Primary batteries have been used in past deep space exploration missions, providing power for periods of several hours to several days of mission operations [1,2]. Future missions planned by the National Aeronautics and Space Administration (NASA) will require more advanced primary batteries to provide power to missions for up to 60 days [3]. The Li/CFx primary battery chemistry has a high specific energy, low self-discharge rate, and a relatively wide operating temperature range which makes it a suitable power source for extended mission durations in deep space, particularly for applications involving low to moderate discharge rates [4]. Li/CFx batteries have not been used in any NASA missions to date, and are now being developed for the Europa Lander mission concept [5]. An extensive evaluation of EaglePicher’s D-sized Li/CFx cells is underway, to benchmark the calendar life performance, radiation tolerance, and performance under different temperatures and currents. One of the unique characteristics of the Li/CFx chemistry is the extremely flat voltage plateau during cell discharge (Figure 1) [6, 7]. As the cell approaches end of life, its voltage quickly drops prior to reaching the 1.5V recommended discharge limit. While the cell’s stable voltage is a key component to the high specific energy of this chemistry, it also makes it challenging to accurately predict the battery’s remaining capacity. Premature battery depletion is a risk to mission success, and improved methods for determining depth-of-discharge (DOD) in this unique cell chemistry are of great interest. A pulse-discharge test method is developed and implemented to investigate the change in cell direct current internal resistance (DCIR) during discharge. Varied responses based on the discharge rate are observed and will be discussed. The resulting data are analyzed for possible correlation with DOD, along with changes in the cell voltage over various segments of the discharge curve. This talk discusses the test method and results achieved using the pulse-discharge test method. References: M. Hofland, E.J. Stofel, R.K. Taenaka, Aerospace and Electronic Systems Magazine IEEE, 11, 14 (1996). P. Dagarin, R.K. Taenaka, E.J. Stofel, Proc. of 31st Energy Conversion Engineering Conference 1996, 1, 427 (1996). P. Hand et al.2022 Planet. Sci. J. 3 22. Frederick C. Krause et al.2018 Electrochem. Soc. 165 A2312. Crum, R. et al 2021 Advanced Technology Developments for Europa Lander and other In-Situ Ocean World Missions. Bulletin of the AAS,53(4). Watanabe, N., & Fukuda, M. (1970). S. Patent No. 3,536,532. Washington, DC: U.S. Patent and Trademark Office. Watanabe, N., Endo, M., Ueno, K., Solid State Ionics, Vol. 1, Issue 5-6 (1980). The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the Europa Lander Pre-Project. Figure 1