New Class L created specifically for lithium-ion battery fires
Iain Hoey
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Why lithium-ion batteries now sit in a new fire class
BS ISO 3941:2026 has been published with a revision that adds a new fire classification for lithium-ion (Li-ion) battery fires.
BSI confirmed the update reflects the growing use of Li-ion batteries across the built environment, from large energy storage systems to electric vehicles and personal mobility devices.
The standard classifies fires according to the nature of the material undergoing combustion.
The latest revision introduces Class L as a dedicated category for Li-ion cells and batteries where no metallic lithium is present.
Specific behaviours recognised in the revision
The revised standard identifies a set of behaviours associated with Li-ion battery fires that differ from existing Class A, B and electrical fire categories.
These include higher energy density, leading to faster heat release and rapid fire growth.
It also highlights thermal runaway, where failure can propagate from cell to cell.
The document notes the release of toxic and flammable gases during venting.
It refers to a potential explosion risk from constrained build-up of vented gases.
The revision points to limited access caused by battery construction.
It lists projectile risk from expelled cells and burning electrolyte.
It also identifies stranded electrical energy that can create a risk of delayed re-ignition.
The standard states: “such a classification is particularly useful in the context of equipment for fire protection and firefighting”.
Implications for risk assessment and fire protection planning
The new Class L category is intended to support those responsible for assessing, insuring and controlling fire risk in settings such as battery energy storage systems (BESSs) and EV charging stations.
The revision sets out that detection and suppression strategies will need to take account of the specific hazards associated with Li-ion batteries.
It also flags the need to manage re-ignition risk and review emergency response plans accordingly.
Fire risk assessments are expected to identify whether Li-ion batteries are present, their type and quantity, and how they are stored and charged.
The standard notes that Class L risks will be relevant across a wide range of premises, including residential buildings, offices, commercial warehouses, industrial sites and transport facilities.
BS ISO 3941:2026 is the UK implementation of ISO 3941:2026, prepared with input from Technical Committee FSH/2, Fire extinguishers.
It replaces the 2007 edition, with the main changes being the addition of the lithium-ion classification and a new Clause 4 setting out hazard information for the fire classes.
These changes formalise how lithium-ion battery fires are categorised and described within the fire classification framework.