An apparatus for charging one or more electric devices. The device may comprise a resonant circuit configured to wirelessly couple to the one or more electric devices, a boost converter configured to convert a system voltage received by the apparatus to an input voltage of a power amplifier that drives the resonant circuit, a sensing circuit configured to detect the input voltage and an associated input current, and a controller. The controller may be configured to receive the detected input voltage and the detected input current, compare the received input voltage and the received input current with stored voltage and current information, identify a difference between the received input voltage and a predetermined voltage according to the stored voltage and current information, and control the boost converter to adjust the input voltage by the identified difference to the predetermined voltage. The adjustment may not need any feedback or communication from the power receiver unit.