The metal air battery using an expanded natural graphite sheet as the cathode and galvanized plate as the anode was investigated as a power units for emergency. The discharge energy increased to 8 mW/cm3 from 5 mW/cm3 by optimizing the thickness of 0.8 mm and density of 0.5 g/cm3 for the expanded natural graphite sheets. The DC voltage proportionally and theoretically increased by increasing the number of battery cells. The capacity of this battery also increased by increasing the cell size for a case of less than 200 cm2. We hope that this battery can be used in many applications, such as a backup power supply, emergency battery, etc., because a DC 12 V output can be obtained for 15 cells connected in series, and the cathode and anode materials can be stored for a long time in the atmosphere.