Implantable medical device power circuits are disclosed. Multiple batteries may be provided, along with a number of switches, enabling a plurality of battery and power circuit configurations to be defined. Configurations of the power circuit may be changed in response to changes in battery status as the batteries are used and/or near end-of-life. Configurations of the power circuit may also be performed in response to changes in device operation. Methods associated with operating such circuits and implantable medical devices are also disclosed.