To study the interaction between the Martian upper atmosphere and the solar wind, the mars orbiter, NOZOMI was launched in July 1998. In NOZOMI, 15 Ah Ni metal hydride (NiMH) cells were used for the first time in the world to meet the requirement of lightweight of the orbiter. Two onboard batteries, each of which consists of 16 NiMH cells connected in series, have been normally operated in a cruise orbit up to the mars for about 3.3 years by means of a management method like charge or reconditioning operations. For future spacecraft, improvement in an increase in the internal pressure of the NiMH cells has been recently made so that charge operations of their onboard batteries may be made removable. This paper describes a development review of the NiMH cells aboard NOZOMI and their on-orbit operations.