An Update on the Performance of Li-Ion Rechargeable Batteries on Mars Rovers
NASA's Mars Rovers, Spirit and Opportunity have been exploring the surface of Mars for the last thirty months, far exceeding the primary mission life of three months, performing astounding geological studies to examine the habitability of Mars. Such an extended mission life may be attributed to impressive performances of several subsystems, including power subsystem components, i.e., solar array and batteries. The novelty and challenge for this mission in terms of energy storage is the use of lithium-ion batteries, for the first time in a major NASA mission, for keeping the rover electronics warm, and supporting nighttime experimentation and communications. The use of Li-ion batteries has considerably enhanced or even enabled these rovers, by providing greater mass and volume allocations for the payload and wider range of operating temperatures for the power subsystem and thus reduced thermal management. After about 800 days of exploration, there is only marginal change in the end-of discharge (EOD) voltages of the batteries or in their capacities, as estimated from in-flight voltage data and corroborated by ground testing of prototype batteries. Enabled by such impressive durability from the Li-ion batteries, both from a cycling and calendar life stand point, these rovers are poised to extend their exploration well beyond 1000 sols, though other components have started showing signs of decay. In this paper, we will update the performance characteristics of these batteries on both Spirit and Opportunity.