The latest version of BS ISO 3941:2026 has been published with a revision to include a new fire classification for lithium-ion (Li-ion) battery fires

The ongoing increase in use of Li-ion batteries across the built environment and society as a whole, from large energy storage solutions to e-vehicles and personal mobility devices, has led to a change in the fire classification standard to reflect the distinct electrochemical hazard they pose.

BS ISO 3941:2026 Classification of fires categorises the different kinds of fires defined by the nature of the material undergoing combustion, with the latest revision introducing Class L as a dedicated fire classification involving Li-ion cells and batteries where no metallic lithium is present.

The new classification was deemed necessary due to the specific behaviours shown by Li-ion batteries in a fire incident. These include:

  • higher energy density, leading to faster heat release and rapid fire growth
  • thermal runaway, where failure can propagate from cell-to-cell
  • toxic and flammable gas release during venting
  • potential explosion risk from constrained vented gas build-up
  • limited access due to battery construction
  • projectile risk from expelled cells and burning electrolyte
  • stranded electrical energy leading to potential delayed re-ignition.

These behaviours have been recognised as specific to Li-ion batteries and can lead to fires that are distinct from the Class A, B, and electrical fires already covered by the standard.

As stated in the standard, “such a classification is particularly useful in the context of equipment for fire protection and firefighting”, and will help those responsible for assessing, insuring, and controlling fire risk in areas such as battery energy storage systems (BESSs) and EV charging stations.  

The new Class L will require specific consideration to be given to the above risks when designing detection and suppression strategies and systems, as well as how to manage any re-ignition risk and re-assess emergency response plans.

Fire risk assessments will need to establish if any Li-ion batteries are present and, if so, both the type and quantity, as well as their storage and charging arrangements. 

With the wide proliferation of Li-ion battery-powered devices, the risk posed by Class L fires will need to be considered and assessed across a wide range of premises. These will include residential buildings, offices, commercial warehousing, industrial sites, and transport facilities where Li-ion batteries may be found.

The revised standard is available here.