In this paper, a wide input range CMOS multi-mode active rectifier is presented for a magnetic resonant wireless battery charging system. The configuration is automatically changed with respect to the magnitude of the input AC voltage. The output voltage of the multi-mode rectifier is sensed by a comparator. Furthermore, the mode of the multi-mode rectifier is automatically selected by switches among the original rectifier mode, 1-stage voltage multiplier mode, and 2-stage voltage multiplier mode. In the original rectifier, the range of the rectified output DC voltage is from 9V to 19V for an input AC voltage from 10V to 20V. In the multi-mode rectifier, the input-range is wider compared to the original rectifier by 5V. As a result, the rectified output DC voltage ranges from 7.5V to 19V for an input AC voltage from 5V to 20V. The proposed multi-mode rectifier is fabricated in a 0.35µm CMOS process with an active area of around 2500µm × 1750µm. When the magnitude of the input AC voltage is 10V, the power conversion efficiency is about 94%.