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 60 mV during the operation, which is within the error expected based on the circuit design and the telemetry conditions. (C) 2012 Elsevier Ltd. All rights reserved.