Fuel cell/battery (FCB) systems are promising power generation/energy storage systems because of their bi-functionality as fuel cells and as secondary batteries. We investigated the required charging after the discharged manganese dioxide (MnOOH) by oxygen gas under the rest condition and during the fuel cell operation mode using manganese dioxide as a positive electrode for the FCB system. Electrochemical characterization was performed using cyclic voltammetry and galvanostatic measurements. Additionally, changes in the crystal structure and the chemical functional groups during the electrode reactions were monitored by X-ray diffractometry and Fourier transform infrared spectroscopy. The results indicated that MnOOH formed via the electrochemical discharge of manganese dioxide (MnO2) and that the oxyhydroxide can be chemically transformed back to MnO2 with gaseous oxygen (O2). The recharged MnO2 can be used as the cathode in a fuel cell with an O2 supply and it can also be electrochemically discharged without an O2 supply. In addition, we confirmed that MnO2 does not convert to Mn3O4 during the charge/discharge cycles if the redox reaction is maintained within a restricted range where a homogeneous process exists between MnO2 and MnOOH. The results in this study suggest that the FCB system can be constructed using MnO2 as the positive electrode and a metal hydride (MH) as the negative electrode, which can be rapidly charged to more than 70% of the theoretical capacity within 10min using pressurized H2 and electrochemically discharged, in an alkaline electrolyte. This system possesses a high-power generation efficiency, a high-energy density and a high load-following capability.