Introduction: Standardizing Safety in BESS
Battery fires are the single biggest barrier to scaling lithium-ion energy storage safely. UL 9540A is the industry’s primary method for evaluating that risk and generating the data used to support deployment decisions.
Battery Energy Storage Systems (BESS) are now core infrastructure across utilities, commercial facilities, and distributed energy networks, with deployments increasing in both size and complexity. Thermal runaway events remain a known risk tied to battery chemistry and system design, with incidents impacting projects, operations, and public safety.
UL 9540A provides the framework used to evaluate how these events develop, how much heat and gas are released, and how failures propagate within a system. In practice, most BESS installations require UL 9540A test data to obtain permitting approval.
With the release of UL 9540A 6th Edition (2026), the standard continues to evolve alongside modern system architectures, ensuring that safety validation reflects real-world deployment conditions.
UL 9540A Test vs UL 9540 Certification
UL 9540A and UL 9540 are closely related but serve very different roles in battery energy storage safety and compliance.
UL 9540A is a Test Method
UL 9540A is a test method used to evaluate how a battery system behaves when a failure occurs.
It focuses on what happens after thermal runaway begins, specifically measuring:
- Fire behavior and intensity
- Heat release during the event
- Gas generation and explosion risk
- Whether failure propagates between cells, modules, or units
The data generated from UL 9540A testing plays a critical role across the industry. It defines key system design parameters, including spacing, enclosure configuration, and fire protection requirements. It also forms the basis for permitting decisions and is used by engineers, developers, and Authorities Having Jurisdiction (AHJs) to evaluate whether a system can be safely deployed.
UL 9540 is a Safety Standard
UL 9540 is the required system-level certification for energy storage systems and is referenced by codes such as NFPA 855 and the International Fire Code (IFC).
It evaluates the safety of the complete energy storage system as an integrated product. UL 9540A testing supports this process by providing the fire performance data used to define installation requirements and safety conditions.
UL 9540 determines whether a system can be deployed, while UL 9540A provides the fire test data that supports those decisions.
Why Thermal Runaway Defines BESS Risk
At the center of UL 9540A is one critical failure mechanism: thermal runaway.
Thermal runaway occurs when a battery cell generates heat faster than it can dissipate it. Once initiated, the reaction accelerates rapidly, leading to:
- Rapid temperature increases
- Release of flammable and toxic gases
- Cell rupture or ignition
The primary risk is not the initial failure itself, but what follows. When heat from a failing cell transfers to adjacent cells, it triggers a cascading event. What begins as a single-cell failure can escalate into a module-level or system-level fire within minutes.
UL 9540A is designed to evaluate this exact scenario by answering two critical questions: how far does thermal runaway spread, and how severe does it become?
How UL 9540A Testing Works
UL 9540A follows a structured, four-level test sequence that scales from individual battery cells to full system installations. Each level builds on the previous one, providing a progressively more complete understanding of how a failure develops and spreads.
Cell Level
Testing begins at the smallest unit: a single battery cell.
At this stage, the cell is intentionally driven into thermal runaway under controlled conditions. This establishes the fundamental characteristics of the chemistry, including:
- Gas composition released during failure
- Flammability properties of those gases
- Baseline thermal behavior and energy release
This data serves as the foundation for all higher-level testing.
Module Level
Cells are then assembled into modules and tested in their production configuration. The focus shifts from individual failure to interaction between cells. This level evaluates:
- Whether thermal runaway propagates between adjacent cells
- Heat release rate (HRR) and intensity of the event
- Gas and smoke generation within a grouped structure
This step begins to reflect how failures behave in real battery assemblies.
Unit Level
At the unit level, the complete battery system is tested as it would be deployed. This includes racks, enclosures, and other internal system components. The goal is to understand system-level behavior, including:
- Fire spread between modules within the unit
- Explosion risk driven by gas accumulation
- Total heat output and overall fire severity
This level provides critical insight into how a single system performs during a failure event, including total heat output and fire intensity, often quantified using HRR, which informs system spacing, suppression design, and protection of adjacent equipment.
Installation Level
The final stage evaluates the system in a full, real-world configuration. Testing incorporates all major deployment elements, including:
- Enclosures and container structures
- Ventilation systems and airflow conditions
- Active fire suppression systems
This level captures how the system behaves under realistic operating conditions. It evaluates overall fire dynamics, gas accumulation, and whether a failure remains contained or spreads beyond the originating unit to adjacent systems or infrastructure.
One critical constraint is that UL 9540A results are only valid for the configuration tested. Changes to enclosure design, system spacing, ventilation, or overall architecture can significantly alter fire behavior. As a result, AHJs increasingly require that test data closely align with the actual installation design.
What’s New in UL 9540A 6th Edition (2026)
UL 9540A has continued to evolve alongside the growth of battery energy storage, with the 2026 update aligning the standard with larger system deployments and more complex code requirements.
The 6th Edition builds on previous revisions by expanding system-level evaluation and formalizing large-scale fire testing as a standard expectation.
Large Scale Fire Testing (LSFT)
The 6th Edition reinforces installation-level evaluation as a central part of the testing process and formally incorporates large-scale fire testing (LSFT) into this framework.
Systems are now expected to demonstrate performance at full scale, including:
- Controlled fire behavior within the system
- Limited propagation beyond the initiating unit
- Stability within realistic deployment configurations
By incorporating large-scale fire testing directly into installation-level evaluation, the standard ensures that fire behavior is assessed in conditions that reflect how systems are deployed in the field, particularly in multi-unit and utility-scale environments.
Expanded Coverage of Battery Chemistries
As energy storage technologies diversify, UL 9540A continues to expand its applicability beyond traditional lithium-ion systems. The updated framework better supports:
- Sodium-ion batteries
- Flow batteries
- High-temperature and alternative chemistries
Each chemistry introduces different thermal and failure characteristics. The updated test method ensures those differences are properly evaluated.
Improved Gas and Explosion Risk Analysis
Gas accumulation and explosion risk are among the most critical hazards during a thermal runaway event. The 6th Edition enhances how these risks are measured and understood, including:
- More precise measurement of hydrogen and hydrocarbon gases
- Improved evaluation of gas accumulation and venting behavior
- Stronger assessment of explosion risk in enclosed or semi-confined environments
These updates provide more accurate data for designing ventilation and explosion prevention systems.
More Consistent and Actionable Data
The updated standard introduces improvements in instrumentation and data collection protocols. This leads to:
- More reliable and consistent results across testing labs
- Better repeatability between test runs
- Data that is easier to apply to system design and permitting
The result is a more predictable pathway from testing to approval.
Alignment with NFPA 855 (2026)
UL 9540A is now more tightly aligned with the latest fire code requirements, particularly NFPA 855 (2026). Test data directly supports:
- Hazard Mitigation Analysis (HMA)
- System spacing and layout decisions
- Fire and explosion protection strategies
Current Safety Standards Are Still Limited in Scope
Standards like UL 9540A are designed to evaluate what happens after thermal runaway, specifically how heat, fire, and gases behave and whether failure propagates through a system.
This propagation-focused approach has improved consistency in system design and permitting, but it remains fundamentally reactive. It prioritizes containment, spacing, and emergency response rather than stopping failure at the cell level. The industry continues to operate under a “let it burn” model, where success is defined by preventing propagation to adjacent equipment rather than stopping propagation entirely.
EticaAG addresses this at the system architecture level. Lithium-ion battery cells are fully submerged in a dielectric, non-toxic, high fire-point fluid that maintains uniform temperatures and eliminates heat transfer between cells. If a single cell enters thermal runaway, the event remains isolated to that cell, preventing propagation to adjacent cells. In parallel, the HazGuard system contains off-gases within the sealed module and neutralizes them into inert outputs, preventing the release of toxic gases into the surrounding environment.
In UL 9540A testing of EticaAG systems, thermal runaway and propagation did not occur under normal immersion-cooled conditions. To complete the test, the fluid must be removed to allow thermal runaway and cell-to-cell propagation.
This exposes a critical gap. Test methods that require propagation to occur cannot fully evaluate systems designed to stop propagation at the source.
A more complete safety framework should prioritize validation of propagation prevention, ensuring systems are tested not only on how failures spread, but on their ability to stop escalation beyond a single cell.
Frequently Asked Questions
What is UL 9540A?
UL 9540A is a fire test method that evaluates how battery energy storage systems behave during thermal runaway, including heat, fire, and gas generation.
Is UL 9540A a certification?
No. UL 9540A is a test method, not a certification, and there is no such thing as a “UL 9540A listed” system.
What is the difference between UL 9540 and UL 9540A?
UL 9540 is the system-level safety certification, while UL 9540A provides the fire test data used to support that certification and define installation requirements.
Why is UL 9540A important for BESS projects?
It provides the data used by AHJs to approve installations and defines system design, spacing, and fire protection requirements.
What does UL 9540A measure?
It measures how heat, fire, and gases behave after thermal runaway begins, including whether failure propagates through the system.
What changed in UL 9540A 6th Edition (2026)?
The update strengthens system-level evaluation with large-scale fire testing, improved gas analysis, expanded chemistry coverage, and alignment with NFPA 855 (2026).
Does UL 9540A prevent battery fires?
No. It evaluates fire behavior after failure begins but does not stop thermal runaway or prevent ignition.
Can UL 9540A evaluate systems that stop thermal runaway?
Not effectively. The test requires thermal runaway to occur, which creates limitations when evaluating systems designed to eliminate propagation or prevent ignition.
