NASA's 2003 Mars Exploration Rovers, Spirit and Opportunity, have been performing exciting surface exploration studies for the past 3 years, providing conclusive evidence for the presence of past water on Mars. Although the rovers are being powered by Li-ion batteries and solar arrays, their critical entry, descent, and landing (EDL) maneuvers were successfully supported by primary lithium-sulfur dioxide batteries. These batteries exhibited voltage delay at the end of cruise, which necessitated a depassivation of these batteries prior to EDL. In the absence of conventional depassivation across a specified load, a new method of depassivation via potentiostatic discharge was employed for the mission. Several simulation tests were performed on cells, cell strings, and battery assemblies, at different potentiostatic voltages and durations to characterize the depassivation process. Effects of repassivation of the lithium anode subsequent to depassivation were also studied, mainly to establish the timeline necessary for depassivation prior to use during the EDL process. Finally, a phenomenological model was developed for the potentiostatic depassivation of Li-SO2 cells, based on dielectric properties of the surface film on Li, which gave voltage predictions in quantitative agreement with the experimental data. The laboratory results obtained were subsequently corroborated by the in-flight data received from the spacecraft. (C) 2007 The Electrochemical Society.