Electrochemical power cells having an open-cell architecture for electrically powering an implantable medical device system include a first and a second electrode assembly, wherein at least one is a biocompatible hermetically sealed anode assembly (e.g., that of a lithium anode assembly). The power cell can be a biological lithium semi-fuel cell in which a bodily constituent partakes in the cell discharge reaction at the cathode as an active reagent. The active cathode reagent can be oxygen supplied from the body. In a particularly suitable application, the biological lithium semi-fuel cell provides electrical power to a cardiac pacemaker device, such as for a novel cardiac pacemaker system.