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In this paper a complete power conditioning system for a vibration energy harvester is presented which operates at ultra-low power levels. The power conditioning system, implemented with discrete components, is self-starting and fully-autonomous, and based upon a full-wave boost rectifier topology. The design utilises the stray inductance of the harvesters coil, eliminating the need for separate inductors, and employs open-loop control which reduces the quiescent power overhead to just 21 ìW, whilst still extracting 84% of the maximum available power from the harvester. The design of the sub-systems, including self-start circuitry, is described in detail, and it is shown that careful active device selection is required to minimise losses. It is experimentally demonstrated that the power converter achieves conversion efficiencies of up to 76% at sub-mW power levels, including quiescent losses. The overall system efficiency peaks at 65% at 0.9 mW, whilst still achieving 51% at 200 ìW. The ability of this system to operate efficiently at ultra-low average power levels opens up new possibilities to further miniaturise vibration harvesters and deploy them into environments with lower vibration levels than is currently possible.