Post-Fire Physical Features of Lithium-Ion Battery Cells

This article examines whether physical features observed on 18650 lithium-ion cells after a fire can establish that a cell started the fire. Controlled fire tests showed these features can also occur when cells are exposed to an external fire, so they are not reliable evidence of ignition causation on their own.

Read the full article, “The Implications of Post-Fire Physical Features of Cylindrical 18650 Lithium-Ion Battery Cells,” published in Fire Technology, for the complete analysis.

Abstract

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.