A new definition of power transmission efficiency for wireless charging systems by using coil arrays

Usage of the coil arrays on both the transmitter and receiver sides is introduced for mid-range wireless power transmission (WPT) by employing the technique of strong magnetic resonant coupling. In this paper, a two-coil array is designed for the power transmitting terminal while another two-coil array is designed for the power receiving terminal. Then based on maximum power transmission theorem, we propose a new definition of power transfer efficiency for the coil array feeding WPT system. By applying the newly defined formula, it is shown that a total power transfer efficiency of over 75% at a transmission separation of 50 cm can be achieved, which is much higher than that of other comparison cases without using coil arrays at both transceiver sides in the meantime. Also, the optimized values of load resistors respectively connected to all the receiver coil elements show that the proposed WPT system is especially suitable for electricity chargers with very low internal resistances such as laptop batteries now in commercial use.