The Japan Aerospace Exploration Agency (JAXA) is developing a lithium-ion secondary battery for deep space missions. Lithium-ion secondary battery was first used for the interplanetary spacecraft, Hayabusa. With a view to future long-term operations on the moon and interplanetary travel, the in-orbit performance of the lithium-ion battery of Hayabusa was examined. The battery cells maintained a constant performance over 2.7 years of operation as Hayabusa travelled to the asteroid Itokawa. To maintain cell conditions. The state of charge was fixed by using a balance circuit. The cell voltages differed by less than 60mV during the operation, which is within the error expected based on the circuit design and the telemetry conditions.