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The 5-minute warning

The 5-minute warning rule is the headline change in GB 38031-2025 over the 2020 edition. It lives in clause 5.2.7 b) and is operationalized in Appendix C. The rule applies to the thermal propagation test (8.2.7.2) and is the single most-cited requirement in the standard for both type-approval staff and pack designers.

The rule, verbatim — clause 5.2.7 b)

b) The battery pack or system or the entire vehicle, after undergoing the thermal propagation analysis and verification as per 8.2.7.2, should:

1) No fire, no explosion;

2) Provide a thermal event warning signal, and the warning signal should be sent no later than 5 minutes after thermal runaway of the battery cell is triggered;

3) For battery pack or system-level testing, provide technical documentation indicating that smoke does not pose a danger to the passenger compartment within 5 minutes before and after the thermal event warning signal is issued; for vehicle-level testing, smoke should not enter the passenger compartment within 5 minutes before and after the thermal event warning signal is issued.

Source: GB 38031-2025 clause 5.2.7 b) (PDF page 12).

What the three sub-criteria mean

The pass criterion is conjunctive — all three must hold.

Sub-criterion Plain reading
5.2.7 b) 1) After triggering thermal runaway in one cell, the pack/system/vehicle does not catch fire and does not explode.
5.2.7 b) 2) A thermal-event warning signal is issued within 5 minutes of the triggering event (not within 5 minutes of fire or smoke — within 5 minutes of TR). The clock starts when TR is triggered in the cell.
5.2.7 b) 3) Smoke must not endanger the passenger compartment in the 10-minute window centered on the warning — 5 min before, 5 min after. At pack/system level this is supported by technical documentation. At vehicle level the criterion hardens: smoke must not enter the passenger compartment in that window.

The "trigger" event is itself defined in the test method (8.2.7.2 / Appendix C.5) — TR is considered triggered when a triggering method (needle, external heating, or internal heating plate) is applied to a centrally located cell, and TR is considered to have occurred when the (a OR b) AND c criteria are met (voltage drop > 25 % OR T at monitor reaches max operating T) AND (dT/dt ≥ 1 °C/s for > 3 s). The warning clock runs from triggering, not from confirmed TR onset.

Engineering note (non-normative): A common misreading is that the 5 minutes runs from the warning to fire/explosion. It does not. The 5 minutes is trigger → warning, and the smoke window is warning − 5 min to warning + 5 min (so it can include time before the warning was even issued). Designers planning the BMS warning logic should treat the trigger event as t = 0 and the warning latency budget as 300 s.

Appendix C — General principles

Appendix C re-states the requirement in slightly different words and applies it to the manufacturer's safety documentation:

After thermal runaway caused by internal short-circuiting of a single cell, the battery pack or system should not catch fire or explode. It should provide a thermal event alarm signal no later than 5 minutes after the thermal runaway occurs. Additionally, within 5 minutes before and after the thermal event alarm signal is issued, the smoke should not pose a danger to the occupant compartment.

Source: GB 38031-2025 Appendix C.1 (PDF page 34).

Engineering note (non-normative): Appendix C.1 says "5 minutes after the thermal runaway occurs", while clause 5.2.7 b) 2) says "5 minutes after thermal runaway of the battery cell is triggered". For a triggering method that brings TR on quickly (e.g., needle), the two are nearly identical. For slower methods (e.g., external heating to threshold), the two clocks can differ. When in doubt, use the stricter reading: trigger-to-warning ≤ 5 min.

Engineering interpretation (non-normative)

Sequence of events in the 5-minute warning rule, with the 10-minute smoke window.

sequenceDiagram
    participant Cell as Triggered cell
    participant Pack as Pack / system
    participant BMS as BMS / BCU
    participant User as Occupant
    Note over Cell: T₀ — thermal runaway triggered
    Cell->>Pack: TR detected (voltage / temp / dT)
    rect rgb(255, 245, 220)
    Note over Pack,BMS: ≤ 5 min budget to issue warning
    Pack->>BMS: Thermal event detected
    BMS->>User: Warning signal (T_warn)
    end
    rect rgb(255, 230, 230)
    Note over Pack,User: 5 min before T_warn ↔ 5 min after T_warn:<br/>smoke must NOT endanger passenger compartment<br/>(vehicle-level: must NOT enter cabin)
    end
    Pack->>Pack: No fire, no explosion required throughout

Two clocks run from T₀ (TR triggered): the warning must fire within 5 minutes, and a 10-minute smoke-safety window centred on the warning must hold.

Pack/system vs. vehicle level — the smoke criterion difference

Test level Smoke requirement
Pack or system Manufacturer provides technical documentation that smoke does not pose a danger to the passenger compartment in the ±5-min window. The pack itself is not in a vehicle, so danger is assessed analytically using the documented vehicle integration.
Vehicle level Smoke must not enter the passenger compartment in the ±5-min window. This is observed directly via the in-cabin video required by C.5.2 d).

Source: GB 38031-2025 clause 5.2.7 b) 3) (PDF page 12); Appendix C.5.2 d) (PDF page 34).

Excluded chemistries

The thermal stability requirements of 5.2.7 do not apply to nickel-hydride (NiMH) battery packs or systems:

After undergoing the thermal stability test as per 8.2.7 (excluding nickel-hydride battery packs or systems), the following requirements apply…

Source: GB 38031-2025 clause 5.2.7 (PDF page 12).

Cross-references