LCC-S vs. LCC-LCC: Efficient Wireless Charging for Underwater Drones Under Seawater Conditions

Battery autonomy is one of the main factors limiting the endurance of autonomous underwater vehicles (AUVs). Conventional charging through electrical connectors is inconvenient in marine environments since connectors are exposed to corrosion and usually require manual intervention or docking procedures. Inductive wireless power transfer (WPT) avoids these drawbacks, although the conductive nature of seawater introduces additional effects, such as eddy current losses and parasitic capacitance between the coils. These effects modify the resonance conditions of the compensation network and, in turn, reduce the transfer efficiency. This paper presents the design and experimental assessment of an inductive charger for a commercial and specific underwater drone operating under seawater conditions. A square coil geometry, selected to match the available installation area on the vehicle, was analysed together with two compensation networks (LCC-S and LCC-LCC) and two coil designs with 20 and 25 turns. Based on an analytical characterisation, their performance was evaluated for different coil separations and operating temperatures. Among the analysed configurations, the LCC-S topology with 25 turns provided the best compromise between efficiency and tolerance to gap variations. A laboratory prototype was subsequently built and tested in saline water with NaCl concentrations of 2%, 3%, and 4%, reaching an efficiency close to 87% at 266 W. These results confirm that the proposed design is suitable for underwater wireless charging under representative marine salinity conditions.