The Implications of Post-Fire Physical Features of Cylindrical 18650 Lithium-Ion Battery Cells

摘要

Fire investigators are trained to apply the scientific method to determine the origin and cause of a fire. They look for patterns that indicate the sequence of involvement of available fuel loads, including considering whether a given fuel load contains enough energy to ignite other fuels. It is common knowledge that cylindrical 18650 lithium-ion (Li-ion) battery cells contain significant electrochemical energy and that they have the potential to fail and cause fires, so they are often considered a potential ignition source. The presence of a damaged 18650 cell at a fire scene poses a challenge to fire investigators because regardless of whether the cell is the cause or a victim of the fire, its stored energy can be released energetically, leaving a burn pattern and rapidly involving other fuel loads. It is therefore desirable to identify post-fire physical features on 18650 Li-ion cells that indicate whether they were the cause or a victim of a fire. This work shows that several features have been incorrectly identified in previous investigations. Expulsion of cell contents, crimp deformation, a flat negative terminal, localized damage in the electrode windings, and a hole in the metal casing have been cited as indications that the cell failed and caused a fire. To test these hypotheses, 18650 Li-ion cells at various states of charge (SOC) were burned in a controlled and repeatable manner. Temperatures were recorded and the experiments were documented with still-photography and video. The post-fire condition of each cell was then characterized with radiography (X-ray), computed tomography, and optical imaging. Each of the post-fire physical features in question occurred in non-defective cells that were victims of controlled fires, thereby demonstrating that these features are not valid indicators of fire causation.