The invention detects foreign objects FO near a primary coil 100 of an induction charger. A sensors 111 of a sensor array 110 output sensing signals in response to magnetically coupling the alternating magnetic field 103 produced by the primary coil. A controller 165 connected to each sensor 111 scans the sensing signals and determines whether there is a foreign object perturbing the magnetic field 103 near a sensor. The magnetic field has a spatial distribution that varies by location across the primary coil area. Each sensor has a magnetic field sensing sensitivity that is inversely proportional to the magnetic intensity of the magnetic field produced by the primary coil at a location of the sensor, to reduce the collective dynamic range of the signals, thereby contributing to maintaining a high accuracy in signal sampling. A reference sensor coil 155 compensates for magnetic field drift of the primary coil.