BlueStar Advanced Technology Corporation (BATC), under contract to the USAF, is developing large Li ion cells and batteries which will satisfy the performance requirements of future Mars Lander and Rover missions. The present scenario for the Lander mission calls for a 28-V, 25-Ah battery which will discharge at the C/5 to C rate and be charged at the C/5 to C/2 rate. The mission requires > 500 cycles at a 50% depth of discharge and an operating temperature of -20°C to + 45°C. The Rover mission calls for a 14-V, 7-Ah battery which will operate in the temperature range -30°C to + 45°C with all other requirements being identical to that of the Lander. Cell designs, characteristics and test data will be presented as well as preliminary battery designs.