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This paper demonstrates a highly efficient, low-voltage ac/dc converter using a voltage multiplier (octupler) circuit architecture intended for vibrational energy harvesting applications where a low-voltage ac waveform is used to charge a battery. The circuit employs output-powered active diodes and does not require any external power supply or startup circuitry. The circuit rectifies and boosts input ac voltages in the range of 0.35-2 V and 20-500 Hz to a dc voltage output, that is, ~8 times higher than the input amplitude. The circuit can cold start from an input voltage of 0.5 V or higher, providing an output voltage sufficient to charge a 3.7-V lithium ion battery. Once started, the circuit can maintain operation at input voltage amplitudes as low as 0.35 V. Over 80% efficiency is achieved from 20 to 100 Hz, with output power ranging from a few microwatts to 1 mW. Furthermore, in testing with an actual electrodynamic (magnetic) vibrational energy harvester that generates >;0.5-V ac output, the circuit delivers power to a lithium ion battery with an efficiency of >;80%.