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The cell voltage imbalance of series-connected energy storage cells such as supercapacitors (SCs) and lithium-ion cells leads to the premature degradation and a decrease in the available energy of the cells. In this study, single-switch equalization chargers using multi-stacked buck-boost converters have been proposed. The single-switch operation can dramatically reduce circuit complexity compared with that of conventional equalizers. Fundamental operating principles of both voltage-equalized and voltage-imbalanced conditions are presented. Experimental charge tests using a single ended primary inductor converter (SEPIC)-based prototype for 4-series connected SCs were performed. The experimental results demonstrated that the proposed equalization charger can charge the series-connected SCs preferentially in the order of increasing cell voltage, and all SCs were charged to a uniform voltage level. Another charge test was performed for 4-series connected SCs containing an aged cell. All the cell could be charged uniformly, regardless of individual cell capacitance.