A resonant circuit can be used in recharging a battery. The resonant circuit includes an inductor, a first capacitor in series with the inductor, and a second capacitor in parallel with the inductor. Upon entering the field of a charging terminal a controller connected to the resonant circuit continually measures loss in the inductor and voltage on a terminal of the resonant circuit. If both are below respective predetermined thresholds, the controller decreases the capacitance of the first capacitor and increases the capacitance of the second capacitor, thereby increasing voltage from the resonant circuit to the battery.