Why Lithium-Ion Batteries Fail — and What Happens When They Do

Lithium-ion batteries power familiar workplace equipment. Understanding how a cell stores energy explains why damage, unsuitable charging or excessive heat can sometimes lead to a serious failure.

Familiar equipment, stored energy

Laptops, handheld scanners, cordless tools and e-bikes all rely on compact rechargeable batteries. Properly designed and used batteries generally operate safely, but their stored energy and internal materials can create a fire hazard if something goes wrong.

This introduction explains the underlying failure process. The workplace examples focus on portable equipment and removable packs; they are not a design or operating guide for vehicle traction batteries or stationary energy-storage systems.

What's actually inside a lithium-ion battery?

A battery contains one or more cells. Each lithium-ion cell has a negative electrode and a positive electrode, an electrolyte that carries lithium ions, and a porous separator that keeps the electrodes apart while allowing those ions to pass.

During discharge, lithium ions move from the negative electrode to the positive electrode inside the cell. Electrons travel through the external circuit to power the device. Charging reverses these directions. Battery literature commonly calls the negative electrode the anode and the positive electrode the cathode, using the names associated with discharge.

The diagram separates these two paths. It is a flat schematic, not a cutaway of a cylindrical cell: real electrode layers can be wound or stacked. The electrodes must remain separated and the cell must stay within its permitted operating limits. Many lithium-ion cells also contain a flammable organic electrolyte.

How does a battery begin to fail?

There is no single cause of lithium-ion battery failure. Sometimes the trigger is obvious, such as a battery being dropped, crushed or punctured. Other problems develop less visibly and may remain undetected for some time.

Physical damage can deform the cell's internal components. If its separator is compromised, the electrodes may come into contact, creating an internal short circuit. Electrical abuse is another potential cause: overcharging, excessive current or unsuitable charging equipment can create dangerous conditions inside the battery.

High temperatures can accelerate unwanted chemical reactions. A battery exposed to excessive external heat may reach a point where its internal materials become unstable. Manufacturing defects can also play a part. Tiny metallic particles, imperfections in separators or other flaws may create the conditions for failure, sometimes long after the cell enters service.

Ageing changes how a battery behaves, too. Repeated charging cycles, storage conditions and operating temperatures gradually affect its internal components. This does not mean an older battery is destined to catch fire, but particular forms of degradation can increase the risk, especially when combined with other stresses.

These different causes can ultimately produce the same problem: excessive heat building up inside a cell. Under certain conditions, that heat can initiate a self-reinforcing chemical reaction.

Thermal runaway: when heat becomes the problem

Every lithium-ion battery produces some heat during normal operation. Usually, that heat escapes into the surroundings, allowing the cell to remain at a stable temperature. An internal short circuit, however, can convert electrical energy into heat much more rapidly than normal.

As the temperature rises, materials inside the cell may begin to break down. Some of these reactions release additional heat, which accelerates further reactions. The result can be a feedback loop in which the cell generates heat faster than it can lose it. Once this process becomes self-sustaining, it is known as thermal runaway.

At that stage, ordinary protective electronics may no longer be able to stop the internal reactions. Temperatures can rise rapidly, potentially reaching several hundred degrees Celsius. Exactly how the failure develops depends on the cell's chemistry, design, condition and state of charge.

Thermal runaway does not always produce visible flames. A battery can therefore be undergoing a serious internal failure before there is an obvious fire to see.

What happens when a cell enters thermal runaway?

As the internal temperature increases, the electrolyte and other materials may decompose, generating gases and causing pressure to build inside the cell. Many cylindrical cells include venting features designed to release that pressure. Other cell formats may swell, rupture or release gases through different failure paths.

Those gases and vapours may contain flammable substances and hazardous decomposition products. Sometimes a cell vents without immediate ignition. In other circumstances, gases ignite as they escape, producing flames or jets of burning material. If flammable gases accumulate in an enclosed space before igniting, they can create a pressure or explosion hazard. Hot particles or other material may also be expelled.

This is part of what makes lithium-ion incidents challenging. The hazard can extend beyond the original device or battery, particularly where other combustible materials or neighbouring battery cells are nearby.

The chain reaction

A single lithium-ion cell is often just one component of a much larger battery. A pack may contain tens, hundreds or thousands of cells connected together. Under normal conditions, they operate as a coordinated energy storage system. During a failure, their proximity introduces an additional concern.

Heat from one failing cell can transfer to neighbouring cells. If those cells become hot enough, they may also enter thermal runaway. This is known as thermal runaway propagation: a failure that begins in one cell can spread through part or all of a battery pack.

Propagation is not inevitable. Cell spacing, thermal barriers, cooling arrangements, chemistry and other aspects of battery design can all influence whether it occurs and how far it spreads. Nevertheless, the potential for a cascading failure is one of the central challenges in lithium-ion battery safety.

Scale matters as well. A small consumer battery and a large commercial battery pack may rely on similar underlying chemistry, but the potential consequences of a failure can be very different.

Why are lithium-ion battery fires different?

Traditional fire safety often focuses on three elements: fuel, heat and oxygen. Removing one can interrupt combustion. Lithium-ion battery failures introduce another complication because, once thermal runaway has begun, internal chemical reactions may continue generating heat even after visible flames have been suppressed.

Some battery chemistries can also release oxygen-containing decomposition products that contribute to combustion. This does not mean lithium-ion battery fires cannot be extinguished. Water and other firefighting approaches can be effective in appropriate circumstances, particularly where cooling is needed. It does mean that extinguishing visible flames and stopping thermal runaway are not necessarily the same thing.

A battery may remain dangerously hot after an initial fire has been controlled. If neighbouring cells have already been heated or damaged, further failure may still be possible. Flammable and potentially toxic gases create additional concerns, especially in charging rooms, cabinets, buildings and other enclosed spaces.

There is also an important distinction between rechargeable lithium-ion batteries and primary lithium-metal batteries. They are different technologies, and their hazards and appropriate response measures should not be treated as interchangeable.

What does this mean for workplaces?

A useful starting point is to identify which batteries are present, where they are charged and what happens when someone reports a concern. A few low-capacity devices, a delivery fleet and a collection of damaged packs create different management questions. The battery type, condition, quantity and surroundings all matter.

For each area, establish:

• Which battery and charger combinations the manufacturer permits.
• Where normal charging and storage occur, and who is responsible.
• How suspect equipment is reported and kept from returning unnoticed to service.
• How staff raise the alarm and leave safely if an incident develops.

Smoke, venting, hissing, fire or rapidly increasing heat require an emergency response. Move away, alert others and follow the site's evacuation arrangements; contact emergency services from a safe location. Do not approach to investigate, unplug or carry an actively failing battery.

Understanding failure supports better decisions about ordinary equipment. It does not make a visual inspection, generic enclosure or single precaution a guarantee of safety.

Sources and next steps

For background on failure and its consequences, see OSHA's lithium-ion battery hazards overview, UL Research Institutes' Fire Safety Research Institute battery fire research and the U.S. Fire Administration's explanation of incident risks.

Continue with the warning signs and the appropriate response.

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Editorial update and source check: October 25, 2026. Prepared by Battery Safe Systems with AI assistance as general education for U.S. workplace readers. Follow manufacturer instructions, site procedures and applicable local requirements; this is not a site-specific assessment or independent certification. The featured image and photographic-style illustrations are AI-generated concepts, not incident photographs or product-performance evidence. The charge-flow diagram is a schematic checked against the linked educational sources.