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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]

Lithium-ion capacitors (LICs) are a hybrid energy storage device combining the energy storage mechanisms of lithium-ion batteries (LIBs) and electric double-layer capacitors (EDLCs), and are considered attractive not only in high-power applications but also as an alternative to rechargeable batteries due to their inherent long cycle life and relatively high energy density. The cycle life testing was performed for commercial-off-the-shelf (COTS) LIC cells procured from three different manufactures, and the cycle life prediction model developed for EDLCs in the previous work was applied to LICs. Based on the resultant capacitance retention trends, the activation energies of degradation ratios were calculated using an Arrhenius equation, whereupon aging acceleration factors were determined. The calculated acceleration factors varied depending on manufacturers, suggesting that a proper aging acceleration factor should be determined for each manufacture cell based on cycle life testing rather than simply applying a rule of thumb which had been accepted for LIBs and EDLCs. The resulting and predicted capacitance retention trends correlated well, verifying that the cycle life prediction model established for EDLCs in the previous work would also be usable for LICs as an alternative to rechargeable batteries. [ABSTRACT FROM PUBLISHER]

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]