Power Battery Thermal Propagation Testing: Standards, Methods, and Pass Criteria

Power battery thermal propagation testing evaluates how thermal runaway initiated in a single battery cell spreads—or is contained—through a module or pack, and it has become a decisive safety gate for electric vehicle homologation in China and beyond. With the release of GB 38031-2025 upgrading the acceptance criterion from a five-minute occupant warning to "no fire, no explosion," manufacturers increasingly rely on accredited third-party laboratories to validate cell selection, pack architecture, and thermal barrier design. This article explains the technical principles, applicable standards, sample requirements, trigger methods, observation criteria, and typical applications of thermal propagation testing for traction batteries.

Technical Overview

Thermal propagation (thermal diffusion) testing is a system-level safety evaluation that determines whether thermal runaway, deliberately triggered in one cell of a lithium-ion battery pack, propagates to adjacent cells and ultimately leads to fire or explosion of the entire pack. Thermal runaway is an exothermic, self-accelerating failure mode initiated by internal short circuit, mechanical abuse, or overheating, during which separator collapse, electrode-electrolyte reactions, and cathode decomposition release large amounts of heat and flammable gas within seconds.

The engineering objective of the test is not simply to observe a fire, but to quantify the capability of the pack design—cell spacing, thermal insulation barriers, venting channels, heat spreaders, and the battery management system (BMS)—to interrupt the chain reaction. A well-designed pack should either confine the failure to the initiating cell or provide occupants with sufficient warning time before any hazardous event.

The test is mandated by major regulatory and voluntary standards frameworks, including:

• GB 38031-2020 – "Electric vehicles traction battery safety requirements," which introduced China's compulsory thermal propagation test with a five-minute warning-before-fire criterion.
• GB 38031-2025 – the revised national standard that tightens the criterion to "no fire, no explosion" for the battery pack or system, applicable to new type approvals from its implementation date.
• UN GTR 20 / UN Regulation No. 100.02 – the global technical regulation on electric vehicle safety, which addresses thermal propagation as a recommended national requirement.
• ISO 6469-1 – electrically propelled road vehicle safety, including thermal propagation protection guidance.
• UL 2580 and IEC 62660-3 – battery pack and cell safety standards covering propagation-related abuse scenarios.
• SAE J2464 / SAE J2929 – abuse response characterization methods widely used in research and development programs.

Testing Samples

Thermal propagation tests are conducted on complete battery-level assemblies, because propagation behavior depends on the module and pack structure rather than the cell chemistry alone. Typical test samples include:

Battery Pack or System Level (Primary Sample)

A complete traction battery pack or system as installed in the vehicle, including the enclosure, internal electrical connections, thermal management components, and venting provisions. BMS communication lines are connected so that fault detection and warning functions can be evaluated in parallel with the mechanical and thermal response. This is the sample level required by GB 38031 and UN GTR 20 type approval programs.

Module Level (Screening Sample)

Battery modules are tested during design development to compare cell spacing, mica or aerogel insulation sheets, and inter-cell thermal barriers before committing to full pack validation. Module tests are faster and more diagnostic, allowing engineers to measure cell-to-cell propagation velocity and peak temperatures under controlled conditions.

Cell Level (Pre-Study Sample)

Single cells and short cell strings are used to characterize the trigger sensitivity, gas generation rate, and heat release profile of the chemistry. These pre-studies establish the thermal runaway onset temperature and confirm criteria—voltage drop threshold and temperature rise rate—that will later be used to verify successful initiation at pack level.

Sample Preparation Requirements

• Samples are typically adjusted to a high state of charge (commonly 100% SOC or the maximum SOC permitted by the BMS), which represents the worst-case stored-energy condition.
• Thermal preconditioning, such as soaking the sample at elevated ambient temperature, may be applied to represent severe operating environments.
• Samples must be structurally and functionally representative of the production configuration, including coolant fill state or a justified equivalent.
• Test orientation should reflect the intended in-vehicle installation orientation unless a more severe orientation is deliberately selected.

Testing Items

A complete thermal propagation evaluation comprises several distinct measurement and judgment items:

• Thermal runaway confirmation – verification that the initiating cell has actually entered thermal runaway, judged by a defined voltage drop (for example, a decline of a specified percentage relative to the initial voltage), a surface temperature rise rate equal to or greater than 1 °C/s, or direct observation of ejecta and smoke.
• Propagation behavior – mapping of cell surface temperatures, propagation sequence, and propagation velocity from the initiating cell to neighboring cells.
• Hazard outcome classification – recording of fire, explosion, or jet flame from any location of the pack; explosion is typically defined as a rapid pressure and energy release causing rupture or projected fragments.
• Occupant warning performance – measurement of the alarm signal issued by the BMS (thermal event warning, smoke detection, gas detection) and verification that the warning precedes any fire or explosion hazard.
• Gas and ejecta analysis – optional characterization of vented gas composition and volume, supporting enclosure pressure design and cabin infiltration studies.
• Thermal barrier performance – temperature difference measured across insulation layers to quantify the effectiveness of inter-cell protection materials.

Testing Methods

The general test sequence follows: sample instrumentation, trigger method application, thermal runaway confirmation, and hazard observation over a defined observation period.

Instrumentation and Setup

• Thermocouples are attached to the initiating cell and to adjacent cell surfaces, barrier surfaces, and representative locations across the pack.
• Voltage monitoring channels record the initiating cell or cell group, and where possible the full pack, at high sampling frequency.
• High-speed and standard video cameras record all faces of the sample from safe distances; explosion-proof enclosures or bunkered test cells protect personnel.
• Gas sensors and sampling probes may be positioned to detect vented electrolyte decomposition products.
• The test is performed in a ventilated, explosion-rated laboratory environment with remote data acquisition.

Trigger Methods

Standards permit one or more initiation methods, provided the chosen method reliably induces thermal runaway in the target cell without mechanically destroying the whole assembly:

• Needle penetration – a steel nail of specified diameter is driven into the initiating cell at a controlled speed, creating an internal short circuit. This method is direct but may be unsuitable for some pack architectures due to accessibility constraints.
• External heating – a film heater, heating plate, or heating rod raises the temperature of the initiating cell at a controlled rate until thermal runaway onset. This is the most widely used method for pack-level testing because it is minimally invasive and reproducible.
• Overcharge trigger – the initiating cell is overcharged beyond its specified limit until internal faults cause thermal runaway; suitable when electrical access to a single cell is available.
• External short circuit of a single cell – used in specific configurations to induce localized overheating.
• Thermal ramp of the whole assembly – used in research contexts to study propagation under oven-like boundary conditions.

Where the pack structure prevents direct access, engineers may drill a minimal access port for the heater or nail, provided the modification does not compromise the propagation path under evaluation.

Confirmation of Thermal Runaway

The test is valid only if thermal runaway of the initiating cell is confirmed by one of the following, consistent with the criteria agreed with the customer and the applicable standard:

• Monitoring voltage of the initiating cell shows a drop of a defined magnitude relative to its initial value.
• The maximum surface temperature rise rate of the initiating cell reaches or exceeds 1 °C/s.
• Observation of ejecta, smoke, or flame is recorded by direct evidence.

Observation Period and Pass Criteria

• Under GB 38031-2020, the pack must provide a thermal event warning to the occupant at least five minutes before any fire or explosion, or must exhibit no fire and no explosion at all during the observation period.
• Under GB 38031-2025, the requirement is tightened: after thermal runaway is triggered, the pack or system must show no fire and no explosion during the observation period, which is defined as at least two hours or until all monitored temperatures and voltages have stabilized, whichever is later.
• Under UN GTR 20 / UN R100.02, a propagation requirement may be applied by contracting parties following the regulation's in-vehicle safety outcome orientation.
• Pass judgment is documented with synchronized temperature, voltage, and video records, and the entire observation is witnessed and reported in a test report usable for type approval or supplier qualification.

Applications

• Vehicle type approval and market access – demonstrating compliance with GB 38031 for new energy vehicles sold in China, and with UN R100-aligned requirements in other jurisdictions.
• Battery supplier qualification – automotive OEMs require pack and module suppliers to pass thermal propagation tests as a condition of design release and mass production approval.
• Thermal barrier material development – comparative testing of aerogel blankets, mica sheets, fire-retardant coatings, and composite heat shields to quantify temperature attenuation between cells.
• Structure innovation validation – verification of cell-to-pack (CTP) designs, large-format cells, and blade-type formats where reduced cell spacing raises propagation risk.
• Energy storage and off-highway adaptation – stationary storage containers, construction machinery, and two-wheeler battery systems increasingly adopt equivalent propagation criteria, referencing standards such as GB/T 36276, UL 9540A-related cell and module evaluation philosophies, and IEC 62619 frameworks.
• Failure analysis and recall support – reproduction of field incidents under controlled conditions to identify root causes and validate corrective actions.

FAQ

What is the difference between thermal propagation and thermal runaway?

Thermal runaway is the failure of an individual cell—an uncontrolled self-heating process. Thermal propagation describes the spread of that failure from one cell to neighboring cells. A battery can experience thermal runaway in a single cell without propagation if the pack design successfully isolates the failure.

Which trigger method is most commonly used?

External heating is the most common method at pack level because it is controllable, reproducible, and minimally invasive. Needle penetration remains valuable for cell and module characterization, while overcharge triggering is used when single-cell electrical access is available.

How does GB 38031-2025 change the pass criterion?

The 2020 edition allowed fire or explosion as long as occupants received a warning at least five minutes beforehand. The 2025 edition upgrades the requirement to no fire and no explosion after thermal runaway is triggered, effectively forcing pack-level containment of the failure event.

How long does a thermal propagation test take?

The active trigger and hazard observation phase typically takes from tens of minutes to several hours, while the full observation period required by GB 38031-2025 extends to at least two hours after trigger or until thermal and electrical stabilization. Including instrumentation, chamber setup, and reporting, laboratories usually schedule one to several days per sample.

Can a pack pass if smoke is released?

Under the tightened GB 38031-2025 framework, fire and explosion are prohibited, while venting and smoke must not create harm to occupants as defined by the standard's outcome requirements. Detailed acceptance of smoke behavior depends on the applicable edition and the agreed test plan.

Is thermal propagation testing required for energy storage batteries?

Stationary storage systems follow a different but related standards ecosystem, such as GB/T 36276 in China and UL 9540A-style escalation testing internationally. Many developers voluntarily adopt EV-style propagation criteria to demonstrate containment, and accredited third-party laboratories can align test plans to the target market's regulatory framework.