Scientists have demonstrated a highly efficient method for wirelessly transferring power to a drone while it is flying. The technology uses inductive coupling, a concept initially demonstrated by inventor Nikola Tesla over 100 years ago. Two copper coils are tuned into one another, using electronics, which enables the wireless exchange of power at a certain frequency. Scientists have been experimenting with this technology for decades, but have not yet been able to wirelessly power flying technology.
http://phys.org/news/2016-10-drones-re-charge-airborne-technology.html