A power-transmitting node for transmitting power wirelessly to a power-receiving node of a wireless charging system, the power-transmitting node comprising: a signal receiver configured to receive signals from the power-receiving node to determine a presence of said power-receiving node; a power transmitter configured to transmit the power wirelessly to the power-receiving node, for charging the power-receiving node, in a power-transfer state; a processor configured to: place the power-transmitting node in a protection state upon detecting, in the power-transfer state, the presence of a foreign object, FO, in addition to said power-receiving node, wherein, in said protection state, the power-transmitting node does not transmit power to the power-receiving node; and measure (206, 214, 220) quality factor, QF, values for the power-transmitting node before and after detecting a presence of the FO, in addition to said presence of said power-receiving node, wherein the measured values comprise: an original QF value, orig_QF, for the power-transmitting node prior to transmitting the power to the power-receiving node; and an initial protection-state QF value, enter_QF, and a current QF value, curr_QF, for the power-transmitting node upon entering the protection state; and use the orig_QF, enter_QF, and curr_QF values to determine whether the FO has been removed without using any factory programmed calibration QF values by evaluating the logic formula: (((orig_QF - enter_QF) > thresh1) AND ((orig_QF - curr_QF) < thresh1)) OR ((abs(orig_QF - enter_QF) < thresh2) AND ((curr_QF - orig_QF) > thresh3)) wherein thresh1, thresh2, and thresh3 are three specified QF threshold levels; and a memory configured to store the QF values. a signal receiver configured to receive signals from the power-receiving node to determine a presence of said power-receiving node; a power transmitter configured to transmit the power wirelessly to the power-receiving node, for charging the power-receiving node, in a power-transfer state; a processor configured to: place the power-transmitting node in a protection state upon detecting, in the power-transfer state, the presence of a foreign object, FO, in addition to said power-receiving node, wherein, in said protection state, the power-transmitting node does not transmit power to the power-receiving node; and measure (206, 214, 220) quality factor, QF, values for the power-transmitting node before and after detecting a presence of the FO, in addition to said presence of said power-receiving node, wherein the measured values comprise: an original QF value, orig_QF, for the power-transmitting node prior to transmitting the power to the power-receiving node; and an initial protection-state QF value, enter_QF, and a current QF value, curr_QF, for the power-transmitting node upon entering the protection state; and use the orig_QF, enter_QF, and curr_QF values to determine whether the FO has been removed without using any factory programmed calibration QF values by evaluating the logic formula: (((orig_QF - enter_QF) > thresh1) AND ((orig_QF - curr_QF) < thresh1)) OR ((abs(orig_QF - enter_QF) < thresh2) AND ((curr_QF - orig_QF) > thresh3)) place the power-transmitting node in a protection state upon detecting, in the power-transfer state, the presence of a foreign object, FO, in addition to said power-receiving node, wherein, in said protection state, the power-transmitting node does not transmit power to the power-receiving node; and measure (206, 214, 220) quality factor, QF, values for the power-transmitting node before and after detecting a presence of the FO, in addition to said presence of said power-receiving node, wherein the measured values comprise: an original QF value, orig_QF, for the power-transmitting node prior to transmitting the power to the power-receiving node; and an initial protection-state QF value, enter_QF, and a current QF value, curr_QF, for the power-transmitting node upon entering the protection state; and an original QF value, orig_QF, for the power-transmitting node prior to transmitting the power to the power-receiving node; and an initial protection-state QF value, enter_QF, and a current QF value, curr_QF, for the power-transmitting node upon entering the protection state; and use the orig_QF, enter_QF, and curr_QF values to determine whether the FO has been removed without using any factory programmed calibration QF values by evaluating the logic formula: (((orig_QF - enter_QF) > thresh1) AND ((orig_QF - curr_QF) < thresh1)) OR ((abs(orig_QF - enter_QF) < thresh2) AND ((curr_QF - orig_QF) > thresh3)) (((orig_QF - enter_QF) > thresh1) AND ((orig_QF - curr_QF) < thresh1)) OR ((abs(orig_QF - enter_QF) < thresh2) AND ((curr_QF - orig_QF) > thresh3)) wherein thresh1, thresh2, and thresh3 are three specified QF threshold levels; and a memory configured to store the QF values.