We present a simple concept for an electret-based capacitive charge pump capable of working at low frequencies (≤1 Hz) with a very low mechanical input energy and a high conversion efficiency. With a charged electret-like elastomer membrane the converter can operate without an initial bias voltage supply. A proof-of-concept demonstrator converts a mechanical input energy of 332 μJ (9.1 μJ/cm²) to an electrical output energy of 206 μJ (5.7 μJ/cm²) per conversion cycle. This gives a total conversion efficiency of 63%.