An example medical device includes a device housing configured to be implantable within a patient, the device housing including an internal surface in contact with a voltaic cell of the battery, and a battery external to the device housing and comprising a battery housing configured to be hermetically sealed. The battery is configured to provide electrical power to an electrical component housed within the device housing, and the battery housing is configured to be attached to the device housing. The battery housing includes an internal surface in contact with a voltaic cell of the battery, and an external surface in contact with the biocompatible electrical insulator. an external surface in contact with the biocompatible electrical insulator.