Batteries currently used as power supplies for measurement while drilling (MWD) equipment in boreholes for oil and gas exploration use a modified lithium/thionyl chloride technology. These batteries are limited to operating temperatures below 200degreesC. At higher temperatures, the batteries and the associated electronics must be protected by a dewar. Sandia National Laboratories has been actively engaged in developing suitable alternative technologies for geothermal and oil/gas borehole power sources that are based on both ionic liquid and solid-state electrolytes. In this paper, we present the results of our studies to date and the directions of future efforts. (C) 2004 Elsevier B.V. All rights reserved.