There is interest in developing a suitable battery system that can be used at temperatures of 250 °C or less to power instrumentation used in oil/gas and geothermal boreholes. The discharge performance of MnO2 cathodes with Li–Mg–B alloy anodes is examined using the LiNO3–KNO3 eutectic electrolyte over a temperature range of 150 °C–300 °C at current densities from 10 to 30 mA cm−2. In this study, we find that the cell can be activated at 150 °C and operate within the desired temperature range without any indication of possible hazards. However, we did observe that temperature and current density significantly affected cell capacity and voltage. Overall, the Li–Mg–B alloy/LiNO3–KNO3/MnO2 system shows great potential for powering instrumentation used in oil/gas and geothermal boreholes.