Lithium-ion batteries now sit at the center of modern energy infrastructure. They stabilize renewable generation, strengthen grid resilience, power data centers, and enable electrification at scale. Deployment continues to accelerate across utilities, commercial facilities, and urban environments.
As adoption expands, safety frameworks must evolve just as quickly.
ISO 3941:2026 introduces Class L, a new fire classification dedicated to lithium-ion battery fires. This update formally recognizes that lithium-ion failures behave differently from traditional fire categories. The hazard originates inside the cell, driven by electrochemical energy, and progresses in ways legacy classifications were never designed to capture.
Understanding Class L is essential for developers, engineers, regulators, and building owners. It clarifies the risk profile of lithium-ion systems and strengthens expectations around how those risks are managed.
What Is a Class L Fire?
Class L applies to fires involving lithium-ion cells and battery systems where no metallic lithium is present.
This includes:
- Grid-scale Battery Energy Storage Systems (BESS)
- Electric vehicle battery packs
- UPS battery installations
- Industrial lithium-ion banks
- Consumer lithium-ion devices
Historically, lithium-ion battery incidents were grouped under electrical fires or flammable liquid behavior, but that categorization never fully reflected reality. Lithium-ion cells contain stored chemical energy that can release internally and rapidly under failure conditions.
Class L formally identifies lithium-ion battery fire behavior as its own category within the ISO framework.
The classification acknowledges that these events originate from internal electrochemical reactions rather than surface fuel combustion. That distinction reshapes how safety is evaluated and how systems must be designed.
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Why Lithium-Ion Fires Required a Separate Classification
Thermal Runaway as a Self-Heating Chemical Event
Lithium-ion battery failures often begin with internal damage, manufacturing defects, overcharging, or mechanical stress. Once the internal temperature rises beyond a critical stability threshold, exothermic reactions accelerate within the cell, generating heat at an increasing rate.
As temperature climbs, reaction rates intensify, which in turn produces even more heat. This self-sustaining cycle rapidly pushes the cell beyond its safe operating limits.
If not contained, the rising heat transfers to neighboring cells, which may then reach their own failure thresholds. This cascading effect can unfold within seconds, driving temperatures above 600°C and initiating widespread propagation within a module.
The energy fueling this process originates inside the battery itself. Because the reaction is internally driven, lithium-ion thermal runaway behaves differently from traditional fires that rely on external fuel sources.
Gas Generation and Explosion Risk
As thermal runaway develops, lithium-ion cells begin to vent a mixture of gases that may include hydrogen, carbon monoxide, hydrocarbons, and hydrogen fluoride. In enclosed battery rooms or containerized BESS installations, these gases can accumulate quickly, especially if ventilation is limited or compromised.
As concentrations rise toward flammable limits, the potential for deflagration increases significantly. A confined ignition event can generate powerful pressure waves capable of damaging enclosures, electrical equipment, and structural components. If ignition sources are present, secondary fire events may follow.
Beyond flammability, the release of toxic and corrosive gases introduces serious safety concerns for first responders and facility occupants. Hydrogen fluoride and other byproducts can impair visibility, affect respiratory health, and contribute to long-term equipment degradation.
Gas generation is a defining characteristic of Class L events and plays a central role in evaluating the full hazard profile of lithium-ion systems.
Re-Ignition and Stranded Energy
Lithium-ion batteries can retain significant stored energy even after visible flames have been extinguished. Beneath the surface, internal reactions may continue within damaged cells, and documented incidents have shown re-ignition occurring hours after the initial event appeared to be under control.
Although cooling reduces external temperature, it does not automatically stop electrochemical reactions taking place inside compromised cells. Residual charge and ongoing internal decomposition can sustain heat generation beyond what is immediately visible.
Class L acknowledges this persistent and delayed hazard profile, underscoring the importance of system-level protection strategies that account for continued reaction potential long after active flames have subsided.
What Class L Changes for Fire Engineering and Codes
Fire Risk Assessments
Under ISO 3941:2026, lithium-ion systems must be explicitly identified within fire risk analyses. Classifying them broadly as electrical hazards no longer reflects the formal recognition of their distinct behavior.
Fire engineering evaluations must now consider the full lithium-ion hazard profile, including:
- Propagation pathways
- Heat transfer dynamics
- Gas concentration thresholds
- Explosion mitigation strategies
- Re-ignition potential
This level of specificity strengthens both system design and regulatory review. When lithium-ion batteries are evaluated as a defined fire class, expectations become clearer and engineering decisions become more deliberate.
Suppression Strategy Reevaluation
Traditional suppression systems are designed to control flame spread and reduce surface temperature. Sprinklers cool exposed materials, clean agents interrupt combustion chemistry, and aerosol systems suppress visible fire. These approaches are effective for many conventional fire scenarios.
Lithium-ion thermal runaway behaves differently because the reaction originates within the cell itself. Heat generation continues internally as long as electrochemical decomposition progresses. Surface cooling may reduce visible flames, yet internal propagation can persist if thermal energy continues to transfer between cells.
Suppression alone does not address cell-level propagation.
UL 9540A testing already evaluates propagation behavior and gas release at the cell, module, and unit levels, providing measurable data on how systems respond during failure. NFPA 855 establishes installation requirements that incorporate ventilation, spacing, gas detection, and hazard mitigation. Class L reinforces the importance of these frameworks by formally acknowledging the unique and internally driven behavior of lithium-ion battery fires.
Alignment with Existing Standards
Class L does not replace existing safety standards. It strengthens alignment between fire classification and established engineering practice.
- UL 9540A evaluates thermal runaway propagation and gas behavior characteristics.
- NFPA 855 governs installation and fire protection requirements for energy storage systems.
- International Fire Code provisions address system placement, ventilation, and emergency response planning.
By introducing a dedicated classification, ISO enhances consistency between how lithium-ion fires are categorized and how they are engineered, tested, and regulated. The result is a clearer safety framework that reflects the true electrochemical behavior of modern battery systems.
The Design Challenge Behind Class L
Class L defines the behavior of lithium-ion battery fires with greater precision. The next question becomes clear.
How should systems be engineered in response?
A lithium-ion safety architecture must be designed around the actual physics of thermal runaway and gas generation. That requires deliberate structural decisions that address both performance and failure conditions from the outset.
A comprehensive approach incorporates:
- Cell-level propagation control
- Continuous thermal absorption
- Module isolation
- Flammable gas concentration control
- Toxic gas mitigation
- Re-ignition prevention
Each of these objectives connects to measurable performance criteria such as heat transfer rates, gas concentration thresholds, and propagation resistance under UL 9540A testing conditions. Together, they shape system design from the earliest planning stages and influence enclosure configuration, thermal management strategy, monitoring systems, and emergency response integration.
Class L clarifies lithium-ion risk. The systems that lead the next phase of energy storage will be those engineered to manage that risk deliberately and measurably.
Engineering for Class L: How EticaAG Architecture Aligns with the New Classification
Class L defines lithium-ion battery fires through three core realities: internal heat-driven propagation, sustained electrochemical reaction, and hazardous gas generation. Once those behaviors are clearly understood, the design objective becomes straightforward. The system must control heat at the cell level and manage gas within the enclosure.
EticaAG’s architecture is engineered around those exact control points.
Stopping Propagation at the Cell Level
Class L recognizes that propagation is driven by heat transfer between adjacent cells. The most effective way to prevent cascading failure is to interrupt that transfer pathway directly.
EticaAG’s LiquidShield immersion cooling BESS architecture fully submerges each cell in dielectric fluid, absorbing heat directly at the source and preventing thermal buildup across the module. Rather than allowing energy to accumulate and transfer between adjacent cells, the system continuously removes heat at the cell surface, maintaining uniform temperature and interrupting the pathway that enables cascading failure.
LiquidShield immersion technology eliminates fire propagation at the cell level.
By absorbing heat at the source and isolating each cell within a dielectric fluid barrier, the system prevents ignition pathways and blocks flame spread between adjacent cells. In the event of internal cell failure, the fluid isolates the cell from oxygen, immediately suppressing combustion at the point of origin.
Controlling the Gas Hazard Profile
Class L highlights gas generation as a defining risk characteristic. Managing gas concentration inside battery enclosures is essential for preventing secondary ignition and protecting first responders.
HazGuard is EticaAG’s integrated toxic gas neutralization system designed specifically for lithium-ion battery environments. Rather than relying solely on dilution through ventilation, HazGuard actively treats hazardous gases within the enclosure.
The system targets toxic byproducts such as hydrogen fluoride and reduces their concentration before they can accumulate to dangerous levels. Only after gases are rendered inert and brought below hazardous thresholds are they safely released from the system.
By actively neutralizing harmful compounds before discharge, HazGuard:
- Reduces flammable vapor concentration buildup
- Prevents secondary ignition conditions
- Mitigates toxic exposure risk for responders and occupants
- Limits corrosive damage to surrounding equipment
- Strengthens enclosure safety during and after an incident
By integrating neutralization and controlled inert release directly into the architecture, EticaAG addresses the full gas hazard profile recognized under Class L.
A Unified Architecture Designed for Class L Realities
Class L makes clear that propagation control and gas mitigation must be engineered together. Both hazards originate from the same failure event, and both require integrated solutions.
EticaAG combines immersion-based thermal management with active gas neutralization in one cohesive system. Heat is absorbed at the cell surface to prevent escalation, while hazardous gases are neutralized and released only once inert.
This unified architecture stabilizes temperature, controls chemical byproducts, and maintains enclosure safety throughout the full lifecycle of an incident.
Class L defines the hazard with precision. Integrated system architecture ensures that it is addressed continuously, not reactively.
What Class L Signals for the Future of Energy Storage
A Higher Standard for Lithium-Ion Safety
The introduction of Class L marks a defining moment in lithium-ion safety. By aligning fire classification with the electrochemical behavior of modern battery systems, it raises expectations for how energy storage must be designed, tested, and evaluated.
Developers and system integrators are now expected to demonstrate validated propagation performance, documented gas behavior analysis, engineered thermal management, and full code compliance. Authorities Having Jurisdiction and insurers are applying this more precise framework as deployments expand across critical infrastructure.
Engineering Precision Sets the Standard
As energy storage scales to support grid resilience and electrification, safety architecture must be embedded from the outset. Systems must demonstrate thermal stability, propagation resistance, controlled gas behavior, and resilience after suppression.
Architectures built around continuous thermal control and integrated gas mitigation align directly with these expectations. Solutions such as LiquidShield and HazGuard support the performance, propagation control, and enclosure safety standards reinforced by Class L.
Class L establishes a clearer benchmark for lithium-ion safety. As deployment accelerates, the systems that meet that benchmark through disciplined engineering will define the next phase of energy storage growth.
