Combined Energy Storage (CES) Device – a power supply unit including a battery and a supercapacitor (SC) connected in parallel. In such a combination the current in the battery sub-circuit cannot exceed a certain value, while all the current peaks are covered by the SC.
Cycle life (or lifetime) – number of charge/discharge cycles of a battery until a significant deterioration of its performance takes place.
Depth of Discharge (DoD) – part of energy (typically in %) that can be taken from the battery or SC without significant deterioration of their performance.
Efficiency, h (or Round Trip Efficiency, RTE) – the ratio between the energy delivered from a power supply unit on the load during discharge (Eout) to the energy put into the power supply unit when charging it (Ein): h = Eout / Ein.
Energy Density – amount of energy (E in W.h, NOTE: 1W.h = 3600 Joules = 3.6 kJ) stored in 1 kg of a battery of SC. For SC: E = 0.5 C.U2, where C is the capacitance in F, and U is the working voltage in V.
Hybrid Energy Storage (HES) Device – a power supply unit comprising one electrode taken from a battery technology, and the second electrode taken from the supercapacitor technology (i.e., a nanoporous carbon electrode).
Maximum Power Density – maximum power (Pmax in W or kW) value per unit mass that can be delivered on the load at a certain efficiency, h. If a SC is discharged from the rated voltage U to the half of U value, the maximum power density is given by the equation: , where Rin is the SC internal resistance in Ohm, and M is the mass in kg. NOTE: as can be seen from this equation, the Pmax value depends strongly on the efficiency, so that the Pmax values at = 0.95 or 0.90 differ from each other by a factor of two.
Primary cell – a type of power supply unit that cannot be recharged and should be changed (at least, partly) after discharge.
Examples: fuel cell, zinc-air cell, various Li-cells (those comprising Li metal or alloy as the anode material).
Secondary cell or rechargeable cell (or simply battery) – a type of power supply unit that can be charged/discharged many times.
Examples: lead-acid, alkaline, nickel-metal-hydride (NiMH), Li-ion.
Time-constant or RC-constant – the product of Rin and C values (in sec).
NOTE: generally, the lower is the time-constant the higher is the power density.