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Wireless power transfer (WPT) has become increasingly widespread in recent years, ranging from electric vehicles, portable consumer electronics, and implantable biomedical devices to flexible/stretchable electronics. The authors here present a WPT system based on the maximum efficiency design principle. Utilising the properties of Helmholtz coils, a near-uniform 3D volume electromagnetic field is generated making it possible to transfer power to multiple devices simultaneously with no requirement of orientation and/or alignment. The system can also provide continuous wireless power to multiple devices even when they are moving, with an efficiency of up to 87%. An automatic frequency tracing technique was developed to compensate for the resonant frequency shifting issue encountered when the number of loadings changes. Meanwhile, a multi-load decoupling control system was also developed, such that each secondary coil receives consistent energy without being affected by the addition or removal of another receiving coil(s). As a proof of concept, the authors successfully charged various numbers of mobile phones simultaneously and transmitted power to single/multiple moving devices wirelessly.

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