The paper aims to provide a more accurate analysis of a magnetostrictive energy harvesting device by proposing a FEM model which, assuming a realistic nonlinear characteristic of the material, is able to describe the harvesting phenomena in presence of the eddy currents induced by the Villari effect. The study is focused on the investigation of the influence of those parameters, such as pre-stress and bias, on the field dynamics and, consequently, on the eddy currents loss phenomena which cannot be disregarded if a reliable prediction of the global performances of these devices is required. The numerical results are computed by considering a compressive stress-driven vibration source and show spatial profiles of the fields, losses, and recovered powers in different operating conditions. Comparisons with the linear model are also provided.