In order to safely operate the supercapacitor (SuperCap) energy storages as peak power units until the end of their lifetime in the real applications, identifying the electrical characteristics of SuperCaps and assessing the energy capacity and energy losses in fast energy conversion are the essential procedures. Until now, most methods are based on the international standard IEC 62391. However, the detected parameters (capacitance and series resistance) are not suitable enough, to precisely represent the energy capacity of the SuperCaps at different voltage levels and to estimate the power losses at different power levels. Therefore, the enhanced methods of characterizing SuperCaps in the fast charging-discharging cycles are developed. Furthermore, the energy losses due to the variation of capacitance during the charging and discharging states are evaluated, and the electrical stability of the SuperCap modules is also observed with respect to the variation of capacitance.