Using ultrasound to power deeply implanted biomedical devices is a promising technique due to its low attenuation in body tissue and its short wavelength that allows precise focusing of the energy. Ultrasound energy harvesting conventionally has been done using lead zirconate titanate (PZT) ultrasound transducers, which uses the piezoelectric effect to convert mechanical vibration to an electrical voltage. However, PZT is typically bulky, and is not bio-compatible, and cannot be monolithically integrated with application-specific integrated circuits (ASIC). In this work, a pre-charged collapse-mode capacitive micromachined ultrasonic transducer (CMUT) was fabricated to harvest ultrasound energy. The pre-charged CMUT has a high power transfer efficiency over a wide bandwidth at optimal loading conditions; 43% at 2.15 MHz and 47% at 5.85 MHz. For the last 1.4 years, the device has been in collapse-mode, and it is still functional without any additional charging. This device will enable the development of smaller implantable biomedical devices in the future.