An electronic device is provided, including a housing having a back surface. The device includes a rechargeable battery, a capacitor, an inductor coil connected to the capacitor, the inductor coil being disposed around an axis oriented perpendicular to the back surface, a rectifier circuit connected to the inductor coil to output a direct current (DC), a DC-DC converter connected to the rectifier circuit, configured to trickle charge the battery with current received from the rectifier circuit, a test load switchably connected to the DC-DC converter, and a feedback circuit configured to detect a voltage level of the test load and provide an indication of the voltage level.