Embodiments herein relate to implantable medical devices including a power subunit with a first biocompatible electrically conductive shell configured for direct contact with an in vivo environment. In some embodiments a lithium anode can be disposed within the first biocompatible electrically conductive shell in direct electrical communication with a feedthrough pin, wherein the feedthrough pin is electrically isolated from the first biocompatible electrically conductive shell. A cathode can also be disposed within the first biocompatible electrically conductive shell and can be in direct electrical communication with the first biocompatible electrically conductive shell. The first biocompatible electrically conductive shell has a positive electrical potential. The implantable medical device further includes an electronics control subunit with a control circuit disposed within a second biocompatible electrically conductive shell. Other embodiments are included herein.