UL9540A-2025 New Standard Officially Released

On March 12, 2025, UL officially released ANSI/CAN/UL9540A-2025 "Battery Energy Storage System Thermal Runaway Propagation Test".

As the world's first special safety specification for thermal runaway propagation of energy storage systems, this revision took 16 months, 27 rounds of technical consultations and cross-continental voting, and the fifth edition was finally officially released.

UL 9540A is not only a national standard that is mandatory for the United States and Canada, but is also widely adopted internationally and is cited in the energy storage system installation regulations of Singapore, Malaysia and Victoria, Australia to cope with specific installation scenarios.

UL9540A levels

When testing energy storage systems in UL 9540A, four levels of testing can be performed: Cell - A single battery cell heats the battery cell in a constant volume combustion bomb and triggers thermal runaway. The gas composition of the thermal runaway is analyzed by gas chromatography, and then the explosion limit, explosion pressure and burning rate of the thermal runaway gas are tested. This part of the test is to establish a repeatable method for forcing the battery into a thermal runaway state. These methods should be used for module, unit and installation level testing.

Module - A collection of connected battery cells. The module level test triggers the thermal runaway of one or more battery cells in the module, and uses a variety of precision gas analysis instruments to comprehensively analyze the gas released by the module after thermal runaway, and evaluate its propagation characteristics and possible fire risks within the module.

Unit - A collection of battery modules connected together and installed in a rack and/or chassis. According to the different installation conditions of BESS units, the test configuration is carried out. By triggering the thermal runaway of one or more battery cells in the module, the heat release rate, gas generation and composition, the hazards of deflagration and splashing, the target energy storage system and wall surface temperature, the heat flux of the target wall and energy storage system and the exit device, and the re-ignition are mainly tested.

Installation - The same setting as the unit test, using an additional fire extinguishing system. Test Method 1-"Effectiveness of sprinklers" is used to evaluate the effectiveness of sprinkler fire extinguishing and explosion protection methods installed according to regulatory requirements. Test Method 2-"Effectiveness of fire protection plan" is used to evaluate the effectiveness of other fire extinguishing systems and explosion methods (such as gas extinguishing agents, water mist system combination systems). Installation level testing is crucial. It simulates the fire risk of the energy storage system in the actual installation and operation environment, and is an important part of the design to verify whether the protective measures are effective enough.

Here is a sneak peek at the summary of key changes to the fifth edition of ANSI/CAN/UL 9450A (March 12, 2025)

1. Test method and measurement updates

FTIR and hydrogen measurement: FTIR (Fourier transform infrared spectroscopy) measurement is changed to optional, and hydrogen measurement requirements in unit-level testing are added (clauses 8.2.14–10.3.13).

Continuous thermal ramp option: A new test method for triggering thermal runaway by continuous thermal ramp is added (7.3.1.5).

Heat flow meter and sampling rate: The use of Gardon heat flow meter is allowed, and the sampling rates for heat flow and wall temperature are revised (6.3, 9.2.15–10.3.10).

Escape path heat flow standard: Update the heat flow measurement requirements for non-residential outdoor wall-mounted systems (9.5.1, 9.5.5).

2. Test Configuration and Equipment Adjustment

Residential Unit Testing: Replace NFPA 286 test room with “test wall” (9.1.2, Figure 9.3).

Thermocouple Location: Revise the placement of thermocouples in battery testing (7.3.1.2, 7.3.1.7–10).

Ground Mount System Exception: Add exception conditions for residential systems (9.2.19–10.3.10).

3. Definition and Process Clarification

Sample Rest Time: Clarify the rest time of samples after conditioning and charging (7.2.2, 8.1.2, 9.1.9).

Battery Charging Method: Refine the battery charging process (7.2.1, 7.2.4).

Test Report Requirements: Clarify the test report specifications for using battery systems as BESS units (7.7.1).

Failure Criteria: Revise the terminology for battery, module, and unit failures (7.3.1.2, 8.2.8–9.1.8).

Term Definitions: Added "Thermal Runaway Propagation" and revised the definition of "Thermal Runaway" (4.16, 4.19).

Residential/Non-Residential Definitions: Clarified the distinction between the two types of use, affecting test configuration and reporting (8.4.1, 10.7.1)

4. New Test Methods

Battery Type Expansion: Added lead-acid battery and nickel-cadmium battery test methods (7.3.3.1–7.10.4) and high-temperature battery test procedures (7.3.4.1–10.11.3).

Flow Battery Revisions: Updated flow battery related requirements (5.4.3, 7.1.1–9.11.1).

5. Performance Standard Revisions

Module Level Performance: Revised the pass criteria for module testing (8.5.1).

Module Surface Temperature Range: Adjusted the measurement range (9.7.3, Table 9.1, 10.5.2).

6. Updates to Reference Standards

Added NFPA 855 as the applicable code (1.2, 3.2).

Replaced UL 1685 with UL 2556: Updated cable standard references (3.2, 10.2.2).

7. Safety and Structural Requirements

Removed non-combustible structural exception: clarified outdoor flame propagation rules (4.16, 9.1.1–9.7.1).

Deflagration risk considerations: added deflagration analysis requirements in Appendix A (A3.3.1).

8. Other Important Updates

Residential Use Alignment: Revised code requirements related to residential uses (1.2, 10.1.1–A2.3.2).

Deleted Residential Installation Restrictions: Removed the statement prohibiting installation in residential units.

Test Report Extensions: Expanded module, unit, and installation level test reports (8.4.1, 10.4.1).

Impact Overview

Increased flexibility: FTIR selectability and thermal ramping methods provide testing flexibility.

Expanded scope of application: Added lead-acid, nickel-cadmium and high-temperature battery tests to cover more technology types.

Enhanced safety: Revised flame propagation rules, added deflagration analysis to reduce the risk of fire spread.

Simplified testing: Residential testing uses test walls instead, which may reduce testing complexity.

This version emphasizes clarity, safety and technical inclusiveness, adapting to the needs of battery technology development and regulatory evolution.

UL 9540A evaluates the system safety of energy storage systems after the battery thermal runaway spreads. It is the reference standard for large-scale fire tests mentioned in NFPA 855 and the only consensus standard recognized in NFPA 855.

The release of UL9540A-2025 marks the strategic upgrade of energy storage safety from "passive fire protection" to "active warning".  If you need to obtain UL9540A test machines or technical support, please contact us!