This study presents the development of a calibratable prototype system for detecting thermal propagation (TP) within a Battery Management System (BMS) for electric passenger vehicles. The primary objective was to identify key factors influencing the software development process, ensuring compliance with the relevant regulation UNECE R100, while maintaining flexibility for various battery types. Given the safety-critical nature of TP detection, the system was designed to be both timesensitive and precise, minimising the risk of false positive alerts to enhance passenger safety. The project scope was confined to batteries with high-reactivity chemistries, such as NMC-811, focusing on triggering timely alerts in the event of TP. A structured software engineering approach was adopted, supported by discussions with subject-matter experts at AFRY Sweden AB. In the absence of a physical battery pack and sensors, simulated data were utilised for functional testing alongside unit testing. This demonstrated the system’s capability to detect TP within 171 seconds of the initial measurement, indicating UNECE R100 requirements could be met with respect to the logical criteria set by AFRY and the data utilised. Although the software was designed to accommodate pressure sensor inputs, such measurements were unavailable for testing. To enable straightforward calibration across various battery configurations, the calibration functionality was developed as a separate, modular class with self-explanatory method names to simplify usage and maintenance. This enhances the system’s overall adaptability and robustness.