Basically, there are three types of devices to treat cardiac diseases and they are implanted inside the human body. They are pacemakers, defibrillator, and ventricular assist devices using the Zn/go HgO batteries. With the introduction of Li/I2 battery, their life of battery extended to 10 years. The chapter deals with this special kind of batteries and their characteristics are presented here. Modern pacemakers use these batteries because they act as micro-computers. The devices utilize an implantable Li–ion battery pack that is recharged through the skin. The Li/I2 battery is explained with figure and the discharge is given graphically. The Li/Silver Vanadium Oxide (SVO) battery is presented and the reactions are explained. The section on other medical applications explains the use in micro-gastrointestinal investigator, implantable gastric stimulation system, self-regulating responsive therapeutic device, and the implantable micro-stimulator. The batteries for special applications are the ones used for micro sensors, and batteries that are used in smart cards and tags. The batteries used in peculiar applications are outlined with examples.