Thermal Propagation in Traction Batteries. Why Is Section 6.15 of UN ECE R100 Becoming a Key Requirement for EV Manufacturers?

What Is Thermal Propagation in Traction Batteries, What Does Section 6.15 of UN ECE R100 Require, and Why Does R100.05 Increase the Importance of Real-World Laboratory Testing?

Daniel Trzciński, Edyta Małecka

8/7/20265 min read

Thermal Propagation in Traction Batteries.

The failure of a single cell within a traction battery can be enough to initiate a chain of events leading to a full vehicle fire. If the thermal management system, module design, and Battery Management System (BMS) fail to contain the event, a localized incident can rapidly escalate into a system-wide safety hazard. Against the backdrop of increasingly stringent type-approval requirements, including the upcoming UN ECE R100.05 series of amendments, thermal propagation is becoming one of the most critical engineering challenges facing the e-mobility industry. How can manufacturers prepare for these new requirements?

What Is the Difference Between Thermal Runaway and Thermal Propagation?

To fully understand the regulatory requirements, it is essential to distinguish between two fundamental phenomena occurring within REESS (Rechargeable Electrical Energy Storage Systems).

The UN ECE R100 Regulation precisely defines these terms:

Thermal runaway means an uncontrolled increase in cell temperature caused by exothermic reactions occurring within the cell.
Thermal propagation means the sequential occurrence of thermal runaway within a REESS, triggered by the thermal runaway of a cell within that system.

For engineers, this leads to a critical question: when a failure occurs, can the battery contain the hazard within a single section, or will it allow the event to spread to adjacent cells through heat conduction, thermal radiation, hot gas flow, flames, or the influence of the module's structural design?

The Critical 5 Minutes. What Exactly Does Section 6.15 of UN ECE R100 Mean?

From the perspective of passenger safety, it is not only the occurrence of a fire itself that matters, but above all the response time of the system. Section 6.15 of UN ECE R100 (applicable to REESS with flammable electrolyte) focuses precisely on this aspect.

The main objective of the requirement is stringent: vehicle occupants must not be suddenly exposed to a hazardous environment caused by thermal propagation triggered by an internal short circuit in a single cell. In simplified terms, the passenger safety logic is as follows:

the event is initiated in a single cell,
sensors and the BMS register deviations,
the system generates a warning in order to alert the vehicle occupants sufficiently in advance.

In accordance with the requirements of the Regulation, at least 5 minutes must elapse between the generation of this warning and the occurrence of a hazardous situation in the passenger compartment, such as fire, explosion, or smoke. This time is absolutely necessary to enable safe evacuation.

To achieve this, the requirement covers two fundamental areas. First, the REESS or the vehicle system must provide a signal that activates the warning in the vehicle. Second, the manufacturer must demonstrate the functions or characteristics of the cell, the REESS, or the vehicle that protect occupants. This means that, among other things, it is necessary to provide a risk mitigation analysis, a description of physical systems, operating logic, and documents confirming the effectiveness of the solutions applied.

This is important because R100 does not reduce thermal propagation solely to a single temperature measurement. The Regulation requires a system-level approach: the cell, module, enclosure, gas pathways, sensors, BMS, warning strategy, and REESS installation in the vehicle must operate as one coherent whole.

A Revolution in Testing: What Does Amendment R100.05 Change?

The upcoming 05 series of amendments to UN ECE Regulation No. 100 (document ECE/TRANS/WP.29/2025/41) drastically changes the rules of the game. However, the most important change is not merely the addition of another test. R100.05 shifts the focus from a safety declaration alone toward verification of the system’s actual behavior.

The key new element is Annex 9K, which formalizes the thermal propagation test procedure. The document specifies four permissible methods for initiating thermal runaway in a laboratory environment:

local, rapid external heating,
use of an internal heater,
mechanical penetration,
laser triggering.

In practice, for most modern Li-ion traction batteries used in category M and N vehicles, the default route for demonstrating compliance will be physical testing in accordance with Annex 9K. Although the Regulation provides for specific cases where a risk management analysis may be applied, in accordance with paragraph 6.15.4, these should not be treated as a standard alternative to physical testing.

This means that manufacturers should already anticipate at the design stage how their solution will be verified: at vehicle level, complete REESS level, or subsystem level.

Why Are Computer Simulations Alone Not Enough?

Thermal modelling and simulations, such as CFD, are essential at the design stage. They make it possible to analyse design variants, predict heat transfer, and reduce the number of costly prototype iterations. Regulation R100 itself allows technical documentation demonstrating the effectiveness of protective measures, including validated simulation methods.

The problem arises when simulation becomes the sole evidence of safety. In accordance with the GTR guidelines for electric vehicle safety, the development of requirements must take into account validation of the entire system. Thermal propagation involves phenomena that are difficult to fully reproduce, such as local cell damage, the operation of safety vents, or actual measurement delays.

This is why physical laboratory testing has particular value: it does not replace engineering analysis, but makes it realistic. Under test conditions, it allows the actual behaviour of the system to be recorded: the BMS response, temperature profiles, event dynamics, the behaviour of neighbouring cells, and whether thermal runaway remains limited to the initiating cell or leads to propagation to subsequent elements of the system.

These observations demonstrate that the outcome is determined not only by cell chemistry, but also by module design, heat transfer pathways, and the system’s ability to provide early warning, thereby verifying in practice the assumptions that cannot be fully reproduced on a computer screen.

Checklist: How Should EV Manufacturers Prepare for Certification?

When designing a module, a battery pack, or an entire electric vehicle, thermal propagation should be considered as a complex system-level event already at the drawing board stage. The documentation and architecture should include:

Safety function map: thermal separators, barriers, enclosure, gas channels, cooling system, and BMS.
Event detection logic: temperature, voltage, current, rate-of-change thresholds, and alarm conditions.
Warning concept: a clear system for notifying the vehicle user.
Assessment of the test level: a decision on whether the test will be performed at vehicle level, complete REESS level, or subsystem level.
Supporting documentation: evidence that the subsystem test result is representative of the behaviour of the complete REESS.
Validation plan: comparison of physical test results with simulation data.

This approach is fully aligned with the direction of R100.05, where verification of thermal propagation includes initial documentation, precise selection of the initiation method, determination of the test level, and strict pass or fail criteria for the test.

Thermal propagation is not merely a type-approval requirement. It is the ultimate maturity test for a traction battery design, showing whether a single event will remain a local incident or become a threat to the entire vehicle.

Section 6.15 of UN ECE R100 already requires manufacturers to demonstrate protection of vehicle occupants against the effects of thermal propagation. The R100.05 series of amendments strengthens this area by introducing a more clearly defined verification process and a greater role for physical testing. For manufacturers and engineers, this means one thing: it is worth designing the REESS in such a way that its safety can not only be described, but also confirmed in a controlled laboratory test.

Are you developing a module, battery pack, or electric vehicle? It is worth planning the thermal propagation assessment already at the design stage. The DLP team supports manufacturers in traction battery safety testing, interpretation of UN ECE R100 requirements, and preparation of the data required for the type-approval process. Contact us to plan the verification and move smoothly through the conformity assessment process.

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