This paper describes the design, modeling and optimization of an efficient ISM band dual patch rectenna capable of achieving more than 80% RF-to-DC conversion efficiency at low/medium power densities. The circuit is based on a full-wave rectifier, designed and optimized at 2.45 GHz with ADS software and the FDTD algorithm. The performances of the rectenna have been accurately predicted using the full-wave 3D-FDTD method extended to lumped linear and non-linear elements. It exhibits 73% (VDC = 1.1 V for RL= 1.2 kΩ) measured efficiency at a low power density of 14 μW/cm2 and 84% (VDC = 1.94 V) at 43 μW/cm2. The differences between the experimental and FDTD simulated efficiencies are less than 3%. The proposed circuit is particularly suitable for low/medium power recycling and power remote supply of wireless sensors, sensor nodes and actuators.