An apparatus for wirelessly transferring charging power is provided. The apparatus comprises a coupler configured to generate a wireless field when driven with a time-varying current. The apparatus comprises a measurement circuit configured to determine a parasitic capacitance between the coupler and ground while the coupler generates the wireless field. The apparatus comprises a controller circuit configured to determine a presence of a foreign object in response to the determined parasitic capacitance satisfying a detection criteria. The detection criteria may comprises a time-varying pattern of the determined parasitic capacitance predetermined to correspond to presence of a presence of the foreign object. The detection criteria may also comprises a threshold change in the determined parasitic capacitance predetermined to correspond to presence of a presence of the foreign object.