This paper presents an optimization study to decrease the operation frequency and increase the output power of a miniature electromagnetic (EM) energy harvester, by incorporating a non-magnetic inertial mass together with the moving magnet. The harvester coil position has been optimized through FEM, and validated through tests. Experimental studies on the inertial mass showed that increasing the magnet size further increases the resonance frequency due to the increased magnetic forces. Conversely, using a non-magnetic mass over the magnet effectively decreases the resonance frequency (27 Hz to 15 Hz), and increases the generated output power. The power output during operation at even lower frequencies is also improved by adding the non-magnetic mass. The optimized 6 cm(3) harvester generates 0.45 V-rms and 110 mu W-rms output power at 15 Hz and 0.7g peak acceleration. (C) 2015 The Authors. Published by Elsevier Ltd.