Why lithium-ion batteries catch fire

Lithium-ion batteries power everything from phones and laptops to e-bikes, EVs and energy storage systems, but when cells are damaged, overheated or charged incorrectly, they can enter thermal runaway and cause fast-moving fires, explosions and reignition risks

Lithium-ion batteries power phones, laptops, e-bikes, e-scooters, power tools, electric vehicles and energy storage systems.

They are popular because they can store a large amount of energy in a relatively small and light package.

That same energy density is also why a failing lithium-ion battery can become dangerous very quickly.

When a battery cell is damaged, defective, overcharged, overheated or used with the wrong charger, it can enter a self-heating failure process known as thermal runaway.

But why do lithium-ion batteries catch fire?

Lithium-ion batteries catch fire because the heat generated inside a failing cell can trigger chemical reactions that release more heat, flammable gases and pressure than the battery can safely contain.

If that process accelerates, the battery can vent, ignite, explode or set fire to nearby materials.

What is inside a lithium-ion battery?

A lithium-ion battery cell contains a positive electrode, a negative electrode, an electrolyte and a thin separator that keeps the electrodes apart while allowing ions to move between them.

During normal use, lithium ions shuttle between the electrodes as the battery charges and discharges.

This process is efficient, which is one reason lithium-ion batteries are used so widely in consumer electronics, mobility devices and larger power systems.

The problem is that some of the materials inside the cell are combustible, and the cell stores a large amount of energy in a confined space.

If a fault causes an internal short circuit or abnormal heating, the cell can begin to break down.

What is thermal runaway?

Thermal runaway is the main reason lithium-ion batteries catch fire.

It is a chain reaction in which a cell becomes hotter, the heat speeds up damaging chemical reactions, and those reactions generate even more heat.

Once that cycle starts and passes a critical point, the battery may no longer be able to control its own temperature.

According to UL Research Institutes, thermal runaway can result in smoke, fire, extremely high temperatures and violent cell venting.

The National Fire Protection Association states that when energy is released in an uncontrolled manner, internal battery components can become flammable and toxic gases.

How thermal runaway begins

The process can begin when a battery is exposed to abuse conditions such as overheating, overcharging, crushing, puncture, manufacturing defects or electrical failure.

These events can damage the separator or other internal components and allow the electrodes to touch.

That creates an internal short circuit, which rapidly generates heat.

If the heat cannot dissipate quickly enough, more materials inside the cell decompose, more gases are produced and internal pressure rises.

At that stage, the cell may vent hot gases, ignite or transfer heat to nearby cells.

Why one failing cell can become a larger fire

Battery packs contain many cells grouped together.

If one cell fails, the heat can spread to adjacent cells and cause thermal runaway propagation through the pack.

This is why fires in e-bikes, electric vehicles and battery energy storage systems can become so intense so quickly.

It is also why lithium-ion battery incidents can involve repeated flare-ups and reignition.

Why lithium-ion batteries catch fire

There is rarely a single answer.

Most lithium-ion battery fires begin with one or more triggering factors that damage the cell, destabilise the chemistry or overwhelm the battery management and charging controls.

Physical damage

Drops, crushing, punctures, vibration and impact can damage the internal layers of a battery cell.

That damage may not always be visible from the outside.

A battery can appear intact, then fail later when the damaged area develops into an internal short circuit.

Overcharging and the wrong charger

Charging a lithium-ion battery with an incompatible, poor-quality or modified charger can expose it to unsafe voltage, current or temperature conditions.

The U.S. Consumer Product Safety Commission advises users to follow the manufacturer’s instructions and only use the charger provided with or recommended for the product.

Incorrect charging can increase the chance of overheating, lithium plating and internal failure.

Manufacturing defects

Contamination, poor assembly, damaged separators or other hidden defects can create weak points inside a cell.

Those defects may remain dormant for some time before causing a short circuit or abnormal heat build-up.

This is one reason quality control, certification and traceability matter.

Exposure to heat

External heat can push a battery outside its safe operating range.

Leaving batteries in direct sunlight, near heaters or inside hot vehicles can raise the cell temperature and increase degradation.

If a battery is already damaged or ageing, that extra heat can be enough to trigger failure.

Poor-quality or incompatible products

Replacement batteries, converted devices and low-cost chargers may not have been designed, tested and certified as a safe system.

The risk becomes higher when batteries, chargers and controllers from different suppliers are mixed together.

For e-bikes, UL Solutions says UL 2849 evaluates the electrical drive train, battery system and charger system as a combined system for electrical and fire safety.

Ageing and repeated stress

All batteries degrade over time.

Repeated charge cycles, deep discharge, poor storage conditions and physical wear can make failure more likely as the battery ages.

A battery that runs unusually hot, charges slowly or swells should be treated as a warning sign.

Why lithium-ion battery fires can be explosive

Lithium-ion battery fires often look different from ordinary combustible fires.

That is because a failing battery can vent a mixture of hot, flammable and toxic gases before visible flames appear.

If those gases ignite, the result can be a jet-like flame, a rapid fire spread event or an explosion.

The National Transportation Safety Board has warned that damaged lithium-ion batteries can create thermal runaway hazards including battery reignition and fire.

UL Research Institutes also notes that thermal runaway can eject gas, particulates and shrapnel.

Why e-bikes and e-scooters are a major concern

E-bikes and e-scooters have made lithium-ion battery fires much more visible to the public.

These devices are often charged inside homes, hallways and flats, which means a battery failure can block an escape route within seconds.

The UK Office for Product Safety and Standards said it received information on 211 fires involving e-bikes or e-scooters in 2024.

The London Fire Brigade said firefighters attended 206 e-bike and e-scooter fires in London in 2025.

According to UK statutory guidance on lithium-ion battery safety for e-bikes, thermal runaway is the core fire risk and safe products should contain mechanisms capable of preventing it.

Why indoor charging increases the danger

Charging concentrates energy into the battery, and many serious incidents happen when devices are left charging unattended or overnight.

The CPSC says people should be present when charging micromobility devices and should never charge them while sleeping or away from home.

The London Fire Brigade also advises people not to charge e-bikes or e-scooters on escape routes and not to leave them charging unattended.

Warning signs a lithium-ion battery may be failing

Battery failures are not always sudden.

In some cases, there are signs that a cell or pack is becoming unstable.

Common warning signs include unusual heat, swelling, bulging, leaking, cracking, hissing, popping, a strong chemical smell, smoke or discolouration.

The U.S. Fire Administration advises people to stop using the battery if they notice an odour, change in colour, too much heat, change in shape, leaking or odd noises.

How to reduce the risk of a lithium-ion battery fire

The best way to reduce the risk is to combine product quality, correct use and safe charging practice.

Choose products from reputable manufacturers, use the correct charger, avoid modifying batteries and stop using any battery that appears damaged.

Store batteries away from extreme heat and keep them clear of combustible materials when charging.

Where possible, buy products that have been independently tested and certified to relevant standards.

For e-bikes and similar products, certification of the full electrical system matters because the battery, charger and drive system interact with one another.

Safe charging basics

Charge on a hard, flat surface.

Keep the device away from exits, hallways and doors.

Do not cover the battery or charger while charging.

Do not use damaged leads, plugs or adapters.

Do not charge under pillows, on beds or near soft furnishings.

Unplug once charging is complete if the manufacturer’s instructions advise this.

What should you do if a lithium-ion battery catches fire?

If a lithium-ion battery starts smoking, hissing or burning, get people out of the area and call the fire service.

Do not pick up the device and do not attempt to move it unless the manufacturer’s emergency guidance explicitly says it is safe to do so.

Response guidance differs by battery size, product type and incident conditions.

For home users, the National Fire Chiefs Council says that in the event of an e-bike, e-scooter or lithium-ion battery fire, people should get out, stay out and call 999.

The U.S. Fire Administration gives similar public advice, urging people to leave, close the door if possible and call emergency services if a battery fire occurs.

Why this issue matters for fire and safety professionals

Lithium-ion batteries are now embedded across transport, consumer products, critical infrastructure and energy systems.

That means battery fire risk is no longer confined to a niche product category.

It affects housing providers, employers, transport operators, insurers, facilities managers, first responders and building owners.

For fire and safety professionals, the challenge is broader than response.

It includes procurement, certification, charging policy, waste handling, staff training, incident reporting and public education.

Conclusion

You should now have a clearer understanding of why lithium-ion batteries catch fire.

The central mechanism is thermal runaway, but the triggers vary.

Physical damage, poor-quality products, manufacturing defects, incompatible chargers, overheating and ageing can all push a battery into failure.

Once that failure begins, a battery can release flammable gases, intense heat and enough energy to ignite nearby materials or spread fire to neighbouring cells.

As lithium-ion batteries continue to spread across homes, vehicles and buildings, understanding how and why they fail is becoming a core part of modern fire safety.