This paper details the analysis and design of an air-core permanent-magnet-based energy-harvest system for use within an in-pipe mobile water-quality sensor. The sensor is a free-moving/free-rotating device of which the energy-harvest system occupies an appreciable volume. Therefore, the electrical and mechanical dynamics of the device are strongly coupled, which poses a modeling challenge. To address this challenge, a rigorous electrical/mechanical model is established to determine expected energy harvest, given the geometric/material parameters and electrical/mechanical inputs. The model was utilized within a single-objective optimization-maximizing energy harvested from impacts-to design a prototype, which was constructed and used for model validation. Subsequently, multiobjective optimizations were performed to establish the tradeoff between energy harvest versus device volume and impact-based versus rolling-based energy-harvest efficacy.