Hi Bouthaina

The main advantage of the supercapacitor is that, contrary to batteries which require complicated methods for indicating the SOC (such as kalman filter), the SOC is proportional to the output voltage. Hence, straightforwardly measure the output voltage.

Since the energy stored in supercaps is a function of capacitance and voltage only and the capacitance is relatively constant, the voltage determines SOC lonely. 

SOC =Vsc ²/Vscmax ²