NFC energy harvesting is the process of turning the 13.56 MHz radio field emitted by an NFC device — usually a smartphone — into usable electrical power. Every NFC link is built on inductive coupling between two small antenna coils, and a passive NFC device sitting in that field can do two things at once: exchange data and draw energy (near-field communication standards, ISO/IEC 18092 and ISO/IEC 14443). A battery-free lock pushes the second capability to its logical end. Th