This paper describes the modelling and simulation of the Electrical Power Subsystem (EPS) of the Thermal Analysis Support and Environment Characterization Laboratory (TASEC-Lab). TASEC-Lab is a university experiment on board a sub-orbital platform. It is designed to measure the convection heat transfer in high-altitude balloon missions. The EPS provides, regulates, and distributes electric power to the different systems, parts, and sensors that compose the TASEC-Lab (e.g., On Board Computer (OBC), temperature and pressure sensors, cup anemometer, GPS, heaters... ). It mainly consists of a Li-ion battery and two DC-DC converters, and they have been characterized by conducting laboratory tests and fitting to experimental data. A real power consumption profile of the first TASEC-Lab’s mission (designed by Universidad Politecnica de Madrid) is used as input to simulate the EPS. The mathematical model is validated by comparison with experimental results.