Why a damaged EV battery can reignite after a crash: thermal runaway and stranded energy
Learn why a battery that looks safe can flare up hours or days later
Safety Behind the Wheel Foundation built this guide from a National Transportation Safety Board (NTSB) safety report, National Highway Traffic Safety Administration (NHTSA) guidance and federal safety standards, and published research on lithium ion battery fires. Every source is listed at the end.
Why “the fire is out” is not always the end
After a collision, most people expect a car fire to work like a campfire. Once the flames are gone and the car is cool to the touch, the danger seems to be over. A damaged electric or hybrid battery does not always follow that pattern.
The National Transportation Safety Board (NTSB), the federal agency that investigates major accidents, studied several electric vehicle fires. It found that a damaged high voltage battery may flare up again, sometimes several times, even after firefighters have put out the flames [1]. The National Highway Traffic Safety Administration (NHTSA) adds a warning. Physical damage to a car or its battery can lead to fire or to toxic or flammable gases, either right away or later [2].
This article explains why. Knowing the reasons makes the strict handling rules easier to understand and helps you ask better questions. If you are recovering from a car accident, this may feel like one more worry. The goal is not to alarm you. It is to help you keep your family and home safe while experts deal with the battery.
Inside a battery cell: the parts that matter
An electric vehicle battery pack is made of many small cells. Each cell is a sealed package with a few key parts:
- A negative electrode and a positive electrode, which store and release lithium.
- A separator, a thin film that keeps the two electrodes from touching.
- An electrolyte, a liquid that lets lithium move between the electrodes. It contains solvents that can give off flammable vapors [1].
Trouble starts when a cell is damaged or overheated. The NTSB notes that a short circuit inside a cell can come from a factory defect or from crushing or puncturing [1]. A short circuit is an unplanned path for electric current. It turns stored energy into heat in a very small space.
Researchers group the triggers for thermal runaway into three kinds of abuse: thermal, mechanical and electrical [3]. A collision can deliver all three at once.
Comparison compiled by Safety Behind the Wheel Foundation
Three kinds of battery abuse in a collision
| Type of abuse | How it can happen in a collision |
|---|---|
| Mechanical | The pack is dented, crushed or pierced by the impact or road debris |
| Electrical | Damaged parts create a short circuit inside or between cells |
| Thermal | Heat from a nearby fire or a failing cell warms healthy cells |
No battery type is fully immune. A 2026 review found that every cell design and chemistry carries some risk of thermal failure, most of all under extreme heat, electrical strain or physical stress [4].
Thermal runaway: heat that feeds itself
Thermal runaway is the core reason a damaged battery can flare up again. The NTSB describes it as a reaction that gives off heat. That heat makes the reaction run faster, which makes still more heat [1]. It is a loop that can keep going on its own.
Researchers describe thermal runaway as a chain of stages [5]:
- The thin protective layer where the negative electrode meets the electrolyte starts to break down.
- The separator shrinks, which can let the two electrodes touch and short out.
- The electrolyte itself breaks down.
- The positive electrode releases oxygen.
Each stage feeds the next, like links in a chain [1, 5].
Here is the key point for families. A damaged battery does not always fail right away. In the NTSB’s cases, batteries flared up again at tow yards and, in one case, days after the collision [1]. NHTSA likewise warns that fire and gas release can be delayed [2].
How one hot cell can spread through a pack
In one vehicle the NTSB studied, the battery held 16 modules with 515 cells in each, more than 8,000 cells in all [1]. The NTSB compares the spread of thermal runaway from cell to cell to a row of falling dominoes [1].
A 2026 engineering review explains how that spread happens [6]. Heat moves through direct contact between cells. It also travels through hot gases moving inside the pack and through radiant heat. Venting gases and flames carry it further still. As the review puts it in technical terms, one failing cell can soon put the whole pack at risk [6].
The pack’s housing matters too. Housings are designed for crash protection, temperature control and fire resistance [7]. The NTSB notes that the battery sits in a case built to resist outside force, placed where a collision is less likely to reach it [1]. That protection works well in normal driving. In a fire, it can make the burning cells hard to reach. Putting out a burning lithium ion battery can take thousands of gallons of water [1].
Stranded energy: the charge with nowhere to go
Stranded energy is the second big reason a damaged pack stays risky. The NTSB explains that when a pack is damaged, the cells and modules that survive intact still hold their charge, and that energy has no way to drain [1].
Federal rules require electric vehicles to protect people from high voltage shock after a crash test, for example by isolating the system [8]. That protection deals with the wiring. It does not empty the cells. Responders can cut special “cut loops” to isolate the battery from the rest of the car. The NTSB points out that this keeps power inside the pack but does not remove it [1].
The NTSB found that this leftover energy creates a risk of electric shock and of reignition and fire [1]. Draining it is not simple:
- If the battery management system, the computer that monitors the pack, still works, specialists can use it to check how much voltage remains [1].
- Some packs have built in discharge ports for specialists. Automakers also make draining tools, but these usually fit one model and work only on an intact pack [1].
- One way to discharge a damaged battery is to sink it in a saltwater bath, because salt water conducts electricity [1].
- At an accident scene, responders have no way to measure the leftover energy. They also cannot drain it, except by slow methods such as letting a battery fire burn itself out [1].
None of these are owner tasks. Until a qualified person has assessed the pack, treat a damaged battery as if it still holds a full charge.
What NTSB investigations found about reignition
The NTSB report describes several collisions that show how this plays out.
Comparison compiled by Safety Behind the Wheel Foundation
NTSB cases where damaged batteries reignited
| Case | What happened after the first fire |
|---|---|
| Lake Forest, California, 2017 | Fire under the car seemed to be out but kept coming back. It flared after the car was pulled onto a driveway and again, for a few seconds, as it was unloaded at the tow yard. Crews used over 20,000 gallons of water across at least 2 hours [1]. |
| Mountain View, California, 2018 | About 20 minutes after the wreck reached the impound yard, an officer heard popping sounds. Less than an hour later, fire crews were called back because the battery had reignited. Five days after the crash, it caught fire yet again [1]. |
| Fort Lauderdale, Florida, 2018 | A chain dragged across the battery case during loading set off a short flare up. Roughly 3 hours after the crash, the case and modules sparked and smoked as they were unloaded at the tow yard [1]. |
These cases teach several lessons. Reignition can happen at the scene, during towing, at the yard, or days later. Moving a damaged pack can set it off. And the people at risk are not only firefighters. In one case, a tow truck driver’s arms were burned, though not badly, as he lowered the burning car from his flatbed. The truck’s controls sat close to the fire [1].
Gases, smoke and residue: the hazards you cannot see
Flames are the most visible danger, but they are not the only one. Research cited by the NTSB found that the flammable gases a damaged battery gives off, such as hydrogen, ethylene, ethane and propane, are the largest fire threat [1]. Damaged battery modules can also vent gas without ever bursting into flames [9].
Some battery gases are also toxic. In lab fire tests of seven types of commercial lithium ion batteries, researchers found the fires can produce a lot of hydrogen fluoride, a toxic gas [10]. They noted that the finding matters most for large battery packs [10]. The NTSB report also lists, among known hazards, the chance that toxic battery gases could overcome people inside the vehicle [1].
The hazards can outlast the fire. Firefighting runoff at one crash scene was tested and found to be toxic [1]. A long term lab study found that even battery modules that vented without igniting released significant pollutants into water [9]. Another lab study found that residue left after thermal runaway kept giving off low levels of chemical vapors. Fabric exposed to those vapors released them again later [11].
For families, this means a few practical things:
- Stay out of smoke from a damaged electric or hybrid vehicle, even if you see no flames.
- Do not touch burned battery debris, fluid or runoff.
- Let trained workers retrieve belongings from a burned or badly damaged car when the yard says it is safe.
- Keep smoky clothing and items out of closed living spaces until they are cleaned.
- If you breathed smoke and feel unwell, seek medical care and tell the provider it came from a battery fire.
Why damaged EV handling rules look so strict
Drive With Care’s guide to what to do with an electric vehicle after a serious crash covers the steps. The table below explains the science behind them.
Comparison compiled by Safety Behind the Wheel Foundation
Handling rules for damaged EVs and the science behind them
| Common handling rule | The battery science behind it |
|---|---|
| Park it outdoors, at least 50 feet from buildings, vehicles and anything that burns [2] | A delayed fire or gas release can start with no warning, so space limits what it can reach |
| Never park it in a home garage [2] | Gases can build up indoors, and a fire there can spread to the house |
| Do not charge or drive it until cleared | Using the pack sends current through cells that may already be damaged |
| Move it as little as possible | In one NTSB case, a chain passing over the case set off a brief reignition [1] |
| Get a trained high voltage technician | Responders at the scene have no way to measure how much energy is left in a damaged pack [1] |
The NTSB found that keeping a 50 foot clear area around a damaged car may not be practical at some tow or storage yards [1]. In one case, the yard was too small, so workers kept the car as far as they could from things that looked flammable [1]. That is a good reason to ask, politely and in writing, where your car is being kept.
What safety standards check, and what is changing
Federal Motor Vehicle Safety Standards (FMVSS) set minimum safety rules for new vehicles. The long standing electric vehicle rule, FMVSS 305, covers three things in a crash: limiting electrolyte spills, keeping the battery in place and preventing electric shock [8]. Spills are checked until 30 minutes after the crash test [8].
In December 2024, NHTSA adopted a new standard, FMVSS 305a, to replace FMVSS 305 [12]. Its stated purpose adds fire, explosion and gas venting to the hazards it addresses [13]. Under the new standard [13]:
- There must be no evidence of fire or explosion from the moment of impact until one hour after the crash test sequence ends.
- Vehicles must warn the driver of a thermal event in the battery within three minutes of its start, with sound and visual signals.
- The rules become mandatory on September 1, 2027, for vehicles with a gross vehicle weight rating of 4,536 kilograms (about 10,000 pounds) or less. Heavier vehicles follow a year later.
NHTSA also created Part 561, which requires automakers to submit standard emergency information for responders [12]. For vehicles built on or after December 22, 2025, automakers must provide rescue sheets and emergency response guides before the first sale [14]. These guides must cover topics such as vehicle fires, towing, and transport and storage [14]. The NTSB had recommended that such guides include details on reignition, stranded energy and safe storage [1].
These are real steps forward. Still, a one hour test cannot rule out every late flare up, and one real world battery reignited five days after a crash [1]. A warning helps a driver get out in time. It does not make a damaged pack safe to keep near a home.
Questions to ask while your car is out of your hands
A damaged car may pass from a tow operator to a storage yard, an adjuster and a repair shop. Battery information can get lost at each handoff. Clear questions help keep everyone safe.
Keep notes of who you spoke with and what they said. Anyone who notices smoke, hissing, popping or a chemical smell from the car should move away and call 911. Drive With Care’s guide to what to do before your wrecked car leaves your hands can help you plan what to collect once the car is safe.
Frequently asked questions
Can a battery catch fire later even if the car never burned in the collision?
Yes. NHTSA warns that a damaged car or battery can give off toxic or flammable gases, or catch fire, some time after the damage occurs [2]. A crushed cell can develop an internal short circuit, which makes heat and can start thermal runaway [1, 5]. A car that looks fine still needs a battery check if the underside or battery area was hit.
How long after a crash can a damaged battery reignite?
There is no simple countdown. In the NTSB’s investigations, batteries reignited at the scene, during towing, at tow yards and, in one case, 5 days after the crash [1]. Because timing is so uncertain, keep the car outdoors and away from buildings until a trained technician has assessed the battery [2]. Safety Behind the Wheel Foundation gives families the same advice.
Does turning the car off or the automatic shutoff make the battery safe?
No. Shutting the car off and the built in high voltage disconnects isolate the cables. They do not drain the cells. The NTSB notes that even cutting a car’s high voltage cut loops keeps energy inside the battery without removing it [1]. The undamaged cells keep their charge, and that stranded energy can still cause shock and reignition [1]. Treat a damaged pack as if it were fully charged.
Why can’t firefighters just use more water?
The cells sit inside a sturdy case designed to resist damage [1, 7]. That protection can also make burning cells hard to reach. In one NTSB case, crews used more than 20,000 gallons, and the fire under the car kept coming back [1]. The emergency guide for one vehicle in the NTSB report tells responders to consider letting the battery burn while protecting nearby property [1]. In another case, crews watched the wreck with thermal cameras, which show heat the eye cannot see, instead of spraying it [1]. These are decisions for trained responders.
Do hybrids and plug in hybrids carry the same risk?
Any vehicle with a damaged lithium ion battery needs the same care. NHTSA’s storage advice applies to damaged vehicles with lithium ion batteries, and its guidance covers both hybrid and electric vehicles [2]. Your owner’s manual will tell you what kind of battery your vehicle uses. When in doubt, tell the tow operator and shop that the car has a high voltage battery.
Will newer cars warn me if the battery starts to fail?
For most passenger vehicles built on or after September 1, 2027, the new federal standard requires a driver warning within three minutes after a battery thermal event begins [13]. The warning must use both sound and visual signals [13]. If you get such a warning, or notice smoke or unusual smells, stop safely, get everyone out and move well away before calling 911.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation urges families to treat any damaged lithium ion battery as live and to keep the car outdoors and away from the home until a qualified technician clears it. A damaged lithium ion battery can hold heat, chemical energy and electrical charge long after a collision. Thermal runaway can keep building inside a cell that looks fine from the outside. Stranded energy keeps that cell’s fuel in place. Together, they explain why a battery can flare up during towing, at a storage yard or days later.
You do not need to become a battery expert. Just remember that a fire that looks out, or a car that is turned off, does not mean the battery is safe. Keep the car outdoors and away from your home, let trained people handle the pack, and ask clear questions until a qualified technician has cleared it.
Related articles
- What to do with an electric vehicle after a serious crash
- Electric and hybrid vehicle safety: a practical guide for families
- Towing, storage fees and getting around after a serious crash
- What to do before your wrecked car leaves your hands
- What to know about car repairs and total loss after a serious crash
- Buying a used EV or hybrid with a past collision: why the battery needs its own inspection
Sources and further reading
- 1National Transportation Safety Board. Safety Risks to Emergency Responders from Lithium-Ion Battery Fires in Electric Vehicles (Safety Report NTSB/SR-20/01). Adopted November 13, 2020. Data year: 2017 to 2018 (crashes investigated). ntsb.gov/…/SR2001.pdf
- 2National Highway Traffic Safety Administration. Electric and Hybrid Vehicles: Battery, Charging & Safety. Accessed October 2026. nhtsa.gov/…/electric-and-hybrid-vehicles
- 3Yuan L, Tang W, Thomas RA, Soles J (National Institute for Occupational Safety and Health). Experimental Study on Suppression of Lithium Iron Phosphate Battery Fires. Mining, Metallurgy & Exploration (indexed in PubMed). Published February 2024. pubmed.ncbi.nlm.nih.gov/39568461
- 4Chen Y, Sun Z, Zong W, et al. Advanced Technologies for Characterizing and Detecting Battery Thermal Failure: A Review. Advanced Science (indexed in PubMed). Published May 8, 2026. pubmed.ncbi.nlm.nih.gov/42101825
- 5Yang JH, Sung JY, Kweon H, et al. A Review of Failure Modes and Safety Strategies of Lithium-Ion Batteries from Materials to Systems. Advanced Science (indexed in PubMed). Published June 30, 2026. pubmed.ncbi.nlm.nih.gov/42376912
- 6Su J, DiFelice A, Mawela J, et al. Engineering Strategies to Suppress Thermal Runaway Propagation in Lithium-Ion Battery: Mechanisms, Metrics, Materials, and Evaluation Methods. Advanced Science (indexed in PubMed). Published July 13, 2026. pubmed.ncbi.nlm.nih.gov/42439374
- 7Bazan P, Przybek A, Łach M, et al. Materials Used in Electric Vehicle Battery Housings: Recycling Pathways and Circular Design — A Review. Materials (indexed in PubMed). Published July 2, 2026. pubmed.ncbi.nlm.nih.gov/42451888
- 8Electronic Code of Federal Regulations. 49 CFR 571.305, Standard No. 305; Electric-powered vehicles: electrolyte spillage and electrical shock protection. Up to date as of October 1, 2026. ecfr.gov/…/section-571.305
- 9Pokorna-Krayzelova L, Kujalova H, Tajnaiova L, et al. Long-term leaching dynamics and ecotoxicity of wastewater from electric vehicle lithium-ion battery fires. Water Research (indexed in PubMed). Published August 18, 2026. pubmed.ncbi.nlm.nih.gov/42632127
- 10Larsson F, Andersson P, Blomqvist P, Mellander BE. Toxic fluoride gas emissions from lithium-ion battery fires. Scientific Reports (indexed in PubMed). Published August 30, 2017. pubmed.ncbi.nlm.nih.gov/28855553
- 11Lin L, Ezekoye OA. Persistent gas hazards partitioned from post-thermal runaway lithium-ion battery residue. Journal of Occupational and Environmental Hygiene (indexed in PubMed). Published May 28, 2026. pubmed.ncbi.nlm.nih.gov/42208077
- 12National Highway Traffic Safety Administration, Federal Register. Federal Motor Vehicle Safety Standards; FMVSS No. 305a Electric-Powered Vehicles: Electric Powertrain Integrity Global Technical Regulation No. 20 Incorporation by Reference (final rule, 2024-28707). Published December 20, 2024. federalregister.gov/…/federal-motor-vehicle-safety-standards…
- 13Electronic Code of Federal Regulations. 49 CFR 571.305a, Standard No. 305a; Electric-powered vehicles: electric powertrain integrity. Up to date as of October 1, 2026. ecfr.gov/…/section-571.305a
- 14Electronic Code of Federal Regulations. 49 CFR Part 561, Documentation for Electric-Powered Vehicles. Up to date as of September 18, 2026. ecfr.gov/…/part-561
This article reflects the safety priorities of electric vehicle fire safety after a collision. It is general education, not legal, medical, insurance or financial advice. Rules and research change, so verify details with official sources. For your own situation, contact your vehicle's dealer or manufacturer, a shop qualified to work on high voltage batteries, your insurer or your local fire department. In an emergency, call 911. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Why a damaged EV battery can reignite after a crash: thermal runaway and stranded energy.” drivewithcare.org, https://drivewithcare.org/why-damaged-ev-battery-can-reignite/. Reviewed October 2026.
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