The need for high power space systems in anticipated within the next 10 to 20 years. Examples are the Space Station, co-orbiting platforms, geostationary orbit (GEO) platforms, and space-based radar satellites and some classified high power missions. Sodium-sulfur (NaS) batteries have been identified as the most likely successor to nickel-hydrogen (NiH 2) batteries for space applications. One advantage of the NaS battery system is that the useable specific energy is two to three times that of NiH 2 batteries. This represents significant launch cost savings or increased payload mass capabilities. NaS batteries support NASA OAST's proposed Civil Space Technology Initiative goal of a factor of two improvement in spacecraft power system performance, as well as the proposed Spacecraft 2000 initiative. The NaS battery operates at between 300 and 400 °C, using liquid sodium and sulfur/polysulfide electrodes and solid ceramic electrolyte [1 – 3]. The transport of the electrode materials to the surface of the electrolyte is through wicking/capillary forces. These critical transport functions must be demonstrated under actual microgravity conditions before NaS batteries can be confidently utilized in space. For Aerospace Corporation, under contract to NASA Lewis Research Center, is currently working on the NaS battery space flight experiment definition study. The objective is to design the experiment that will demonstrate operation of the NaS battery/cell in the space environment, with particular emphasis on evaluation of microgravity effects. Experimental payload definitions have been completed and preliminary designs of the experiment have been defined.