e-Book: An Inside Look at the Updates to UL 9540A:2025

Introduction to UL 9540A

UL 9540A, the Standard Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, is a critical Standard that addresses the safety of battery energy storage systems (BESS) by evaluating the thermal runaway fire propagation potential. This Standard has evolved significantly throughout its editions, with the fifth edition released in March 2025.

The latest version incorporates valuable insights from past testing experiences and incidents to enhance the fire safety measures for energy storage technologies that are increasingly used across various sectors.

Key enhancements in UL 9540A:2025

New definitions

One of the significant updates in UL 9540A:2025 is the introduction of several new definitions that clarify key terminologies related to battery storage systems. Understanding these terms is vital for compliance and safety evaluation.

• Cell, initiating – Refers to the cell that experiences a fault leading to thermal runaway
• Thermal runaway propagation – Describes the energy transfer from a cell undergoing thermal runaway that may induce similar conditions in adjacent cells
• Residential vs. nonresidential use – These definitions help clarify equipment suitability based on building types

Updates to measurement requirements

Accurate measurements are crucial in evaluating the risks associated with thermal runaway. The latest updates have enhanced how measurements are taken and reported. New updates cover the various topics, including:

Refinements to testing requirements

Cell-level testing requirements

Enhancements in cell-level testing are pivotal for accurately simulating real-world conditions.

• Prismatic cell testing – These cells must be constrained during testing to represent their conditions in actual BESS environments.
• Thermocouple placement – Clear guidelines on thermocouple placements safeguard effective temperature measurement during testing.

Module-level testing requirements

An important focus of UL 9540A:2025 is the propagation of thermal runaway between cells within a module.

• Cell-to-cell propagation – Tests must be conducted under conditions where at least one additional cell enters thermal runaway during the evaluation.
• Number of cells for testing – Sufficient cells must fail to trigger thermal runaway; for higher-capacity cells, a minimum of three cells is necessary.

Unit- and installation-level testing

Updates also include testing at both the unit and installation levels, enhancing the comprehensive evaluation process whether   each component functions optimally under expected conditions.

Conclusion: Elevating safety standards

The updates to UL 9540A:2025 are integral to promoting safety in the ever-evolving domain of battery energy storage systems. By refining testing methodologies and clarifying definitions, UL Solutions is setting a stronger foundation for assessing thermal runaway risks. These enhancements not only address current challenges but also adapt to the technological advances within the battery storage landscape.

As stakeholders in the energy sector, understanding these updates allows for better compliance, improved safety protocols and ultimately, the advancement of innovative energy storage  tools that align with today's safety standards. Compliance with UL 9540A:2025 is crucial for safe performance of battery energy storage systems, whether you are a manufacturer, installer or end-user.