A control-based analysis and characterization of a free-piston Stirling engine is presented, and proposed as a lightweight power supply for untethered robots. Typically, such devices are designed from the point of view of a thermodynamic cycle in terms of traditional thermodynamic equations of state. Such equations of state are independent of time and therefore lend little insight when dynamic elements are incorporated into the design. The approach presented here is from a system dynamics and control perspective. Equations of state are replaced by dynamic system modeling elements. Utilizing these dynamic elements, control concepts are applied to evaluate a given configuration and ensure an unstable oscillatory response and therefore transform heat into useful work. A simulation of a commercially available free-piston engine is presented, and standard control design tools are applied to its linearized model. The results show promising potential in utilizing small-scale free-piston Stirling engines as portable power supply for robotic systems.