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• A system-level thermal-electrochemical model of thermal batteries during activation stage is built. • Fuse strip has important effects on the activation process of thermal batteries. • The difference of internal resistance between the unactivated and activated state can reach 12 orders of magnitude. As the primary reserve batteries widely used in military domain, the intricate thermal-electrochemical behaviors and tiny time scale of the activation process make the development of thermal batteries a pain-stacking work, as lacking of the precise model to guide the design process. In this study, a system-level thermal-electrochemical model for evaluating the main technical indexes of LiSi/FeS 2 thermal batteries during activation stage is built, which comprehensively considers the main physical and chemical processes such as combustion, heat transfer, phase change, ion transport, voltage variation and so on. The activation characteristics of two types of 5-cell thermal batteries are predicted by this model, and the activation voltages of the two batteries are tested to validate the model. The model possesses the high precision with error less than 5 % by comparing the simulation results of activation voltage with that of experiment results. Specifically, the model reveals that the instantaneous temperature of pyrotechnic pellet after combustion is around 1570 K, and the electrolyte becomes molten and its internal resistance sharply decreases to mΩ level when the temperature reaches its melting point. The variation process of activation voltage is mainly dominated by the geometry size of cell stack and the thickness ratio of pyrotechnic pellet to single cell, besides, the activation time can be considered to be the peak value time of activation voltage. Fuse strip has important effects on the activation process of thermal batteries, especially in the rapid increase period of activation voltage, and the heat transfer is the time-limited step of the activation process. The difference of internal resistance between the unactivated state and activated state of thermal batteries can reach 12 orders of magnitude. These results indicate that our model can perform well in evaluating the activation process of thermal batteries and guiding the optimal design. [ABSTRACT FROM AUTHOR]