The EnerCage system is designed to enable electrophysiology experiments of any duration in experimental arenas of any size. Optimized overlapping planar spiral coils (PSCs) provide closed-loop wireless power and data to a mobile unit on a small, freely behaving animal subject. The system tracks the location of the mobile unit with an array of 3-axis magnetic sensors and only activates the closest PSC, thereby reducing power dissipation and heat within the cage. A control algorithm collects data from all the sensors, organizes it by physical location, and systematically compares the magnitudes of each to two thresholds to determine the location of a small magnetic tracer embedded in the mobile unit. An in vivo experiment was conducted for one hour using a 1.03 × 0.9 m2 EnerCage prototype in a physiology lab, where the system was able to continuously deliver 20 mW to a dummy load and maintain a steady 4V supply on the rat headstage (mobile unit).