Why the Hazards of Lithium-Ion Batteries Require a Different Approach to Safety

White electric car engulfed in bright flames inside a dimly lit concrete parking garage.

Understanding Thermal Runaway to Build Safer Battery Energy Storage Systems

Editor’s Note: The following article is based on the FDNY Plenary Session, “Considerations for promoting the safe utilization of various lithium-ion battery technologies in the advancement of public policy initiatives in today’s world,” presented at ISPCE 2026 in May 2026.

ithium-ion batteries have become so common that they are easy to take for granted. They power phones, laptops, tools, e-bikes, and electric vehicles, and are associated with convenience and portability. That familiarity makes it easy to overlook the amount of energy these batteries store and how quickly that energy can be released when something goes wrong.

New York City began seeing a sharp increase in lithium-ion battery fires beginning in early 2021. Investigations showed that these fires were occurring at an alarming rate, particularly in micromobility devices such as e-bikes and e-scooters. Fires involving lithium-ion batteries were growing rapidly, producing severe conditions earlier than expected and contributing to deaths and injuries across the city. Data from the Fire Department of the City of New York (FDNY) later documented 268 lithium-ion battery-related fires in 2023 and 277 in 2024, with 18 deaths in 2023 before prevention efforts began to reduce that number.

The severity of these incidents also showed in the types of rescues firefighters were being forced to perform. One of the clearest examples was the increased use of the life-saving rope, a high-risk technique used only in extreme circumstances to lower a firefighter from a floor above to rescue someone trapped at a window. In normal conditions, this is a rarely used tactic. It is the kind of rescue that might happen once a decade. Seeing multiple deployments in a single year showed lithium-ion battery fires were trapping occupants before they could reach an exit more frequently than other fire types.

Like micromobility devices, battery energy storage systems (BESS) share the same underlying fire hazards. These systems generally use the same lithium-ion technology, but they can scale that energy storage into larger, more complex installations that support buildings, infrastructure, and power systems. As the scale increases, the importance of managing the hazard increases with it. Conditions that drive battery failure do not change, but the consequences depend more heavily on how the system is designed, installed, used, and controlled.

Therefore, safe deployment of BESS depends on how these systems are built to perform, not only during normal operation but also when something goes wrong. Understanding how lithium-ion batteries function, how they fail, and how system design influences those outcomes is essential to evaluating risk and supporting safe installation and operation.

Known Hazards

Battery-powered products are filling the space between small consumer devices and large stationary systems. Portable power packs, battery‑supported appliances, and plug‑in photovoltaic systems with portable storage all raise similar questions about stored energy, use environment, and safety expectations. As lithium-ion batteries appear in more forms, the safety discussion becomes less about product category and more about how the system stores energy, controls failure, and interacts with its surroundings.

The Battery Fundamentals Behind the Risk

A lithium-ion battery stores and releases energy by moving lithium ions between two electrodes inside a cell. The basic internal structure includes a cathode, an anode, a separator, and an electrolyte. The cathode is typically made from metal oxide-coated aluminum foil. The anode is usually carbon-coated copper foil. The separator is a very thin plastic layer that keeps the two electrodes apart while still allowing ionic movement. The electrolyte is usually made up of an organic solvent, lithium salts, and additives that support charge transfer.

That arrangement allows energy to move efficiently during charging and discharging, which is one reason lithium-ion batteries became so central to modern electrical products. The battery can be recharged many times, can deliver power quickly, and can be built in different shapes for different applications. Pouch cells are common in smartphones and laptops. Prismatic cells are used in some electric vehicles and large-scale battery energy storage systems. Cylindrical cells, including the commonly used 18650 format, are used in power tools, e-mobility devices, and some laptops and vehicles.

Thermal Runaway

Lithium-ion batteries can fail for many different reasons. While the initiating causes may vary, many serious battery safety incidents involve a phenomenon known as thermal runaway.

Thermal runaway occurs when an internal short circuit or other condition destabilizes the cell, causing an uncontrolled charge movement across the separator between the anode and cathode. This uncontrolled flow of electricity creates a lot of heat. As the flow continues, the cell generates heat faster than it can dissipate it. That process accelerates internal decomposition, releases more heat, and pushes the cell further into failure. Once thermal runaway begins, it often spreads to adjacent cells. That chain reaction is known as propagation.

This is the point at which lithium-ion batteries become fundamentally different from many other electrical hazards. The battery is not simply overheating from the outside. It is generating heat internally in a self accelerating way. That process can continue until all affected cells are consumed or until sufficient cooling slows the reaction. In large systems, the difference between a single failing cell and a propagating event is one of the most important risk distinctions to understand.

The Many Paths to Battery Failure

Conditions contributing to the failure of lithium-ion batteries have several general categories, including:

Manufacturing or design defects;

Environmental conditions; and

Mechanical damage or electrical abuse.

Manufacturing-related defects can include contamination inside the battery, misalignment of electrodes, or loose parts. Environmental conditions can include extreme temperatures, moisture intrusion, flooding, or exposure to corrosive substances. Mechanical damage, such as crushing, penetration, excessive vibration, dropping, or heavy impact, may create internal short circuits that can increase the likelihood of thermal runaway. Electrical abuse includes overcharging, over-discharging, current imbalance, or external short circuits.

A failure event can be the end result of damage or instability that developed earlier. A system can look fine from the outside while internal conditions are gradually moving it toward failure. That is why the battery management system (BMS), thermal controls, and protective enclosures are so important in a BESS. They are part of the effort to prevent hazardous conditions from developing or progressing unnoticed.

When One Failure Creates Several Hazards

Once battery failure begins, fires can look different depending on fuel load, physical architecture, heat intensity, etc. What makes lithium-ion battery incidents particularly challenging is that several hazards can develop at the same time, often interacting with one another in ways that are not typical of other fire events.

One of the most immediate concerns is flammable gas production. When a battery fails, it can release large amounts of flammable and toxic gases before flaming ever begins. In a confined or partially enclosed space, those gases can accumulate and create conditions for ignition or deflagration (an over-pressure condition caused by ignition of a flammable gas at subsonic speed that may cause physical harm to surrounding people or structures). This introduces both a fire hazard and a pressure hazard. The presence of flammable gas means that an incident may escalate rapidly if ignition occurs, even if visible fire initially appears limited.

A failing battery can also produce flame jetting, where a concentrated flame plume is released from the cell. This creates a direct path for ignition beyond the original point of failure. Rather than spreading gradually, fire is forced outward in a way that rapidly ignites nearby materials like furniture or adjacent batteries and can also ignite any unburned gas released by the battery. In systems with multiple cells or modules, this jetting increases the likelihood that surrounding components become involved early in the event, leading to propagation.

Another complicated factor is cell and material ejection. The battery failure can expel hot parts of the battery and its contents outward, spreading new ignition points away from the original battery pack. Lithium ion battery fires can generate their own oxygen to support combustion, making suppression much more challenging. This takes the incident from a contained fire within a device to a rapidly spreading room fire and complicates containment efforts.

Adding to the complications is the fact that lithium-ion batteries can generate their own oxygen source, which results in a self-propagating fire. These combined factors create an intensity that exceeds typical fire behavior found with combustibles like paper or cardboard. This rapid escalation of the fire helps explain why FDNY units were encountering advanced fire conditions upon arrival, even for fires that were immediately reported.

The speed of that development reduces the margin for response and increases the likelihood that occupants are exposed to life-threatening conditions before they can exit. After a fire appears to be extinguished, individual cells within a battery may remain damaged but have not yet completely failed. Damaged batteries can retain a significant amount of stranded electrical energy, leaving them susceptible to delayed failure or restrike. Those cells may subsequently enter thermal runaway, resulting in reignition, and can also pose electrical shock hazards within damaged modules. This is why managing the aftermath of a battery incident requires continued caution even after visible fire has been controlled.

Managing lithium-ion battery risk requires a multifaceted and holistic approach involving quality design and manufacturing, third-party certification, user education, and emergency response. The FDNY experience reinforced the value of a coordinated approach that included regulation and policy work, public education, enforcement, operational learning, and technical expertise. As incidents increased, different parts of the department worked with other city agencies to roll out prevention initiatives, educate the public and advocate for new legislation. FDNY also drew on partnerships with battery scientists and standards experts to better understand the hazard and apply that knowledge to prevention and response tactics.

Risk Mitigation for Lithium-Ion BESS

BESS risk depends on the likelihood of that event occurring and the consequences that would result. The coordination mentioned in the previous section is especially important for mitigation of BESS risk. Model Codes and safety standards provide a critical framework for safe design and installation, but they are only one part of the broader safety system.

Risk mitigation extends beyond the BESS. It extends into the communities where BESS are used. Community risk reduction, or CRR, is a strategy used by fire departments and public safety agencies to lower local risks, prevent injuries, and reduce property damage before disasters happen. The five areas, also called the Five Es of CRR, are education, engineering, enforcement, economic incentive, and emergency response, and no single strategy is sufficient on its own.

Engineering solutions determine how BESS are designed, manufactured, and protected. Education initiatives help owners, operators, and users understand safe operation and maintenance practices. Enforcement activities support compliance with codes and standards and promote consistent application of requirements. Economic incentives push the market toward investment in safer products, certified systems, and risk-reduction measures. Emergency response planning prepares responders to manage battery incidents safely and effectively. Together, these Five Es create a layered approach to risk mitigation that helps to address both the likelihood of a BESS-related event and the potential consequences if one occurs.

For BESS, battery hazards may begin with chemistry, but effective risk mitigation depends on a coordinated framework of engineering, education, enforcement, economic incentives, and emergency response working together to reduce both the likelihood and consequences of an incident.

BESS Design as the First Layer of Protection

The safety features built into a BESS are intended to reduce the likelihood of battery failure and to limit the consequences if failure still occurs. The battery management system (BMS) monitors and controls battery operating conditions, helping to prevent electrical and thermal abuse that could lead to battery failure. The venting system helps manage pressure and flammable gas release during abnormal conditions. The enclosure protects the system from physical damage and water intrusion. Together, these features are designed to reduce the kinds of damage that can trigger thermal runaway.

That design logic is important because thermal runaway begins when abnormal conditions destabilize the battery and initiate self-heating reactions. A large stationary system therefore must be designed to resist several pathways to failure at once. The protective features built into the system are not simply conveniences or performance enhancements. They are part of the safety architecture that allows the system to operate with a large amount of stored energy.

At the same time, these features shape how the system behaves during failure. A weatherproof enclosure protects the batteries from outside conditions during normal operation, but it also changes what happens once fire begins inside the system. The enclosure, the controls, and protection systems intended to mitigate the risks all become part of the way the installation is expected to manage a thermal runaway event. At that point, safety is no longer only about preventing battery damage. It is also about whether the system can govern the consequences of that cascading damage.

That is why system design and compliance cannot be separated. The product may include the right protective features, but those features only support safety if they are properly demonstrated to promote the right outcomes, and then the product is installed in a way that allows them to perform as intended.

Turning Battery Hazards Into Installation Requirements

The hazards associated with lithium-ion batteries become meaningful from a compliance perspective only when they are translated into requirements that can be applied consistently. It is not enough to know that thermal runaway can occur, that gas can accumulate, or that propagation can spread to nearby cells. Those conditions inform decisions about installation, separation, gas management, product evaluation, and fire testing.

Standards and codes are intended to work in concert. Safety standards address the products and systems. Model codes reference product standards and address safe installation, inspection, use, and maintenance as appropriate.

By using known battery behavior, technical committees can create consensus-based requirements that can be used by manufacturers, designers, installers, owners, and authorities having jurisdiction (AHJ). In the built environment, battery hazards cannot be managed through general awareness alone. They should be reflected in the requirements and regulations that govern how systems are classified, evaluated, installed, and maintained and, for BESS, how they are decommissioned at end of life.

This is especially important in BESS because the thermal runaway hazards are interconnected and the risks to people, buildings and infrastructure are more severe than a fire involving ordinary combustibles. Thermal runaway affects heat release. Heat release affects surrounding exposures. Gas release affects enclosure pressure and deflagration risk. Propagation affects whether the event stays localized or expands through the system. A codes and standards framework with a systematic safety approach provides a consensus-based path for hazard mitigation through a common structure allowing the industry to move innovative new technologies such as BESS into the built environment with consistency and safety for building occupants and emergency responders alike.

The Role of NFPA 855 in BESS Installation

For stationary energy storage systems, NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, provides the core installation framework, references certification to UL 9540, and specifies large scale fire test in accordance with UL 9540A.

Large-Scale Fire Testing

Why System Certification and Fire Testing Support the Safety Case

A safe installation framework depends on comprehensive system evaluation and large-scale fire testing (LSFT). For BESS, that is where ANSI/CAN/UL 9540, Energy Storage Systems and Equipment, and ANSI/CAN/UL 9540A, Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, becomes central.

UL 9540 is a product certification standard that evaluates energy storage systems and equipment as complete systems. That system-level approach matters because the safety of a BESS depends on how battery modules, controls, power conversion elements, and protective features work together. A stationary energy storage installation has to be evaluated as the integrated system it actually is, not as a collection of unrelated components. That system-level evaluation supports a more meaningful safety basis because it reflects how the equipment will be installed and used.

Unlike UL 9540, UL 9540A is not a product certification standard. It provides the test methodology used to evaluate fire and explosion hazard characteristics of BESS that are capable of thermal runaway. That testing matters because it characterizes what happens when a failure actually occurs. At the cell level, it examines whether a cell can enter thermal runaway and what kind of gases are produced. At the module level, it examines the tendency for propagation within the module and through the unit, and the severity of heat, flame, and flammable gas release. At the installation – LSFT level, it examines whether fire from one BESS unit can expose adjacent BESS units or nearby structures and whether proposed separation distances and protection strategies, including deflagration mitigation, are adequate. The unit-level test of UL 9540A is waived for commercial and industrial applications unless otherwise required in UL 9540.

The cell-module-unit (residential)-installation LSFT test progression is important because it provides a standardized evaluation of battery behavior into installation insight. Instead of relying on design intent alone, LSFT uses actual data from a developed fire condition inside of a BESS for evaluation to support decisions about how the installation should be configured and protected. The installation level – LSFT of UL 9540A is especially valuable because it brings the exposure question into focus. It helps engineers and AHJs determine whether nearby structures are adequately protected, whether adjacent systems are sufficiently separated, and whether the prescribed fire protection plan can mitigate the fire and deflagration risks originating in the batteries.

This is one of the clearest ways the safety framework supports confidence. A listed system and a tested installation give regulators, designers, and owners something more substantial than reassurance. They provide a data-driven way to connect specific BESS behavior to site‑specific decisions.

Code Compliance and Certification

Where Installation Determines Real‑World Safety

A certified system and a compliant installation test provide confidence in the safety and performance of the BESS. The certification reflects the basis on which the system was evaluated. The installation determines whether the real-world conditions still match that basis. Compliance with NFPA 855 and certification in accordance with UL 9540 requires the installation location, separation, ventilation, and surrounding exposures to align with how the system was evaluated (including UL 9540A LSFT). AHJs and BESS designers/owners/operators use required code compliance initial commissioning of a BESS to help ensure the installation has the full margin of safety that the design and testing were meant to provide.

This matters most in the areas where the safety framework is built around. Flammable gas management features depend on enclosure behavior and ventilation conditions. Separation distances only provide the intended protection when they are maintained. Fire protection measures only support the system if they reflect how a battery-originated event is expected to develop. None of these can be treated as minor site details. They are part of the conditions under which the system is supposed to remain safe.

This is why safe deployment depends on alignment. The battery chemistry, the system architecture, the protective features, the product evaluation, the fire test data, the installation conditions, and the code requirements must all support one another.

Importance of Inspection, Testing, and Maintenance (ITM)

Why Safety Should Extend Beyond the Day of Installation

BESS safety continues after commissioning. Safe operation depends on maintaining the conditions that support safe performance over time. Monitoring matters because the BMS must keep the batteries operating properly through many cycles of charge/discharge and detect harmful conditions. Maintenance matters because fire protection and deflagration mitigation systems within the enclosure must remain reliable. Replacement practices matter because component compatibility still affects system behavior. Operational decisions matter because the system must continue functioning within the assumptions behind its design and evaluation.

A Stronger Basis for Confidence

BESS allows energy to be stored and used when it is needed, and that value becomes more important as electrification expands and as buildings, facilities and infrastructure rely more heavily on managed energy systems. This is especially true with BESS supporting the rapid construction of data centers that have significant electrical power demand and where downtime is costly and disruptive. Long-term confidence in BESS depends on whether safety remains tied to actual system performance.

That means the system must perform during normal charging, storage, and discharging. It also must perform when a battery is damaged, when thermal runaway occurs, when gases are generated, when heat threatens to propagate, and when the installation has to limit the consequences of internal battery failure. Codes, standards, system certification, and LSFT support that level of performance by connecting battery behavior to product evaluation, installation requirements, and site conditions.

The strongest BESS deployments are the ones where the system has been evaluated as a complete system, the hazards have been characterized through meaningful LSFT, the installation reflects the assumptions behind that evaluation, and the governing requirements are applied in a way that preserves the intended safety performance of the system. That is what creates a stronger basis for confidence as energy storage becomes a larger part of the built environment.

Key Takeaways for Safer BESS Deployment

Several lessons stand out as lithium-ion batteries move into larger and more complex applications:

The hazard begins with the battery but does not end there. The way cells are arranged, monitored, cooled, enclosed, and installed determines whether a failure remains localized or expands into a larger event.
Safety depends on more than one mitigation strategy. Education, engineering, enforcement, economic incentives, and emergency response each play a role. The most effective safety framework is collaborative and layered.

Codes and standards support confidence when they are connected to real system behavior. System certification, fire testing, installation requirements, and life‑cycle management practices help translate battery hazards into requirements that can be applied consistently.
BESS safety depends on preserving the assumptions behind the design. The installation, operation, and maintenance of the system must support the product and performance evaluations.

Conclusion

Lithium-ion batteries are found in many consumer and commercial products and systems. Their continued safe deployment depends on more than battery technology alone. A strong safety foundation begins with quality design and manufacturing practices that help prevent defects and minimize the conditions that can lead to failure. Independent third‑party certification provides confidence that products and systems have been evaluated to established safety standards, while compliance with current codes and installation requirements helps ensure they are deployed as intended. Ongoing inspection, testing, and maintenance of battery‑operated equipment and products are equally important to preserve safety throughout their life and to identify developing issues before they become hazardous.

The safe use of lithium-ion battery technologies also relies on informed users, owners, operators, and emergency responders. User awareness, proper operating practices, and routine maintenance help reduce the likelihood of incidents, while emergency response planning prepares responders to manage the unique hazards associated with lithium-ion battery failures. Together, quality manufacturing, third-party certification, code compliance, ongoing maintenance, user education, and emergency preparedness create a comprehensive safety framework that supports the responsible adoption of lithium-ion battery technologies across consumer, commercial, and industrial applications.

Acknowledgement

The authors acknowledge the contributions of Chief Matt Quinn of the Fire Department of the City of New York (FDNY) to the development of this article. Insights shared through a combined IEEE‑ISPCE presentation and his experience addressing lithium‑ion battery and battery energy storage system hazards provided valuable technical and operational perspective.

References

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The Authors
Bruce Johnson is Regulatory Services Manager at UL Solutions, and a Distinguished Member of Technical Staff at the William Henry Merrill Society. Johnson can be reached at bruce.johnson@ul.com.

LaTanya Schwalb is a Principal Engineer, Energy & Industrial Automation, at UL Solutions, and is also a Distinguished Member of Technical Staff at the William Henry Merrill Society. Schwalb can be reached at latanya.schwalb@ul.com.

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