An apparatus and a method for wirelessly charging are disclosed. An accessory device is detachably mounted on an electronic device. The accessory device includes a housing including a first face configured to oppose a face of the electronic device when the housing is mounted on the electronic device, a second face opposite to the first face, and a third face extending in a first direction from the first face so as to enclose at least a portion of a side face. The accessory device further includes a battery and at least one conductive pattern electrically connected to the battery. The accessory device can transmit power to the electronic device in a wireless manner and/or receive power from an external device in a wireless manner. Areas of the first face and the third face of the housing of the accessory device are formed of a non-conductive material.