The paper presents a set of experimental investigations related to the dynamic behavior of supercapacitors. The experimentally observed results are then used as inputs for the development of an improved version of one of the most common supercapacitor RC-equivalent circuit models. The key improvement concerns the accurate modeling of the diffusion phenomenon of the supercapacitor residual charge during charging/discharging and rest phases. The experimental procedure needed for evaluating the parameters of the proposed model is also given. The accuracy of the obtained model is, then, experimentally validated for different cycles characterized by different dynamics.