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In order to maximize power density, the resonant frequency of an energy harvester should be equal to the source excitation frequency and the electrical damping set equal to the parasitic damping. These parameters should be adjustable during device operation because the excitation characteristics can change. This brief presents, for the first time, a power electronic interface that is capable of continual adjustment of the damping and the resonant frequency of an energy harvester by controlling real and reactive power exchange between the electrical and mechanical domains while storing the harvested energy in a battery. The advantages of this technique over previously proposed methods are the precise control over the tuning parameters of the electrical system and integrated rectification within the tuning interface. Experimental results verify the operation, and the prototype system presented can change the resonant frequency of the electromechanical system by ±10% and increase the damping by 45%. As the input excitation frequency was swept away from the unmodified resonant frequency of the harvester, the use of the tuning mechanism was shown to increase real power generation by up to 25%. The prototype harvester is capable of generating 100 mW at an excitation frequency of 1.25 Hz.