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For many years, wireless RF power transmission has been investigated as a viable method of power delivery in a wide array of applications, from high-power space solar power satellites to low-power wireless sensors. However, until recently, efficient application at the low sub-milliwatt power levels has not been realized due to limitations in available control circuitry. This paper presents a “smart” microcontroller-based power management system with online power stage efficiency optimization and maximum power point tracking (MPPT). The system is experimentally evaluated using a new, more accurate four-quadrant rectenna model and circuit realization that enables rigorous testing of the power management system for a wide range of rectenna arrays and power characteristics. Hardware results are presented with online optimization over a converter input power range from 10 μW to 1 mW. Results are also shown based on the application of harvesting RF power from a nearby cellular tower, where the power management system collects up to seven times more energy when compared to a direct battery connection.