Battery energy storage systems (BESS) using lithium-ion batteries (LIB) are required for storing electric power derived from renewable energy sources. The system consists of many components such as cells with thermal runaway (TR) risk, parameter sensors, and cooling units. In addition, these components have relations with each other. Therefore, the system can cause a large-scale fire or explosion due to failures of the relation between system components including the TR propagation through cells. For the safe design and operation of BESS, international safety standards recommend a system-level risk analysis that considers these characteristics and hazards of BESS. However, the system-level risk analysis is difficult by using document-based qualitative methods described in the standards because the methods can’t consider the characteristics of BESS well. Therefore, this study focused on development of a risk analysis method using 1D model, which can model relations between system components, and aimed to identify system-level risk scenarios caused by combined failures. To identify the risk scenarios by combined failures, we constructed a battery module model and failure models using the 1D model. From the calculation result of the cell temperature profile, we determined whether to occur TR in the case of assuming combined failures. In conclusion, the developed method can identify risk scenarios and combined failures that cause TR.