ISO Created a Fire Class for Lithium-Ion Batteries, and the Newest US Battery Code Chapter Does Not Cover Stationary Storage

ISO 3941:2026 introduces Class L, a dedicated fire classification for lithium-ion battery systems that contain no metallic lithium. Its scope covers grid-scale battery energy storage, electric vehicle packs, uninterruptible power supply installations, industrial lithium-ion banks and consumer devices. A failing lithium-ion cell had previously been described using categories built for other fuels, and Class L is the first classification written around what such a cell actually does.

The three behaviors. The classification is constructed around failure modes that existing classes do not describe. First, internal electrochemical propagation that continues independently of any external fuel. Second, venting of hydrogen, carbon monoxide, hydrocarbons and hydrogen fluoride, which can accumulate to explosive concentrations. Third, re-ignition hours after visible flame has stopped.

The consequence drawn from those behaviors is that conventional sprinkler and clean-agent suppression is treated as insufficient on its own. Cell-level thermal management is now expected alongside it, on the reasoning that heat transfer between cells continues regardless of conditions in the surrounding room.

What the classification requires. Fire risk assessments must explicitly identify lithium-ion systems and evaluate propagation pathways, gas concentration thresholds and re-ignition potential. That is a documentation obligation rather than an engineering specification. Class L does not name an extinguishing agent, a suppression system, a separation distance or a permitting condition. It gives fire marshals, insurers and building owners a vocabulary that separates lithium-ion from everything else in an occupied building’s fire plan.

A classification arriving through ISO does not, by itself, bind an authority having jurisdiction in the United States. Taxonomies acquire force through downstream instruments: adopted fire codes, extinguisher rating standards and insurance policy forms. The status of each of those in the US market is a separate question from the publication of the standard, and it is not settled by it.

The code that did move. The International Code Council published Lithium-Ion Battery Safety in the Built Environment on July 23, 2026, an early release of Chapter 43 of the forthcoming 2027 International Fire Code. The early release was issued specifically so that local jurisdictions could adopt or reference the provisions ahead of full publication of the code. Chapter 43 covers the manufacture, storage, use and recycling of lithium-ion batteries across a broad sweep of building activity.

It does not apply to stationary energy storage systems. Those remain governed by the referenced NFPA standard, NFPA 855, with UL 9540A large-scale fire test data as the underlying evidence base.

The result is a scope boundary that will be easy to misread at a permitting counter. The most visible US code movement on lithium-ion this year addresses batteries in a building as cargo, inventory and equipment, and stops short of the category that is wired into the electrical system. Jurisdictions that pre-adopt Chapter 43 will have a new chapter on the shelf that does not govern the wall-mounted or floor-standing battery in front of them.

Two clocks running at different speeds. NFPA and ICC operate multi-year revision cycles with public comment. Insurance carriers rewrite property forms on their own schedules. Neither code body has Class L in an adopted document, and the classification therefore has no US enforcement pathway at present.

That asymmetry suggests the first place Class L appears with commercial consequence in the United States is a carrier’s policy form rather than a code section: an exclusion, a warranty condition or a rate factor referencing the classification. That is an inference about sequencing rather than a reported development, though the underlying mechanism is not hypothetical. Underwriting has already functioned as the binding constraint on commercial battery projects in cases where the permitting path was clear.

What the classification does to documents. The practical content of Class L is a list of questions an assessment must answer. Propagation pathways, gas concentration thresholds and re-ignition potential are the quantities that a UL 9540A large-scale fire test is designed to measure at cell, module and unit level. A project with that test data can answer a Class L framed question with test results. A project without it can answer with a datasheet, which is a different category of answer.

This is why the classification matters commercially before it matters legally. It does not change what a system must be. It changes what a system must be able to demonstrate, and it hands the parties asking (fire marshals, insurers, building owners) a named standard to ask under.

Checkable from here. Three developments would resolve the open questions. Whether NFPA takes up Class L in a future revision cycle. Whether the ICC addresses the classification when the full 2027 International Fire Code publishes, and whether the stationary-storage exclusion in Chapter 43 survives that publication unchanged. And whether a US commercial property carrier writes the letter into a policy form before any code body adopts it.

For a commercial building in the United States evaluating stationary storage today, none of this alters the approval route. That route runs through NFPA 855 and UL 9540A, as it did before ISO 3941:2026 published, and as it will continue to do through the 2027 code cycle given Chapter 43’s stated exclusion. What has changed is the vocabulary available to everyone reviewing the file.

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