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This study proposes a direct-current (DC) driving impedance matching method for rectennas for the wireless charging of implantable lithium-ion (Li-ion) rechargeable batteries using the medical implant communication service band. Unlike the traditional method, which uses only the DC resistive load or battery potential, the proposed method adopts the output DC current of the dual-diode rectifiers to establish a simple equivalent model for wireless battery charging. Using this model, the optimised impedance matching network of the rectenna is derived to maximise efficiency within the charging range of a Li-ion battery. For 10 dBm input radio-frequency (RF) power, the highest RF-to-DC conversion efficiency of the traditional matching method is 22.3%. For the proposed method, the RF-to-DC conversion efficiency is > 75% in the potential range of 3.7 to 4.2 V, which is the range used for over 90% of the charging duration. The best battery charging efficiency was 76.2% at a transmission distance of 50 cm.