UL 9540A explained: how battery fire testing works from cell to installation

UL 9540A measures how thermal runaway develops and spreads in a battery energy storage system (BESS). Its test report gives designers evidence for evaluating the tested configuration and its installation conditions.

UL 9540 addresses the safety of the complete energy storage system. A UL 9540A report supports that assessment; it does not by itself establish a UL 9540 system listing. Individual test levels still have criteria that determine how results are evaluated and what testing follows. UL's explanation of the two standards describes their roles.

Start with the applicable edition

The official UL catalog dates the sixth edition to March 13, 2026. A published edition and the edition used for a particular project can differ. Confirm the project basis with the test laboratory and the authority having jurisdiction (AHJ).

The required sequence depends on the edition, chemistry and application. Containment at one level does not automatically remove higher-level testing. A report should identify the edition, test configuration and reason for any omitted level.

The four test levels

The levels connect cell behavior to the system around it. They are a framework for reading the report; the applicable method sets the actual test program.

Use this table as a reading checklist, alongside the standard's public scope and UL's overview of the test levels.

In the sixth edition, non-residential systems generally move from module testing to installation-level large-scale fire testing. Unit testing remains relevant for residential systems and evaluating active propagation-prevention systems. The applicable provisions determine the route for a specific design. UL edition guidance

Installation testing and large-scale fire testing

The sixth edition changes installation-level testing to examine a developed fire and post-deflagration conditions. Which protection systems operate depends on the test configuration. The UL fifth- and sixth-edition comparison describes that change.

Large-scale fire testing evaluates the exposure of neighboring systems and proposed separation distances under forced-fire conditions. Read the results together with the tested arrangement. UL large-scale fire testing overview

For a proposed site, ask:

• Do the cell, module and enclosure match the report?
• Do separation distances and adjacent exposures match the tested arrangement?
• Which detection, ventilation and suppression systems operated during testing?
• Which assumptions need a project-specific engineering assessment?

The NFPA 855 installation framework connects this evidence to site design. Applicable requirements and the AHJ determine the project's acceptance conditions.

Chemistry and operating conditions

Use results that represent the selected cells and system configuration. A chemistry label alone cannot determine gas release, propagation or the required separation.

The UL fifth-edition update includes method changes for lead-acid, nickel-cadmium, flow and high-temperature sodium batteries. High-temperature sodium and ambient-temperature sodium-ion are different technologies; the applicable method must be checked for the selected battery.

Record the cell condition and operating assumptions behind each report. Changes to the cell, enclosure or protection design should be reviewed with the laboratory to determine whether the existing evidence still applies.

Plan the evidence alongside the design

1. Agree the test basis early. Record the applicable edition, system configuration and required levels with the laboratory and AHJ.
2. Keep the full report. Track the sample identity, setup, results and limitations alongside the design revision.
3. Check changes explicitly. Review cell substitutions, spacing changes and enclosure changes against the tested configuration.
4. Plan operational checks. Define inspections, fault reporting and emergency procedures for the installed system.
5. Confirm documentation needs. Agree the project-specific package with the AHJ and insurer before submission.

For AI data centers, the battery's condition in operation also matters. Wattality is developing Sense to track cell-level measurements and support diagnostics. Our technology architecture is designed to carry that information into battery and facility decisions. These measurements complement the installation's safety assessment; they do not establish fire-test performance.

This article explains the test framework for general understanding. Project decisions follow the applicable edition, full test report and requirements of the authority having jurisdiction.