EV Battery Fires and Thermal Runaway

EV battery fires present hazards that differ from conventional vehicle fires in ways that matter operationally. The energy source is different, the fire behavior is different, and the hazards do not necessarily end when visible flames are suppressed.

This article explains what thermal runaway is, how battery fires develop and behave, why reignition is a credible post-suppression hazard, and what first responders need to know to manage EV battery fire incidents safely. It is intended to complement Electric Vehicle Electrical Safety for First Responders and Recognizing High-Voltage Components in Electric Vehicles, which cover operational response principles, HV component recognition, and extrication considerations.

What Is Thermal Runaway?

Thermal runaway is a self-sustaining chain reaction within a lithium-ion battery cell in which heat generation exceeds the cell’s ability to dissipate it. As the cell overheats, internal chemical reactions accelerate, generating more heat, which drives further reactions — a cycle that can propagate to adjacent cells and ultimately to the entire battery pack.

The process can produce:

• Intense heat
• Flammable and toxic gases
• Fire
• Explosive venting

Thermal runaway is not unique to EVs — it can occur in any lithium-ion battery system. In an EV traction battery, however, the scale of the battery pack means that thermal runaway involving a significant portion of the pack can produce sustained, high-intensity fire that differs substantially from a conventional vehicle fire.

NTSB Safety Report SR-20-01 identifies thermal runaway as a primary hazard in EV battery fires and documents associated risks including electric shock, fire, reignition, and stranded energy remaining in the battery after apparent suppression.

What Triggers Thermal Runaway?

Thermal runaway in a traction battery may be triggered by:

• Physical damage — crush, penetration, or deformation of battery cells from collision impact
• Internal short circuit — cell failure from manufacturing defects, degradation, or damage
• Overcharging or charging system faults
• External heat — fire exposure from another part of the vehicle or an external source
• Electrical faults within the battery system

Critically, thermal runaway may not begin immediately after the triggering event. NHTSA notes that physical damage to the vehicle or HV battery may cause immediate or delayed toxic and flammable gas release and fire. A vehicle that appears stable after a collision may develop thermal runaway minutes, hours, or longer after the initial incident.

Cell Failure and Propagation

A lithium-ion traction battery pack contains many individual cells grouped into modules. When one cell enters thermal runaway, the heat it generates can raise the temperature of adjacent cells to the point where they also enter thermal runaway — a process called thermal propagation or cell-to-cell propagation.

Battery manufacturers design packs with thermal management systems and physical barriers intended to slow or limit propagation. The effectiveness of these measures varies by design and may be compromised by crash damage, fire exposure, or flooding.

From a responder perspective, the key implication is that thermal runaway involving one area of the battery pack does not necessarily remain confined to that area. A fire that appears localized may involve additional cells or modules as propagation continues — which is one reason why apparent suppression of visible flames does not establish that the battery is stable.

Crash Damage and Internal Battery Failure

Collision forces can damage traction battery cells and modules in ways that are not externally visible. A battery pack that shows no obvious external damage may have sustained internal cell damage sufficient to initiate thermal runaway.

Indicators that may suggest battery involvement after a collision include:

• Smoke from the vehicle or undercarriage
• Unusual odors — described variously as chemical, sweet, or burning
• Hissing, popping, crackling, or gurgling sounds from the battery area
• Heat from the undercarriage or floor
• Fluid leakage from the battery area
• Visible deformation of the undercarriage in the battery pack location

However — the absence of these indicators does not establish that the battery is undamaged or that thermal runaway will not develop. NHTSA’s guidance is explicit: physical damage may cause immediate or delayed hazard development. Responders should not clear a damaged EV based on the absence of immediate warning signs.

Stored and Stranded Energy

The traction battery in an electric or hybrid vehicle stores significant electrical energy by design. That energy does not dissipate when the vehicle is turned off, when the 12-volt system is disconnected, or when the vehicle is involved in a fire.

NTSB SR-20-01 specifically identifies stranded energy — electrical energy remaining in the battery after a fire or other incident — as a hazard for responders and recovery personnel. Even a battery that has been involved in fire may retain substantial stored energy, presenting both shock and reignition hazards.

Responders should not assume that a battery involved in fire has been discharged or rendered electrically inert. For a detailed explanation of HV shock hazards associated with damaged EV batteries, see Recognizing High-Voltage Components in Electric Vehicles.

Immediate vs. Delayed Fire Development

EV battery fires do not always develop immediately after the triggering event. This is one of the most operationally significant characteristics of EV battery incidents.

A vehicle involved in a collision may show no signs of fire at the scene — and develop thermal runaway and fire hours later, during towing, in storage, or at a salvage facility. NHTSA and NTSB documentation supports the possibility of delayed fire development following battery damage.

This has direct implications for:

• Scene release — a damaged EV that shows no fire at the scene should not be released to towing without communicating the possibility of delayed fire development
• Towing and transport — tow operators should be informed of EV involvement and potential delayed hazard
• Storage — damaged EVs may require specific storage conditions and separation from other vehicles
• Monitoring — continued observation of a damaged EV before and during towing and storage handoff

Battery Venting and Gas Release

As cells heat during thermal runaway, they may vent flammable and toxic gases before or during fire. Venting may produce:

• Hissing or high-pitched sounds
• White or gray smoke
• Chemical odors
• Visible vapor or mist

Gases released during battery venting may include hydrogen fluoride and other toxic compounds, as well as flammable gases that can ignite. Responders should treat smoke and vapor from a battery fire as potentially toxic and position personnel accordingly.

Venting may occur before visible fire develops, providing a warning — but venting can also occur rapidly and without extended warning. Responders should not assume that a venting battery will provide sufficient warning time before ignition or escalation.

Warning Signs — and Their Limitations

Responders should be alert to indicators of battery thermal involvement:

• Smoke from the vehicle, undercarriage, or wheel wells
• Unusual chemical, sweet, or burning odors
• Hissing, popping, crackling, or gurgling sounds
• Heat from the floor, undercarriage, or battery area
• Visible swelling or deformation of battery components
• Fluid leakage from the battery area
• Sparking or arcing

These indicators, when present, warrant immediate attention and conservative scene management.

However, their absence does not establish battery safety. Thermal runaway can develop without extended visible warning. A battery that appears undamaged and shows no warning signs may still be in the early stages of a process that will produce fire later.

Responders should not use the absence of warning signs as a basis for concluding that a damaged EV battery is safe.

High-Voltage Shock Hazards in Battery Fires

A burning or fire-damaged EV battery retains electrical hazards alongside thermal hazards. Fire damage may compromise insulation, expose conductors, and create new electrical fault paths — while the battery continues to store significant energy.

Responders working near a burning or fire-damaged EV should be aware that:

• Electrical shock hazards may be present in areas affected by fire
• Water used for suppression may contact energized components
• Fire-damaged orange cabling and HV warning labels may no longer be visible or intact
• The hazard area around a burning EV includes both thermal and electrical exposure zones

For arc flash hazard awareness relevant to damaged HV systems, see Arc Flash Awareness for First Responders. For broader guidance on PPE selection at electrical emergency scenes, see PPE for Electrical Emergencies.

Battery Fire Characteristics

EV traction battery fires have characteristics that differ from conventional vehicle fires in ways that are operationally relevant:

Heat output — Battery fires can produce intense, sustained heat. The energy stored in a large traction battery pack can sustain fire for extended periods.

Reignition potential — As documented by NTSB, EV batteries can reignite after apparent suppression. This is a defining characteristic of EV battery fires that distinguishes them from most conventional vehicle fires.

Toxic gas production — Battery fires produce toxic combustion products including hydrogen fluoride and other compounds. Respiratory protection is essential.

Suppression challenges — Conventional vehicle fire suppression approaches may not be sufficient to cool a burning battery pack to a stable temperature. The battery’s thermal management and physical structure affect how suppression agents reach burning cells.

Duration — EV battery fires may burn longer than conventional vehicle fires and may require sustained suppression efforts.

These characteristics do not mean EV battery fires cannot be managed — they mean that management requires awareness of how these fires behave and access to vehicle-specific guidance.

Fire Suppression

Fire suppression tactics for EV battery fires vary by vehicle, battery architecture, battery location, degree of involvement, available resources, and incident conditions. There is no single universal procedure that applies to all EV battery fires.

NHTSA and NTSB support the use of manufacturer-specific guidance. Some authoritative sources and manufacturer ERGs indicate that large, sustained water application may be needed to cool lithium-ion battery fires — but the specific approach, including application method and duration, depends on the vehicle and situation.

Responders should:

• Consult the manufacturer ERG or rescue sheet for vehicle-specific fire response guidance whenever available
• Follow department SOPs and incident commander direction
• Not assume that tactics effective for one EV model will be equally effective for another
• Maintain awareness that suppression of visible flames does not establish that the battery is stable

Tactics that involve penetrating the vehicle structure or battery enclosure to apply suppression agents directly to battery cells have been discussed in some manufacturer and fire-service guidance. Whether such tactics are appropriate depends on the specific vehicle, the manufacturer’s guidance for that vehicle, department SOPs, and incident conditions. Responders should not improvise penetration tactics without vehicle-specific authoritative support and department authorization.

Reignition After Apparent Suppression

Reignition is one of the most significant operational hazards in EV battery fire response. NTSB Safety Report SR-20-01 documents instances of EV batteries reigniting after initial fire suppression — in some cases after the vehicle had been moved or was in storage.

Why Reignition Occurs

When visible flames are suppressed, the battery cells may still contain stored energy and may still be at elevated temperatures. Cells that have not yet entered thermal runaway may do so as heat continues to transfer through the pack. The suppression of surface fire does not necessarily cool the interior of the battery pack to a stable temperature.

The result is that a battery that appears extinguished may reignite — sometimes after a significant interval.

Operational Implications

• Apparent extinguishment is not confirmation of battery stability. Continued monitoring is required.
• Scene release should not occur based solely on the absence of visible fire after suppression.
• Towing and storage personnel must be informed of reignition risk before the vehicle is released.
• Monitoring requirements and duration vary by vehicle, battery condition, and incident circumstances. There is no universal monitoring period that applies to all EV battery fires. Consult the manufacturer ERG and department SOPs.
• Storage location for a post-fire EV should account for reignition risk. Vehicle-specific and department guidance applies.

The transfer of reignition risk information to towing, transport, and storage personnel is an operational responsibility that belongs to incident command — not an optional step.

Damaged EVs With No Active Fire

A damaged EV that shows no active fire is not necessarily safe. This is a distinct scenario that requires its own operational awareness.

Responders should not assume:

• No smoke means no battery damage
• No fire means no thermal-runaway risk
• Vehicle shutdown removes stored energy from the traction battery
• Absence of visible battery damage establishes that the battery was not involved
• A stable-appearing vehicle will remain stable

A battery that has sustained internal damage from a collision may be in the early stages of thermal runaway without producing visible external signs. NHTSA’s guidance supports the possibility of delayed hazard development after physical damage.

For damaged EVs with no active fire, responders should:

• Treat the vehicle as a potential thermal runaway hazard
• Maintain appropriate distances and hazard zones
• Consult the manufacturer ERG for the specific vehicle
• Monitor for developing warning signs — smoke, odors, sounds, heat
• Communicate battery damage potential to towing and storage personnel before vehicle release
• Follow department SOPs for damaged EV handling

The absence of fire does not eliminate the need for careful scene management and handoff.

Flooded and Submerged EVs

Submersion does not de-energize the traction battery. A flooded or submerged EV retains stored electrical energy and may present both shock and thermal runaway hazards.

Water intrusion may damage battery cell insulation and internal components, potentially creating conditions that increase thermal runaway risk — particularly after the vehicle is removed from water and conditions change. The probability and timing of thermal runaway following submersion depend on the specific battery design, the extent of water intrusion, and other factors that are not externally assessable.

Responders should not overstate the certainty of thermal runaway following submersion, but should treat a flooded EV as a potential HV and fire hazard and communicate submersion history to towing and storage personnel.

For broader guidance on electrical hazards in flooded environments, see Electrical Hazards in Flooded Buildings. For post-disaster scenes where EV submersion may occur alongside other electrical hazards, see Electrical Safety Following Storm and Disaster Damage.

Extrication When Battery Damage Is Suspected

When battery damage is suspected — based on collision severity, battery location relative to impact, or warning signs — extrication operations should account for the possibility of thermal runaway development during or after the operation.

Key considerations:

• Retrieve the manufacturer rescue sheet for the specific vehicle before extrication begins whenever possible
• Identify battery pack location relative to the impact and extrication work areas
• Avoid cutting, spreading, or penetrating areas where the battery pack is located unless the rescue sheet identifies those areas as safe
• Brief all extrication personnel on battery location and thermal runaway risk
• Maintain monitoring for warning signs throughout extrication operations
• Have suppression resources positioned and ready

Extrication urgency must be weighed against the hazards present. Incident command is responsible for that assessment based on conditions, available information, and department SOPs.

For detailed guidance on HV component locations and extrication implications, see Recognizing High-Voltage Components in Electric Vehicles.

Scene Control and Personnel Positioning

EV battery fire scenes require conservative personnel positioning that accounts for both thermal and electrical hazards.

Scene control considerations include:

• Establish and maintain hazard zones that reflect the possibility of reignition, venting, and electrical exposure — not just the current fire perimeter
• Limit personnel in the immediate area of the burning or fire-damaged vehicle to those with a specific operational role
• Position apparatus to allow rapid repositioning if conditions change
• Account for toxic gas production — respiratory protection and upwind positioning where possible
• Communicate EV involvement to all incoming units, mutual aid, and incident command as early as possible
• Brief arriving personnel on battery location, fire status, reignition risk, and HV hazards

For broader electrical scene control principles applicable to EV incidents, see Electrical Hazards During Emergency Response and the Complete Guide to First Responder Electrical Safety.

Manufacturer ERGs and Rescue Sheets

NHTSA maintains a publicly accessible database of manufacturer Emergency Response Guides and rescue sheets. These documents contain vehicle-specific fire response guidance that cannot be reliably generalized across makes and models.

For EV battery fire incidents, the manufacturer ERG may contain:

• Vehicle-specific fire suppression guidance
• Battery location and approximate dimensions
• Information on suppression agent application
• Post-fire monitoring guidance
• Towing and storage requirements specific to the vehicle

Retrieving the ERG for the specific vehicle — by make, model, and year — should be an early incident command priority. A rescue sheet or ERG for a similar model should not be substituted for the correct document, as fire response guidance may differ between models and model years.

Incident Command Considerations

Incident command at an EV battery fire has responsibilities that extend beyond the active fire phase:

• Assign ERG/rescue sheet retrieval as an early priority
• Communicate EV involvement and battery fire status to all sectors, mutual aid, and arriving units
• Establish hazard zones that account for reignition, venting, and electrical exposure
• Maintain accountability for all personnel operating near the burning or fire-damaged vehicle
• Plan for extended operations — EV battery fires may require sustained suppression and monitoring
• Coordinate scene release — do not release the scene based solely on apparent suppression; confirm monitoring plan and handoff
• Execute towing and storage handoff — ensure reignition risk, battery condition, and vehicle-specific handling requirements are communicated to recovery personnel before vehicle release
• Document and communicate battery fire history for towing, transport, and storage personnel

Towing, Recovery, and Storage

The hazards associated with an EV battery fire do not end when suppression operations conclude. Stranded energy, reignition risk, and toxic residue transfer with the vehicle.

Before releasing a post-fire EV to towing or recovery personnel, incident command should communicate:

• That the vehicle is an EV, HEV, or PHEV
• That the battery was involved in fire
• That reignition is a credible post-suppression hazard
• The vehicle-specific towing and storage requirements from the manufacturer ERG
• Any specific observations about battery condition, suppression history, and warning signs

Towing and storage of post-fire EVs may require specific procedures — including restrictions on transport method, storage location, and proximity to other vehicles or structures. These requirements vary by manufacturer and vehicle condition. Tow operators should be directed to the manufacturer ERG and their own applicable guidance.

Do not assume a tow operator is aware of EV battery fire hazards or reignition risk unless that information has been explicitly communicated.

What Responders Should Not Do

• Do not assume the battery is safe because visible flames are suppressed. Stranded energy and reignition risk persist after apparent extinguishment.
• Do not assume no smoke means no thermal runaway. Thermal runaway can develop without extended visible warning.
• Do not assume the vehicle is electrically safe because it is on fire or has been in a fire. Fire-damaged batteries retain stored energy.
• Do not open, dismantle, drill, or penetrate the traction battery unless explicitly supported by vehicle-specific authoritative guidance and department SOPs.
• Do not cut HV cables.
• Do not attempt to discharge the traction battery.
• Do not open HV junction boxes, inverter housings, or battery enclosures.
• Do not treat 1000V-rated insulated hand tools as authorization for traction battery service.
• Do not release a post-fire EV to towing without communicating reignition risk and battery condition.
• Do not apply one vehicle’s suppression or monitoring procedure to a different make or model.

Dangerous Assumptions

Summary

EV battery fires and thermal runaway present hazards that require specific operational awareness:

• Thermal runaway is a self-sustaining process that can propagate through a battery pack and may develop immediately or with significant delay after the triggering event
• Apparent suppression of visible flames does not establish battery stability — stranded energy and reignition risk persist
• Damaged EVs with no active fire are not necessarily safe — thermal runaway may develop after the initial incident
• Fire suppression tactics vary by vehicle — consult the manufacturer ERG and follow department SOPs
• Reignition risk and battery condition must be communicated to towing, transport, and storage personnel before vehicle release
• Vehicle-specific guidance from the manufacturer ERG is the most reliable source of fire response information for the specific vehicle involved

Related Resources

• Electric Vehicle Electrical Safety for First Responders
• Recognizing High-Voltage Components in Electric Vehicles
• Arc Flash Awareness for First Responders
• PPE for Electrical Emergencies
• Electrical Hazards in Flooded Buildings
• Complete Guide to First Responder Electrical Safety