Can a battery energy storage system be “UL 9540A certified”?
No: UL 9540A is a test method, it produces data, and nothing about performing UL 9540A testing certifies or lists a battery energy storage system. In battery energy storage disputes, the UL 9540A report is produced more often than any other document, and it is the one most often misdescribed. A party says the system was "UL 9540A certified," or that it "passed 9540A," and the sentence is taken to establish that thermal runaway could not propagate.
What is the difference between UL 9540 and UL 9540A?
UL 9540 is a product safety standard to which an energy storage system can be listed, while UL 9540A is a test method from which no listing results. UL 9540, Standard for Energy Storage Systems and Equipment, is a product safety standard: a system evaluated to UL 9540 can be listed and carry a certification mark. The third edition of UL 9540 was published June 28, 2023.
UL 9540A, Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, is a different kind of document. UL states plainly that UL 9540A is a test method and not a certification standard, so no listing results from performing the testing, and the Sustainable Energy Action Committee’s informational bulletin on UL 9540 and UL 9540A says the same. The fifth edition of UL 9540A was published March 12, 2025 as ANSI/CAN/UL 9540A; the sixth edition was published March 13, 2026.
What does UL 9540A testing actually do?
UL 9540A testing produces characterization data on thermal runaway and fire propagation, organized in levels: cell, module, unit and installation. Cell-level UL 9540A testing characterizes the thermal runaway behavior of a single cell and the composition and flammability of the gases it vents. Module-level testing looks at whether runaway propagates cell to cell and at module heat and gas release rates. Unit-level and installation-level testing address spread between modules, fire protection performance, and system heat and gas release.
The intended output of UL 9540A testing is characterization data that a designer or code official uses to evaluate separation distances, explosion control, ventilation and suppression for one installation.
What does it mean when a system “passed” UL 9540A?
At most, “passed” can mean that a defined configuration met the UL 9540A performance criteria for the levels at which it was tested, and meeting those criteria is not a listing. UL 9540A does carry performance criteria, and they are level-specific. UL’s own summary of the fifth edition of UL 9540A lists module-level criteria — cell vent gas is nonflammable, no spread of flame outside the module, module exterior surface temperature does not exceed the cell venting temperature — together with revised unit-level criteria and an installation-level criterion on recording reignition after sprinkler operation is discontinued. A UL 9540A report records what a defined configuration did against the criteria for the levels at which it was tested, and says nothing about certification.
The structure of UL 9540A changed with the sixth edition, which UL has given an effective date of January 1, 2027. UL describes the difference this way: under the fifth edition, testing could stop once the unit-level performance criteria were met, while under the sixth the installation-level large-scale fire test is conducted and that test “has pass-fail criteria that must be met to comply.” Which edition a UL 9540A report was run under therefore governs what “passed” can mean, and the edition is on the report.
How do NFPA 855 and the International Fire Code use UL 9540A test data?
Under NFPA 855 and the International Fire Code, the pattern is that UL 9540A test data is what allows a departure from prescriptive defaults, such as larger stored energy per unit or reduced separation, subject to the authority having jurisdiction. UL 9540A is referenced by NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, and by the International Fire Code.
That structure locates the question. UL 9540A data does not certify a battery energy storage system. It supports an installation-specific judgment made by someone else, and that judgment, its inputs, and whether the installation as built matched them are separate items of evidence.
Does a UL 9540A test report describe the battery energy storage system as it was installed?
Not necessarily: a UL 9540A test report describes the configuration that was tested, and an installation years later may differ in any element of that configuration. The configuration a UL 9540A report describes is a cell, a module, an enclosure, a spacing, a state of charge, an initiation method and a ventilation arrangement. Cell suppliers change, modules are revised, and racks go in closer than the tested spacing or in a room whose ventilation was never part of the test.
So the first forensic question about a UL 9540A report is not what it concluded but what it tested, and whether that configuration is the one that burned. A mismatch between the tested and installed configurations does not establish causation. It does establish that the UL 9540A report cannot be read as describing the system in question.
Why does state of charge matter when reading a UL 9540A report?
State of charge matters because thermal runaway severity is not a fixed property of a cell, and a single test condition characterizes a single condition. Golubkov and colleagues ran temperature-ramp tests on commercial 18650 cells with LFP and NCA cathodes preconditioned across states of charge from 0 to 143 percent, covering the working window and overcharge, and reported that charged cells “went into a thermal runaway, when heated above a critical temperature,” with the runaway parameters depending on state of charge and differing between the two cell types (RSC Advances 5, 57171-57186, 2015).
Cell vent gas matters for the same reason. Baird, Archibald, Marr and Ezekoye evaluated methods for estimating lower flammability limit, laminar flame speed and maximum overpressure for cell vent gas, to quantify how chemistry and state of charge affect explosion hazard for confined cells (Journal of Power Sources 446, 227257, 2020).
What can a UL 9540A report establish about a battery energy storage system fire, and what can’t it?
A UL 9540A report can establish what a defined configuration did under a defined initiation, but it cannot establish that a particular fire began in a cell, that it began in that system at all, or that a specific defect existed. A UL 9540A report can also establish what gases that configuration produced, whether propagation occurred at the level tested, and what the permitting record was built on. Where the as-built battery energy storage system matches the tested configuration, a UL 9540A report is a sound reference point for expected behavior.
Origin and ignition source in a battery energy storage system fire are answered by scene examination, the physical state of the modules and the surrounding compartment, the battery management system and site data, and the sequence recoverable from that hardware.
What does evidence preservation look like after a battery energy storage system fire?
Evidence preservation after a battery energy storage system fire covers the affected modules and the adjacent unaffected ones, kept separately and handled as holding stranded energy. It also covers the enclosure with its vents, deflagration panels and suppression components as found, and battery management system logs and site monitoring data pulled before retention windows close.
After a battery energy storage system fire, the paper record matters as much as the hardware: the full UL 9540A test report with its edition and configuration, the listing and installation instructions, and the permit file.
Where are expert opinions that rely on a UL 9540A report challenged?
Opinions about a battery energy storage system fire that rely on a UL 9540A report are predictably challenged on five points: that a test method was described as a certification; that a report from one edition was read as though it carried another edition’s criteria; that the tested configuration was never compared with the installed one; that state of charge and initiation method were left out of that comparison; and that origin was inferred from test data rather than hardware.
Work that names the standard by designation and edition, states what the UL 9540A test characterized and what it did not, and keeps the installation question separate from the origin question survives those challenges. A sentence saying the system passed does not.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.