We propose a quasi-steady nonlinear circuit model for the solar wind-magnetosphere-ionosphere (SW-M-I) coupling to study the observed saturation of polar cap potential. The oval conductance is shown to be a nonlinear circuit element since it increases with increasing dayside reconnection E field driving the proposed circuit. Oval conductance is produced by precipitating particles energized by enhanced sunward convection in the plasma sheet driven by reconnection at the dayside magnetopause and in the plasma sheet. The asymptotic saturation potential is shown to increase with (1) decreasing internal resistance of the dynamo region, (2) increasing length of dayside reconnection line, (3) increasing ratio of nightside to dayside reconnection potentials, and (4) increasing ratio of nightside to dayside internal resistances.