Comparative study on the fire behavior of 18650 NCM lithium-ion batteries subjected to bottom and lateral heating sources

Thermal runaway of 18650 LiNi 0 . 5 Co 0 . 3 Mn 0 . 2 O 2 (NCM) batteries poses a high fire risk because it is often accompanied by violent jet flames, rapid temperature rise, and toxic gas release. However, the extent to which different external heating modes alter the resulting fire behavior remains unclear. In this study, the fire behavior of 18650 NCM battery modules under bottom heating (BH) and lateral heating (LH) was comparatively investigated in a confined chamber. Continuous heating time (CHT), defined as the duration for which external heating was maintained after the first appearance of a visible flame, was set at 0, 30, and 60 s. The combustion process, flame dimensions, mass loss, temperature characteristics, and gas emissions were analyzed. The results showed that BH produced markedly more severe combustion than LH, as indicated by a higher peak gas-phase temperature of 734.8 °C, a greater total mass loss of 145.5 g at CHT = 60 s, a wider high-temperature influence zone, and a higher mass-loss rate. In contrast, LH triggered earlier thermal runaway but resulted in weaker combustion, with a lower peak gas-phase temperature of 384.2 °C and a final mass loss of 51.3 g. With increasing CHT, the maximum flame height under BH increased from 0.401 to 0.764 m, whereas under LH it first increased and then decreased, peaking at 0.577 m at CHT = 30 s. In addition, the sum of the species-specific peak concentrations of the monitored hazardous gas species under BH decreased by approximately 60%. These findings highlight the importance of heating direction in battery-pack layout, thermal management, and fire warning and suppression design.