A method performed by device battery pack (B) coupled to a non-contact charger (A), the method comprising: receiving, by a secondary coil (Scoil1) of the battery pack (B), wireless power induced via an electromagnetic wave from a primary coil of the non-contact charger (A), rectifying, by a rectification unit coupled to the secondary coil (Scoil1), the wireless power to generate rectified power, controlling, by a foreign object detection unit , a switch (Q3) of the battery pack (B) for transferring the rectified power to a power system in the battery pack (B), and detecting by the foreign detection unit an instantaneous power at the same time that the battery pack (B) containing the secondary coil is coupled to the wireless charger (A) and characterised in that the controlling of the switch (Q3), by the the foreign object detection unit includes: allowing a no load state to be maintained by maintaining the switch (Q3) in an 'OFF' state for a certain period of time, and allowing the no load state to be switched into a load state by maintaining the switch (Q3) in an 'ON' state after the no load state has been maintained for the period of time, thereby informing the primary coil of the non-contact charger (A) through a load modulation performed by the switch (Q3) that the battery pack (B) containing the secondary coil has been placed on the non-contact charger (A) and, at the same time, applying power to the power system . receiving, by a secondary coil (Scoil1) of the battery pack (B), wireless power induced via an electromagnetic wave from a primary coil of the non-contact charger (A), rectifying, by a rectification unit coupled to the secondary coil (Scoil1), the wireless power to generate rectified power, controlling, by a foreign object detection unit , a switch (Q3) of the battery pack (B) for transferring the rectified power to a power system in the battery pack (B), and detecting by the foreign detection unit an instantaneous power at the same time that the battery pack (B) containing the secondary coil is coupled to the wireless charger (A) and characterised in that the controlling of the switch (Q3), by the the foreign object detection unit includes: allowing a no load state to be maintained by maintaining the switch (Q3) in an 'OFF' state for a certain period of time, and allowing the no load state to be switched into a load state by maintaining the switch (Q3) in an 'ON' state after the no load state has been maintained for the period of time, thereby informing the primary coil of the non-contact charger (A) through a load modulation performed by the switch (Q3) that the battery pack (B) containing the secondary coil has been placed on the non-contact charger (A) and, at the same time, applying power to the power system . allowing a no load state to be maintained by maintaining the switch (Q3) in an 'OFF' state for a certain period of time, and allowing the no load state to be switched into a load state by maintaining the switch (Q3) in an 'ON' state after the no load state has been maintained for the period of time, thereby informing the primary coil of the non-contact charger (A) through a load modulation performed by the switch (Q3) that the battery pack (B) containing the secondary coil has been placed on the non-contact charger (A) and, at the same time, applying power to the power system . <EOS>