Systems and designs for charging a battery in a wireless power transfer system are provided, which may include any number of features. In one embodiment, an implantable wireless power receiver is coupled to a battery with a rectifier circuit. The receiver can include battery monitor circuitry configured to measure a parameter of the battery. The receiver can be configured to wirelessly communicate the parameter of the battery to the transmitter, which can control delivery of power from the transmitter to the receiver based on the measured battery parameter. Power can be delivered to the battery without utilizing a battery charge control circuit in the device or receiver. Methods of use are also provided.
