High-power, large-capacity reserve thermal batteries are useful power sources for the actuators of guided weapons and propulsion for the underwater vehicles. However, unexpected thermal runaway and explosions can occur due to the thermal design error. Preventing such accidents is crucial and arduous challenge in developing high capacity, reliable thermal batteries. To do so, the heat balance between the heat source and the electrode/solid-electrolyte components must be carefully considered when designing a thermal battery. Herein, the optimal discharge temperatures and heat balances of thermal batteries are obtained through the discharge experiment of unit cells with Li(Si) and pure-Li anodes (LAN): (Li(Si): 500 degrees C/100 cal g(-1), LAN: 550 degrees C/110 cal g(-1)). The thermal properties of two types of thermal batteries (150 V, 80 cells, 1 kWh) with Li(Si) and LAN are also analyzed using COMSOL Multiphysics software. Furthermore, to compare the performance, two high-power, large-capacity thermal batteries with the same heat-source energy (110 cal g(-1)) are fabricated and discharged at 40 A over 600 s. The experimentally evaluated temperatures of the cathodes, electrolytes, heat sources, and other components are found to be consistent with the simulation results suggesting that this study provides a complete and accurate computational methods to design high capacity thermal batteries.