Examining Safe Intervention Windows for 18650 and 21700 Lithium-Ion Batteries under Heat Interruption Experiments

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Abstract

This study experimentally quantifies the safe intervention window (the time between the earliest thermal runaway indicator and ignition) for cylindrical lithium-ion batteries exposed to constant external heating. A total of 77 experiments were performed on 18650 (nickel-cobalt-aluminum oxide, 3500 mAh) and 21700 (nickel-manganese-cobalt oxide, 5000 mAh) batteries using a 41 W heating pad with four state of charge (SOC) conditions. When heating was terminated after safety valve breakage, ignition was preventable only within approximately 195 s to 270 s for 18650 batteries, but just about 90 s and 40 s for 21700 batteries with 25 % and 50 % SOC, respectively. At higher SOCs (75 % and 100 %), ignition occurred even when heating was stopped within 10 s and some 21700 batteries exhibited delayed ignition after a temporary cooling period of about 200 s. A joint analysis of intervention time, cutoff temperature, and temperature-rate slope identified predictive thresholds that distinguish between no-ignition and delayed-ignition outcomes. Although this study applies only to single-cell configurations under natural convection, the results provide quantitative insights that link early-stage detection to actionable response timing, providing a quantitative framework for preventing ignition in lithium-ion batteries. Under continuous heating, the delay time between safety valve breakage (SVB) and ignition ranged from 228 s to 312 s and 66 s to 148 s for 18650 and 21700 batteries, respectively. Higher SOCs also yield faster transitions. When heating was intentionally stopped, ignition could be fully prevented only within 195 s to 270 s after SVB for 18650 batteries. Beyond this period, either delayed ignition or ignition occurred. In contrast, 21700 batteries exhibited significantly shorter safe intervention windows of about 90 s at 25 % SOC and 40 s at 50 % SOC. Ignition was unavoidable at 75 % and 100 % SOC even when heating was stopped within 10 s of SVB. Results further show that surface temperature at interruption and post-intervention temperature rate provide predictive thresholds for delayed ignition behavior. These findings provide quantitative insights into the timing of intervention before ignition, offering valuable data and insights for the design of early-warning systems, emergency shutdown algorithms, and thermal management controls.

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Keywords

Fire risk reduction, Fire fighting, Fire detection and Fire

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