation analysis is made, and a dc equivalent circuit is derived to describe their charging and equalization behaviors. Experimental charge tests were performed separately for series-connected SC modules and lithium-ion batteries from voltage-unbalanced conditions. The least charged module/battery was charged preferentially; as a result, all of the modules/batteries were charged to a desired uniform voltage level while eliminating the voltage imbalance. [ABSTRACT FROM PUBLISHER]

The cell voltage imbalance of series-connected energy storage cells, such as supercapacitors (SCs) and lithium-ion cells, causes premature deterioration and a decrease in the available energies of the cells. Various equalization techniques have been developed for diminishing such imbalances. However, since the number of switches, sensors, and/or multiwinding transformers present in conventional equalizers is directly proportional to the number of series connections of the cells, the circuit complexity and cost of the equalizers are prone to increase with the number of series connections. In this paper, single-switch cell voltage equalizers using multistacked buck-boost converters, such as the single-ended primary inductor converter (SEPIC), Zeta, and Ćuk converters, are proposed. These equalizers consist of passive components and a single switch, significantly reducing the complexity of the circuit when compared with that of conventional equalizers. In addition, when the proposed equalizers operate in discontinuous conduction mode, feedback control is not required to limit currents flowing through cells and circuit components. The proposed equalizers are compared with conventional topologies in terms of the number of active and passive components required. Operating analyses were conducted under both cell-voltage-balanced and -imbalanced conditions. Experimental equalization tests were performed for four series-connected SCs using the SEPIC-based single-switch equalizer. The energies of the series-connected SCs were preferentially redistributed by the equalizer to the cell(s) having the lowest voltage, resulting in the elimination of the cell voltage imbalance and subsequent uniformity of the cell voltages. [ABSTRACT FROM PUBLISHER]