What is the UL 9540A Test Method?

UL 9540A is a safety standard for energy storage systems and equipment, developed by UL as a test method to evaluate thermal runaway and fire propagation in battery energy storage systems. It is widely recognized by relevant authorities.

Authoritative U.S. industry codes—such as the National Electrical Code (Article 706), the International Residential Code (Section R327), the International Fire Code (Energy Storage Systems section), and NFPA 855 from the National Fire Protection Association—all require UL 9540A testing for energy storage systems.

The results of the UL 9540A test provide critical safety insights for energy storage system purchasers, architects, and local fire departments, including:

• • Installation best practices for energy storage systems
  • Ventilation requirements
  • Effectiveness of fire suppression systems
  • Firefighting strategies and emergency response planning

How to Conduct a UL 9540A Test.

The UL 9540A test method comprises four progressive stages: the cell level test, module level test, unit level test, and installation level test.

Cell Level Test

This test is conducted on the smallest individual battery cell within the Battery Energy Storage System (BESS). A reliable and repeatable method is used to induce thermal runaway inside a pressure vessel.

During thermal runaway, the gases released from the cell are collected for analysis. The composition of these gases is evaluated, and synthetically reproduced gases are used for the following assessments:

• • Lower Flammability Limit (LFL) test
  • Maximum Explosion Pressure (Pmax) test
  • Burning Velocity test

Module Level Test

This test is performed on a complete battery module placed under a smoke collection hood. Using the same induction method as in the cell-level test, multiple cells within the module are triggered into thermal runaway to evaluate cell-to-cell propagation within the module.

Gases released during this event are collected by the hood and analyzed. The analysis includes:

• • Chemical heat release measurement (based on oxygen consumption)
  • Vent gas composition analysis using an FTIR spectrometer
  • Hydrocarbon content analysis using a Flame Ionization Detector (FID)
  • Hydrogen gas content analysis
  • Smoke release quantification

Unit Level Test

This stage involves testing fully assembled BESS units installed per the manufacturer’s instructions, in a configuration representative of actual deployment.

For floor-mounted indoor BESS units, an initiating BESS unit is placed adjacent to a target unit, with instrumented walls around them. The initiating unit includes all components representative of a full BESS system. A hood is installed above to collect combustion gases for analysis. The initiating unit is forced into thermal runaway using the same method applied in the module-level test, aiming to evaluate module-to-module propagation.

Gas and thermal data collected during this test are analyzed for:

• • Convective heat release
  • Chemical heat release (oxygen consumption-based)
  • Vent gas composition via FTIR
  • Hydrocarbon content via FID
  • Smoke generation
  • Temperature and heat flux on surrounding instrumented walls

Installation Level Test

This final stage evaluates the performance of fire and explosion mitigation systems—such as sprinklers or fire suppression systems—in the actual installation environment. It does not apply to residential BESS.

The test is conducted similarly to the unit-level test, but includes additional elements like sprinkler systems and flame indicators installed above the initiating BESS unit.

Graduated Testing Approach

UL 9540A is a tiered test method, progressing from the smallest component (cell) to the full system (installation). If a system passes the current level of testing, the next level may not be required.

Additionally, test parameters are continuously applied across levels. For example, thermal runaway data obtained from the cell-level test is used to define conditions for the module-level test, and so on up to the installation level. This continuity enhances both the repeatability and reproducibility of results, while also improving overall test safety.