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Wireless power transfer systems are finding increasing applications which can enhance the living standard of the general public. Typical examples are wireless charging of mobile phone batteries and car batteries. Thanks to the advent of power electronics in the past few decades, some wireless charging systems are now emerging in the commercial sector. However, the transfer efficiency and transfer distance of these chargers are limiting the technology to a few specific applications only. In order to increase the transfer distance, some researchers have successfully proposed to use relay coils to improve their wireless power transfer efficiency. In this paper, the role of relay resonator is examined critically. The reported findings indicate that it is not always desirable to have relay coils in wireless charger as the relay resonator may increase, or sometimes reduce, the overall power transfer efficiency. The findings also …