A wireless power system has first and second electronic devices. The first electronic device may receive power from an alternating-current-to-direct-current power converter. A magnetic sensor and/or other sensing circuitry may detect when a second electronic device is attached to the first electronic device. In response to detecting device attachment and receipt of power from the power converter, the first electronic device sends test wireless power signals to the second electronic device. During reception of the test signals, the second electronic device adjusts a ballast load so that output current from a rectifier in the second electronic device flows through the ballast load. The output of the rectifier is measured and compared to a threshold voltage. If the rectifier output voltage exceeds the threshold, the first electronic device is directed to alert the user that wireless charging of the second electronic device is commencing.