Abstract: The activation of a thermal battery (TB) depends on the internal heat source to melt insulating solid electrolyte to highly ionic conductive melten state. A 2D model of a thermal battery is developed to simulate the heat transfer during TB activation based on multiphysics coupling software COMSOL.The heat releasing process of a pyrotechnic system is simulated with the user-defined heat source function. The temperature probes are set in the seventh unit cell and adjacent components at the center of TB stack. Temperature distribution, temperature curves of unit cell, and melting phase change are calculated.The time when the solid electrolyte melts enough to connect the cathodes and anodes is taken as the symbol of TB's activation to predict the shortest activation time. Two TBs are tested to verify the predicted results. Furthermore, an igniting tape built-in TB model is developed to study the influence of igniting tape's position on the activation time. The results show that the temperatures at the ends of TB stack are higher to compensate for heat dissipation during discharge. The current collectors can withstand thermal impact. The probes maintain at 560 ℃ in a short time. The predicted activation time is about 45 ms, the test activation times are 42 ms and 47 ms，indicating the model and prediction method are of high accuracy. The activation time of the built-in TB reduces to 30 ms.

Key words: thermalbatteryactivation, heattransfersimulation, temperaturedistribution, phasechange, activationtime