A charging time control method for wireless power transfer systems with a secondary-side hysteresis output power control is presented. It is a primary-side control method that adopts the combined use of three concepts, namely 1) an intermediate capacitor in the receiver circuit as a power flow indicator, 2) hysteresis switching actions of a shunt decoupling power switch in the receiver circuit to regulate the dc voltage of such intermediate capacitor, and 3) the turn-on and turn-off times of the decoupling switch detected on the primary side for closed-loop control. This method has the advantage of eliminating the need for 1) precise information of the mutual inductance between the transmitter and the receiver coils and 2) wireless communication system for feedback purpose. Practical results obtained from a hardware prototype are included. They confirm the proposed operating principles and indicate that the method can automatically lead to optimal energy efficiency operation.