Effect of trigger cell position on thermal runaway propagation in lithium-ion battery modules within branched tunnels

Lithium-ion battery (LIB) fires in branched tunnels present complex safety challenges due to confined geometry and limited ventilation, yet their thermal runaway (TR) behaviour remains insufficiently understood. This study investigates the influence of state of charge (SOC) and trigger cell position on TR characteristics of lithium iron phosphate (LFP) cells and battery modules under tunnel-like conditions. A series of controlled experiments was conducted in a scaled branched tunnel using single LFP cell and battery module arranged in 3 × 3 cells. TR was initiated at different SOC levels and module positions (#5, #7, #8), while ceiling temperature and radiation heat flux were monitored using thermocouples and thermal radiometer. Results reveal distinct TR dynamics that the single-cell TR exhibits a single peak temperature stage, whereas module TR produces multiple peaks accompanied by intermittent jet flames, significantly prolonging event duration. Maximum ceiling temperature for modules exceeds that of single cells, with deeper trigger positions amplifying thermal severity. SOC strongly influences thermal response, with ceiling temperature growth rates reaching 94% for single-cell TR and 44% for module TR. Radiation heat flux increases with SOC and is highest when the trigger cell is located deeper within the module. A predictive model for maximum ceiling temperature and longitudinal temperature decay is proposed for both single-cell and module TR scenarios. These findings enhance understanding of LIB fire behaviour in complex tunnel environments and provide actionable insights for tunnel ventilation design, emergency response planning, and battery safety standards.