Future NASAs planetary exploration missions require batteries that can operate at extreme temperatures and deliver high specific energy and energy densities. Conventional aerospace rechargeable battery systems, such as Ni-Cd, Ni-H-2 and Ag-Zn, are inadequate to meet these demands. Lithium ion rechargeable batteries were therefore chosen as the baseline for these missions. The 2003 Mars Exploration Rover (MER) mission plans to deploy twin rovers onto Mars, with the objectives of understanding its geology, climate conditions and possibility of life on Mars. The spacecraft contain various batteries, i.e. primary batteries on the lander, thermal batteries on the back shell and rechargeable batteries on the Rovers. Significant among them are the Li ion rechargeable batteries, which are being utilized for the first time in a major NASA mission. The selection of the Li ion battery has been dictated by various factors, including mass and volume constraints, cycle life, and its ability to operate well at sub-zero temperatures (down to -30 degreesC), at moderate rates. This paper describes the selection criteria, design and performance of the three battery systems on 2003 MER mission. (C) 2003 Published by Elsevier Science B.V.