You've probably seen the headlines: an electric vehicle bursting into flames in a parking garage, or an e-bike igniting while charging overnight—like the tragic Reims apartment fire that claimed four lives. Behind these dramatic incidents lies a chemical process called thermal runaway—a chain reaction that can turn a battery into an inferno in seconds.
Understanding thermal runaway isn't just for engineers and firefighters. If you own an EV, e-bike, laptop, or even a phone, knowing how this phenomenon works can help you recognize warning signs and respond safely when things go wrong.
What Is Thermal Runaway?
Thermal runaway is a self-accelerating process where rising temperature triggers reactions that generate even more heat. In lithium-ion batteries, this creates a dangerous feedback loop: heat damages the battery's internal structure, which releases more heat, which causes more damage—and the cycle continues until the battery fails catastrophically.
Think of it like a snowball rolling downhill. A small disturbance at the top becomes an unstoppable force by the time it reaches the bottom. Except instead of snow, we're talking about temperatures that can exceed 600°C (1,112°F) and toxic gases that can ignite explosively.
According to SAE International standards, thermal runaway is defined as a condition where the battery's internal temperature increases uncontrollably due to heat generation exceeding heat dissipation. Once initiated, the process becomes self-sustaining and extremely difficult to stop.
How Thermal Runaway Happens: The 3 Stages
Thermal runaway doesn't happen instantaneously. It progresses through three distinct stages, each more dangerous than the last.
Stage 1: Onset (70-150°C / 158-302°F)
The process begins when something causes a battery cell to heat beyond its normal operating range. Common triggers include:
- Physical damage from a crash or impact
- Internal short circuits from manufacturing defects
- Overcharging beyond safe voltage limits
- External heat exposure (hot car interiors, direct sunlight)
- Age-related degradation of internal components
At this stage, the solid electrolyte interface (SEI)—a protective layer inside the battery—begins breaking down. This breakdown generates heat and releases gases, but the reaction can still potentially be stopped if the heat source is removed.
Stage 2: Acceleration (150-250°C / 302-482°F)
As temperatures climb past 150°C (302°F), the separator between the battery's electrodes begins melting. This thin plastic film normally prevents the positive and negative sides from touching. When it fails, internal short circuits form, dumping massive amounts of energy into heat.
The electrolyte—the liquid that allows ions to move inside the battery—starts decomposing and vaporizing. These vapors are highly flammable. Pressure builds inside the cell as gases accumulate.
Research from UL's Electrochemical Safety program shows that at this stage, temperatures can rise at rates exceeding 10°C per second. The reaction becomes self-sustaining—even removing the original heat source won't stop it.
Stage 3: Propagation (250°C+ / 482°F+)
This is the catastrophic phase. The battery cell ruptures, venting flammable gases that ignite on contact with air. Temperatures can spike to 600°C (1,112°F) or higher within seconds.
In multi-cell battery packs—like those in EVs, which can contain thousands of cells—heat from one failing cell spreads to neighbors. This cell-to-cell propagation can create a cascade effect, with cells failing in rapid succession like dominoes.
The National Fire Protection Association (NFPA) notes that EV battery fires can burn for hours and require 10,000 to 40,000 gallons of water to extinguish—compared to about 1,000 gallons for a typical gasoline car fire.
Thermal Runaway in Electric Vehicles
EV batteries present unique challenges compared to smaller lithium-ion devices. A typical EV battery pack contains 1,000 to 8,000 individual cells, stores enough energy to power a house for several days, and weighs 400-1,000 pounds.
This massive energy density means that when things go wrong, they go very wrong. But it's important to maintain perspective: NHTSA data shows EVs catch fire at lower rates than gasoline vehicles. The difference is in how those fires behave once they start.
Common Causes of EV Thermal Runaway
Crash damage: High-speed impacts can penetrate battery enclosures or crush individual cells, triggering immediate or delayed thermal runaway. Some incidents have occurred hours or even days after the initial accident.
Manufacturing defects: Microscopic metal particles contaminating battery cells during production can create internal shorts. Several major recalls have addressed this issue.
Charging issues: Using non-approved chargers, charging in extreme temperatures, or system failures that allow overcharging can all initiate thermal runaway.
Battery degradation: As batteries age, internal resistance increases and dendrites (metallic growths) can form, increasing thermal runaway risk.
For a comprehensive overview of EV fire safety and containment strategies, see our EV Fire Blanket Guide.
Why EV Battery Fires Are So Dangerous
EV fires present challenges that make them fundamentally different from traditional vehicle fires.
Reignition Risk
Perhaps the most alarming characteristic of battery fires is their tendency to reignite. Even after flames are extinguished, damaged cells can continue heating and re-enter thermal runaway hours or days later. Fire departments now routinely monitor EV fire scenes for 24-48 hours after initial suppression.
Toxic Emissions
Burning lithium-ion batteries release a cocktail of toxic gases including hydrogen fluoride, carbon monoxide, and hydrogen cyanide. These emissions require firefighters to use self-contained breathing apparatus and create hazardous zones around burning vehicles.
Water Requirements
While water can cool battery fires and slow propagation, it cannot extinguish the electrochemical reactions occurring inside cells. NFPA guidelines recommend massive water application—often 10 times what's needed for a gasoline fire—to cool the battery pack and prevent cell-to-cell spread.
Suppression Challenges
Traditional fire suppression methods have limited effectiveness against thermal runaway. The battery generates its own oxygen through chemical decomposition, making smothering techniques ineffective. This is why specialized EV fire blankets focus on containment rather than complete suppression.
Signs of Thermal Runaway Starting
Recognizing early warning signs can provide critical time to respond safely. Watch for:
- Hissing or popping sounds: Gas venting from cells as pressure builds
- Unusual odors: A sweet, chemical smell (often described as similar to nail polish remover) indicates electrolyte decomposition
- Smoke or vapor: Any visible emissions from a battery pack require immediate action
- Battery swelling: Bulging or deformation of the battery case signals internal gas buildup
- Rapid temperature rise: If a battery feels hot to the touch during normal operation or charging, disconnect immediately
- Error messages: Battery management system warnings about temperature or cell imbalance
If you notice any of these signs, don't wait. Move away from the vehicle or device, get others to safety, and call emergency services.
How to Respond to EV Thermal Runaway
If you witness a battery fire beginning, your priority is safety—not saving the vehicle.
Immediate Actions
- Evacuate immediately: Move at least 30 meters (100 feet) away from the vehicle. Battery fires can produce explosive gas releases.
- Call 911: Specifically mention it's an EV or lithium battery fire. This information helps dispatch appropriate resources.
- Warn others: Alert anyone nearby to move away from the area.
- Stay upwind: Position yourself where smoke and fumes blow away from you.
Containment Options
For smaller lithium battery devices (e-bikes, scooters, power tools), a high-temperature fire blanket can help contain the fire and slow propagation. For step-by-step guidance by device type, see our guide on how to put out a lithium battery fire. For vehicles, specialized EV fire blankets exist that can limit flame spread while firefighters respond.
Do not attempt to extinguish an EV fire with a standard fire extinguisher—it won't work and puts you in danger. Water can help if applied in large quantities from a safe distance, but household amounts are insufficient.
What NOT to Do
- Don't try to open the hood or access the battery compartment
- Don't assume the fire is out just because flames aren't visible
- Don't stay near the vehicle to record video
- Don't attempt to drive or move a vehicle showing thermal runaway signs
For detailed guidance on fire blanket use and emergency response, visit our Fire Blanket Guide.
Preventing Thermal Runaway
While thermal runaway can't be completely eliminated, proper practices significantly reduce risk.
Charging Best Practices
- Use only manufacturer-approved charging equipment
- Avoid charging in extreme temperatures (below 0°C/32°F or above 35°C/95°F)
- Don't charge to 100% for daily use; 80% is optimal for battery health
- Unplug when charging is complete rather than leaving connected indefinitely
- Charge in well-ventilated areas away from flammable materials
Storage Considerations
- Park EVs away from structures when possible, especially in enclosed garages
- Maintain batteries at 40-60% charge for long-term storage
- Keep vehicles in temperature-controlled environments when feasible
- Inspect batteries regularly for damage or swelling
Maintenance Awareness
- Follow manufacturer service schedules
- Address battery warning lights immediately
- Have batteries inspected after any significant impact
- Replace batteries that show signs of degradation
For home fire safety equipment, check out our FAQs on fire blanket selection and maintenance.
The Future of Battery Safety
Battery technology continues advancing, with new designs specifically targeting thermal runaway prevention.
Solid-state batteries replace liquid electrolytes with solid materials, eliminating the flammable component that contributes to thermal runaway severity. Several manufacturers expect commercial solid-state EVs by 2028.
Improved battery management systems now monitor individual cell temperatures and can isolate failing cells before propagation occurs.
Cell-to-cell barriers in modern battery packs use fire-resistant materials to slow thermal propagation, providing more time for evacuation and emergency response.
Until these technologies mature, understanding thermal runaway and having appropriate safety equipment remains essential for anyone using lithium-ion battery technology.
Want the complete picture?
Read our comprehensive EV Fire Blanket Guide
