UN Regulation 100 - Battery Design

UN Regulation 100 covers uniform provisions concerning the approval of battery electric vehicles with regard to specific requirements for construction, functional safety, and hydrogen emission. In March 2025, the 195th session of the UNECE World Forum for the Harmonisation of Vehicle Regulations (WP.29) adopted the 05 series of amendments (Rev.5), the most significant update to the regulation since its introduction, entering into force on 26 September 2025. The new series focuses specifically on thermal propagation within the Rechargeable Electrical Energy Storage System (REESS) and introduces a system-level safety demonstration approach.

Published by: UNECE World Forum for the Harmonisation of Vehicle Regulations (WP.29)

Current edition: 05 series of amendments (Rev.5) — adopted March 2025, entered into force 26 September 2025

Supersedes: Rev.3 (04 series) — 2022; Rev.2 remains referenced in some existing EU type approvals

Status: Mandatory for UN type approval in contracting parties. Compliance with the 05 series becomes mandatory for new type approvals from 1 September 2027 and for all type approvals from 1 September 2029. Existing Rev.3 approvals remain valid for vehicles already on the market until the transition deadline.

Scope

The sections and tests cover:

Electrical: external short circuit, over-charge, over-discharge, over-current
Mechanical: vibration, mechanical shock, mechanical integrity
Environmental: thermal shock and cycling, over-temperature
Abuse: fire resistance, thermal propagation

Key Updates — 05 Series of Amendments (Rev.5)

The 05 series introduces mandatory thermal propagation testing for the REESS and reorganises how vehicle manufacturers demonstrate system-level safety:

Mandatory thermal propagation requirement: where a thermal runaway occurs in one battery cell within the REESS, the regulation now requires manufacturers to demonstrate that the resulting thermal propagation to adjacent cells does not endanger vehicle occupants — giving them time and warning to safely evacuate.
Four alternative trigger test methods: manufacturers may choose from four physical test methods to induce thermal runaway in a single cell (e.g. internal short circuit, overheating, mechanical penetration, or overcharge-type triggers), each of which can be carried out at complete vehicle level, complete REESS level, or REESS sub-system level, as appropriate to the design.
System safety demonstration approach: Rev.5 shifts the standard toward a broader system-level safety case focused on controlling thermal runaway, rather than isolated component pass/fail criteria alone.
New REESS installation section: a dedicated new section addresses the approval of REESS installation within the vehicle, covering how the battery system integrates with the wider vehicle structure and safety systems.

This timing is notable for manufacturers selling into both the EU and US markets: the mandatory compliance date for the 05 series (1 September 2027 for new type approvals) lines up closely with the US FMVSS 305a mandatory compliance date, also 1 September 2027. Both regulations converge on REESS thermal-propagation testing at a similar point, meaning pack designs validated for one will likely need to be assessed against the other’s specific test protocol rather than assumed equivalent.

China Market Cross-Reference

China’s mandatory GB 38031-2025 (effective 1 July 2026) already requires a 2-hour no-fire/no-explosion thermal propagation result — a materially stricter bar than the qualitative “no propagation endangering occupants” language in UN R100 Rev.5. Designers targeting global platforms should treat GB 38031-2025 as the more conservative thermal-propagation design target of the two.

Disclaimer: this is not a definitive guide, just an indication and it is important that you always go back to the source for legislation.