Lithium-ion batteries are increasingly present on-board French Navy ships and in the power supply of devices in our daily lives. As current events show, the normal or abusive use of a lithium-ion cell can lead to a “thermal runaway” reaction. In order to understand the triggering conditions, as well as the consequences of thermal runaway in a large volume confined chamber (48 m3) and the influence of the number of cells involved, a large-scale test campaign, focusing on the study of the behavior of different groups of high-power lithium-ion packs exposed to a heat source (hydrocarbon pool fire), and its consequences, has been carried out. Four test configurations were selected, involving 14, 48, 96, and 192 18650 lithium-ion cells, leading to energy capacities of 154, 528, 1056, 2112 Wh respectively. The measurements focused on the mass loss of the groups of cells, gas and wall temperatures during thermal runaway and combustion. Finally, a two-zone model has been extended to simulate lithium-ion battery fires. A combustion test involving 192 cells was successfully modelled. Regarding the influence of the number of cells involved, the test results showed that the thermal consequences are proportional to the energy capacity involved.