Creare is developing a miniature, low-power, free-piston energy conversion system. Our converter is designed to transform thermal energy from radioisotope heater units into on-demand electricity essential to space exploration probes, unmanned surface rovers, small landers, small satellites, and similar small-scale systems operating in darkness. We have achieved a simple system design with a single moving part that requires no recuperator and no regenerators or valves. Our converter technology promises a high-efficiency system in an extremely compact enclosure. This work describes preliminary design, analysis, and testing efforts for our miniaturized converter. We fabricated a laboratory-scale prototype and acquired experimental data at prototypical temperatures to validate our performance models. Our numerical model was able to accurately predict converter losses. In doing so, we also demonstrated the feasibility of our novel thermodynamic cycle through the generation of net positive pressure-volume work of the system at its design temperature (~873 K). These results have been used to guide subsequent converter design modifications. Future work includes the fabrication, testing, and detailed performance assessment of a complete prototype converter.