This paper describes an effort to develop long-life, multi-tap thermal battery technology with a minimal weight and volume. The effort has several challenging goals. Some of the development goals include an activated life of at least one hour, four voltage sections, and the ability to sustain significant pulse loads at the end of life. In order to meet these goals, advanced materials were chosen for development. The thermal battery chemistry developed consists of lithium–silicon anodes, low-melting eutectic electrolyte/separators, and cobalt disulfide cathodes. Besides evolving the electrochemistry for this battery, there are several other design challenges such as fine-tuning the heat balance so as to allow the battery to sustain the extended duration discharge. In addition, to minimize volume, the battery can is configured in a tapered shape and consequently requires a tapered Min-K™ sleeve for insulation. A new igniter design is also being used. Finally, extremely narrow voltage ranges for each of the four voltage taps have contributed to the challenges facing development engineers. This paper includes a summary of the battery design and presents test data from pre-prototype units.