Effective thermal management systems, designed to handle the impacts of thermal runaway (TR) and to prevent cell-to-cell propagation, are key to safe operation of lithium-ion (Li-ion) battery assemblies. Critical factors for optimizing these systems include the total energy released during a single cell TR event and the fraction of the total energy that is released through the cell casing versus through the ejecta material. A unique fractional thermal runaway calorimeter (FTRC), designed to characterize said critical factors, was utilized to examine the TR behavior of 18650-format Li-ion cells representing a variety of manufacturers, chemistries, capacities, and safety features. Primarily, the impacts of bottom vent (BV) safety features, varied cell casing thickness, and Dreamweaver cellulose based separators were assessed for select cell types. A subset of cells also had an imbedded internal short circuiting (ISC) device to allow examination of TR behavior initiated at lower temperatures (i.e. closer to field failure conditions). The impact of bottom rupture on TR behavior was also examined for experiments that resulted in this non-standard failure mechanism. Statistical analysis of the results for each cell configuration reveals that a lognormal distribution effectively characterizes the variation of total TR energy release. Typically 20%–30% of the total energy yield is released through the cell casing with the remainder through the ejecta material. Higher energy cells tend to exhibit more violent ejections and less predictable TR events. Inclusion of a BV feature reduces the overall severity of the TR event and can increase the predictability. Results also suggest that the magnitude of the TR event may not be directly proportional to the stored electrical energy, but rather has additional dependence on other design and failure mode factors.