Experimental study on the radiative heat flux from flaming 18650- and 26650-type LFP battery cells

Fires from lithium-ion cells are a major heat transfer pathway, promoting thermal runaway propagation and spreading flames to surrounding cells. This study presents new experimental insights into the fire behaviour of lithium iron phosphate (LFP) cells by quantifying radiative heat flux, gas temperature, and cell surface temperature during venting and flaming. The key contributions are: (i) the first spatially and time-resolved measurements of radiative heat flux from single-cell LFP fires, revealing non-uniform radiation with peak intensities varying up to ∼10× across the horizontal plane; and (ii) a controlled comparison of 18650 and 26650 cylindrical cells, demonstrating fundamental differences in combustion behaviour and peak radiative output. Fully charged cells were heated under constant power to induce thermal runaway, with and without an external ignition source. Three gas emission stages were identified: (1) steady emission post-venting, (2) increased gas production with self-heating, and (3) rapid gas release at thermal runaway. Forced ignition sustained flames in stages 1 and 3 for 18650 cells, whereas 26650 cells maintained combustion throughout. Peak radiative heat fluxes reached ∼150 kW/m2 for 18650 cells and 350–500 kW/m2 for 26650 cells. These findings quantify the dynamic flammability of off-gases and highlight the importance of spatially resolved measurements for fire hazard assessment.