This work adopts high-frequency inductive power transfer (HF-IPT) to design an efficient WPT system for low-power sensors and robots in a fresh water environment, and explores the effect of fresh water on coil coupling at different distances. The system configuration is composed of a load-independent Class-EF inverter, PCB coils and a Class-D rectifier with an available gap up to 100 mm. In the experiments, the prototype achieves a peak system efficiency of 62.8% with a power delivery of 8.0 W at a 4.5 cm air gap. In a fresh water environment, it maintains Zero Voltage Switching (ZVS) with a gap varying from 4 cm to 6 cm, achieving a peak system efficiency of 57.1 % over a wide power range from 3.9 W to 7.1 W. The experimental results show that the load-independent Class-EF inverter can maintain ZVS in fresh water around 4.5 cm even without any tuning. For this phenomenon, the effect of fresh water on coils is further studied by measurement and simulation, and the results are in good agreement. In addition, the experimental results show that when the coupling coefficient is less than 0.05, the prototype has higher system efficiency and power output in freshwater than in air.
