UL 9540A: 2025 Test Standard Introduction

On March 12, 2025, UL Solutions officially released ul 9540a ed5: 2025. After a year and a half of discussions and debates, 28 revision topics were discussed, with 1 unresolved and 27 topics agreed upon and released.

This revision clarifies more testing details and terms. Taking module thermal testing as an example, the following important changes were made:

1. It is clarified that if there is an insulating layer or insulation sheet between cells, it needs to be removed before placing them into the heating plate. An example diagram has been added:

2. The definition of "thermal runaway propagation" is added: The energy released by one or more cells experiencing thermal runaway, which then transfers to other cells causing them to experience thermal runaway, without any additional triggering mechanisms (such as heating plates). Simply venting is not considered thermal runaway.

3. The stopping conditions for module thermal runaway tests are clarified. If the cells have not achieved thermal runaway propagation, testing should be repeated until other cells are forced into thermal runaway. If thermal runaway propagation between cells cannot be achieved by adding additional trigger cells, then it can be concluded that thermal runaway propagation will not occur.

Friendly Reminder: Currently, the number of additional trigger cells required to verify whether thermal runaway can be triggered is not clearly defined for module testing (the original draft suggested 50% of cells, but this was removed in the final version).

In addition, there are many other detailed revisions, such as clarifying definitions for residential and non-residential use, modifying test setups for module and cell-level testing, and adding module, cell, and installation-level test reports, as well as thermocouple placement. For more details, please contact MCM, and we will provide you with a comprehensive explanation.

Related Standards Updates

UL 9540B introduces fire (ignition) testing for residential energy storage products;

NFPA 855 Energy Storage Installation Standard will release a new version in 2026, adding requirements for large-scale fires;

UL 9540, released on March 7, 2025, includes Appendix H - Residential Energy Storage Installation Guidelines.

Looking at the revision trends of the North American energy storage standard system, it is evident that local fire departments and AHJ (Authorities Having Jurisdiction) are more focused on the safety of residential energy storage and the consequences and impacts of energy storage thermal runaway, rather than simply whether thermal runaway occurs during testing. Manufacturers need to be aware of this and focus on the inherent safety of the cells while using external designs to meet safety requirements.