A hydrogen energy storage system for portable/mobile applications such as personal power sources and unmanned underwater vehicles is developed. An application-oriented design and system integration strategy are newly suggested to maximize energy density while incorporating conventional technologies for the electrolyzer (Ely), the metal hydride (MH), and the polymer electrolyte membrane fuel cell (PEMFC). To improve both the energy density and usability, the systems for charging and discharging are separated. The charging component is composed of a water Ely (0.5 Nm(3) h(-1)) and an MH cooling device as one system. The discharging component consists of an MH (900 NL H-2), a PEMFC stack (50 W), and a power conditioning system (PCS) as a single system. The MH material and engineering properties are investigated to find an MH that is suitable for the target system. The hybrid design and operating strategy of the PEMFC and PCS are developed to maximize energy density. The prototype system provides a nominal power output of 31.5 W at 12 V for 38 h with one recharging. We find it significant that the discharging component shows an energy density of 410 Wh L-1, which is twice that of conventional energy storage systems at the 2.9-L level.