  As a long-range high-power wireless power transfer (WPT) technology, resonant
beam charging (RBC) can transmit Watt-level power over long distance for the
devices in the internet of things (IoT). Due to its open-loop architecture, RBC
faces the challenge of providing dynamic current and voltage to optimize
battery charging performance. In RBC, battery overcharge may cause energy
waste, thermal effects, and even safety issues. On the other hand, battery
undercharge may lead to charging time extension and significant battery
capacity reduction. In this paper, we present an adaptive resonant beam
charging (ARBC) system for battery charging optimization. Based on RBC, ARBC
uses a feedback system to control the supplied power dynamically according to
the battery preferred charging values. Moreover, in order to transform the
received current and voltage to match the battery preferred charging values,
ARBC adopts a direct current to direct current (DC-DC) conversion circuit.
Relying on the analytical models for RBC power transmission, we obtain the
end-to-end power transfer relationship in the approximate linear closed-form of
ARBC. Thus, the battery preferred charging power at the receiver can be mapped
to the supplied power at the transmitter for feedback control. Numerical
evaluation demonstrates that ARBC can save 61% battery charging energy and
53%-60% supplied energy compared with RBC. Furthermore, ARBC has high
energy-saving gain over RBC when the WPT is unefficient. ARBC in WPT is similar
to link adaption in wireless communications. Both of them play the important
roles in their respective areas.
