A wireless power transmitting device transmits wireless power signals to a wireless power receiving device by supplying drive signals with a duty cycle to a wireless power transmitting coil. The wireless power receiving device has a rectifier and a wireless power receiving coil that receives wireless power signals having the duty cycle from the wireless power transmitting device. The rectifier is coupled to an integrated circuit such as a battery charger integrated circuit. The amount of current drawn by the integrated circuit from the rectifier is adjustable. During operation, control circuitry in the wireless power receiving device sets the current to multiple different values while using sensor circuitry to measure output power from the rectifier. A satisfactory value for the duty cycle can be identified by adjusting the duty cycle while observing when peaks in the output power arise as a function of the different current values.