The emergence of Plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) as a viable means of transportation has been coincident with the development of lithium-ion battery technology and electronics that have enabled the storage and use of large amounts of energy that were previously only possible with internal combustion engines. However, the safety aspects of using these large energy storage battery packs are a significant challenge to address. For example an unintentional sudden release of energy, such as through a thermal runaway event, is a common concern. Developing thermal management systems for upset conditions in battery packs requires a clear understanding of the heat generation mechanisms and kinetics associated with the failures of Li-ion batteries.Although every effort is made to avoid thermal runaway situations, there can still be upset and unforeseen instances where a cell or a pack would reach a sufficiently high temperature to initiate exothermic reaction(s) that often are initially slow to develop. Properly designed thermal management systems should be able to lower pack temperatures, effectively slowing down or stopping these exothermic reactions. On the other hand, a poorly designed system could let the temperature rise to a point where a thermal runaway becomes inevitable.In this work, a framework for assessing the efficacy of a thermal management system is presented. In particular, the cells can be tested in an Accelerating Rate Calorimeter (ARC) in order to quantify the slow exothermic reactions that are typically the precursor to a thermal runaway event. Using the test data, numerical models of the cell or cell packs, along with its thermal management system can be developed. Specifically, a three-dimensional (3D) computational fluid dynamics (CFD) model and an in-house one dimensional (1D) heat transfer model are proposed and their results compared. The paper shows how both the models are viable tools to simulate various thermal upset conditions and assess the performance of the thermal management in avoiding thermal runaway.