In this brief, a linear-based battery charger is proposed, designed, fabricated in a 180 nm SOI process, and measured. The design is aimed to charge low-power wearable or implantable medical devices through a wireless power transfer (WPT) link at a transfer distance of several centimeters, where the available power is in the range of a few milliwatts. A novel self-turn-on/off circuit was implemented, preventing quiescent power consumption while the WPT link is inactive. The compact proposed circuit requires a remarkable low chip area of 0.023 mm 2 . In measurements, using a 3.7 V (nominal) lithium-Ion battery, a configurable charging current from 500 μA to 20 mA was achieved, while presenting a competitive efficiency with the state-of-the-art. The automatic transition between the constant current (CC) and constant voltage (CV) charging phases is shown in measurements.