We present a basic model of supercapacitor based on a theoretical analysis discussing the frequency dependence of the electric permittivity. Directly from the Maxwell’s theory, an equivalent parallel capacitance and conductance is deduced having frequency dependence. Dynamic behavior is assessed in the frequency range up to 20 Hz which is typical for hybrid vehicle applications. In order to investigate the equivalent capacitance and conductance variation in the frequency range 1 mHz–20 Hz, an impedance spectroscopy method has been adopted. We provide a detailed description of the measurement procedure and present the results obtained on a 120 F Maxwell double layer capacitor. Finally, the model response calculated through the Fourier analysis is tested under square and triangular waveforms; results obtained perfectly match the actual measurements.