Why thermal propagation is the headline of GB 38031-2025¶
What is this: the rationale for why clause 5.2.7 b) — "no fire, no explosion" after single-cell thermal runaway — is the most consequential change in the 2025 revision.
The risk model¶
Lithium-ion battery packs fail catastrophically when a single cell goes into thermal runaway (TR) and the heat from that cell ignites adjacent cells, which ignite their neighbours, until the entire pack is involved. The energy released by a fully-propagated 50–100 kWh pack is enormous — comparable to a fuel-tank fire, but harder to extinguish because of the oxidiser carried in the cathode.
The standard targets the internal short-circuit as the initiating event. From C.1:
"After thermal runaway caused by internal short-circuiting of a single cell, the battery pack or system should not catch fire or explode."
Internal short-circuits are the realistic worst case because they:
- Are not detectable by the BMS until cell voltage starts to drop, by which time TR may already be unavoidable for that cell.
- Can be triggered by manufacturing defects (lithium plating, conductive particle contamination, separator damage), mechanical insult (bottom impact, intrusion), or thermal stress (cooling-system failure).
- Cannot be prevented at 100 % reliability across the pack lifetime, even with state-of-the-art cell quality.
The standard therefore accepts that one cell will run away and asks: does the pack design contain it?
The 2025 step-up¶
GB 38031-2020 required a 5-minute warning before any thermal event reached the cabin. It did not require that the pack survive without fire or explosion. The 2025 revision adds two new bars:
| Condition | 2020 | 2025 |
|---|---|---|
| Warning before thermal event | Required (≥ 5 min before) | Required (≤ 5 min after TR triggered) |
| No fire / no explosion at pack level | Not required | Required (5.2.7 b) 1) |
| Smoke condition in cabin / 10-min window | Not specified | Required (5.2.7 b) 3) |
This re-anchors the test: passing now requires demonstrably containing the TR event, not just warning the occupants in time to escape.
Precedent from real-world incidents¶
The 2025 revision was driven by a multi-year pattern of EV pack fires in China, with the most-cited categories being:
- Charging-related events — cells already at high SOC failing during or just after a fast-charge cycle.
- Underbody-impact events — road debris damaging the pack baseplate, with a delayed fire hours or days later. (This is also why bottom impact, 8.2.16, is a new test.)
- Spontaneous events at rest, with no trigger identified externally — i.e. internal short-circuits.
In each category, the failure mode that drove regulator attention was propagation beyond the originating cell. A single-cell failure that vents but does not propagate is not a public-safety incident; a fully-involved pack fire is.
What "no fire, no explosion" demands of pack design¶
To meet 5.2.7 b) 1), pack designs typically need some combination of:
- Inter-cell thermal barriers (mica, aerogel, ceramic-coated separators between cells or modules) to slow heat transfer from the runaway cell to its neighbours.
- Directed venting so that hot ejecta from a failing cell goes out of the pack (downward or sideways into open space) rather than into adjacent cells.
- Pack-level vent paths that open before internal pressure causes the pack housing to rupture.
- BMS detection that issues the thermal-event warning fast enough — the (a OR b) AND c runaway-confirmation rule is the lower bound on what the BMS must catch.
None of these are mandated by the standard; the standard sets the outcome and leaves the design path to the OEM. The OEM documentation package is where those design choices are recorded.
Source: GB 38031-2025, clause 5.2.7 b) (PDF p. 12) and Appendix C section C.1 (PDF p. 34).