Lithium-ion battery fire risks are currently undermanaged in transit operations. At current fleet scales the magnitude of these risks is relatively small; however, there is no widespread understanding of how these risks will be magnified when fleet size increases. Lithium-ion battery fires, while less probable than fires due to combustion engines, are far more difficult to extinguish and may be many times more destructive and dangerous. Damaged cells in the battery can lead to thermal runaway, a phenomenon in which a failure in the architecture of a battery cell (e.g., a short) causes the heat of the battery to rapidly increase, releasing flammable gas, which then ignites, triggering similar events in adjacent cells. The ensuing fires are exceedingly difficult to extinguish and must be addressed with significant quantities of specialized fire suppressants. Moreover, there are several documented instances of stranded energy remaining after the fire is extinguished, causing batteries to reignite even after being initially suppressed by first responders.
The industry has largely addressed this risk by incorporating rigorous early detection and protection protocols in battery management systems that prevent thermal runaway when the battery pack is physically compromised through improper use or external impact. However, some fires still occur spontaneously and there is little consensus in the transit or Electric Vehicle (EV) communities regarding the best practices for managing this risk. Moreover, a greater risk of fire may arise when numerous electric buses are consolidated in a confined bus garage. In this scenario, the probability of a fire compounds with the introduction of additional buses as the resulting impact of such a fire would increase dramatically should a single bus fire spread to an adjacent vehicle. Furthermore, exogenous fires could initiate a fire in a nearby bus, turning a conventional fire into a lithium-ion battery fire of much greater severity.
With national news reporting on several instances of high-profile electric vehicle models catching fire in recent months, the risk of lithium-ion battery fires is a valid concern for transit agencies that are considering whether to electrify their bus fleets. At present, there is no comprehensive review of these risks, best practices for their mitigation on the part of fleet operators, strategies for suppressing and containing the spread of fires when they do arise, or policies for managing electric vehicles after the fire is extinguished to prevent reignition. Among the existing research, there has never been a focus on transit bus fleets or thermal events that occur within a bus maintenance or storage facility. Such a review would assuage agency concerns by objectively evaluating the risk of such fires, providing concrete steps to lessen this risk, and preparing transit agencies for the unlikely event that such a fire occurs on their premises. It would also help to lessen the risk of a high-profile lithium-ion fire that could take an incalculable toll on the industry-wide effort toward fleet electrification.
The objective of this research is to identify, evaluate, and summarize best practices for fire risk mitigation and suppression with regard to battery electric buses, with a particular focus given to agencies that store and charge their buses in indoor facilities, as well as the technical, economic, and institutional barriers to implementing these solutions. The anticipated products will be a guide to the potential origins of battery electric bus fires, an analysis of the potential of such fires to spread to other vehicles or reignite after suppression, an evaluation of various risk mitigation efforts, and recommended best practices for fire risk management.