Thermal Runaway Testing for New Energy Batteries: Standards, Methods, and Safety Evaluation

Thermal runaway testing is a critical safety evaluation for lithium-ion batteries used in electric vehicles and energy storage systems, assessing how a battery cell reacts to abuse conditions and whether thermal propagation can be contained. This testing is mandated by international standards such as GB 38031, UL 9540A, IEC 62660, and SAE J2464 to prevent fire and explosion hazards. In this article, we provide a comprehensive overview of thermal runaway test samples, testing items, methodologies, and applications across the new energy industry.

Technical Overview

Thermal runaway is a self-accelerating exothermic failure mechanism in lithium-ion batteries, initiated by internal short circuits, mechanical deformation, overcharging, or external heating. Once triggered, the exothermic decomposition of the separator, electrolyte, cathode, and anode materials generates heat faster than it can be dissipated, causing a rapid temperature rise, gas venting, and potentially fire or explosion.

Thermal runaway testing evaluates three key aspects of battery safety: the ease of triggering thermal runaway (trigger threshold), the behavior of the cell once thermal runaway occurs (temperature, pressure, gas composition), and whether thermal propagation to adjacent cells can be prevented or delayed long enough for occupants or operators to escape. With the growing deployment of electric vehicles and grid-scale energy storage systems, regulators worldwide have incorporated thermal runaway and propagation requirements into mandatory safety standards, making this testing essential for market access and product certification.

Modern thermal runaway tests are conducted in purpose-built explosion-proof chambers equipped with heating plates, nail penetration devices, data acquisition systems, gas analyzers, and high-speed imaging. Accredited third-party laboratories typically perform these tests to demonstrate compliance with regulatory and industry standards.

Testing Samples

Cell-Level Samples

Individual lithium-ion cells in cylindrical, prismatic, or pouch formats represent the fundamental unit of thermal runaway evaluation. Cell-level tests determine the trigger threshold, maximum surface temperature, temperature rise rate, and vent gas characteristics. Fresh cells, aged cells, and cycled cells may all be evaluated, as degradation can significantly lower the thermal runaway trigger point.

Module-Level Samples

Battery modules, consisting of multiple cells interconnected with busbars and enclosed in mechanical structures, are tested to evaluate thermal propagation between adjacent cells. Module-level tests verify whether heat insulation materials, spacing designs, and venting channels effectively delay or prevent cascade failure.

Pack-Level Samples

Complete battery packs, including battery management systems (BMS), cooling circuits, and protective housings, are evaluated for system-level safety. Pack tests assess whether the thermal event warning function provides sufficient escape time, typically five minutes or more as required by certain regulations, and whether fire or explosion occurs outside the enclosure.

Energy Storage System Units

For stationary battery energy storage systems (BESS), rack-level and cabinet-level units are tested to evaluate fire propagation behavior in an installation-representative configuration. These tests support fire safety planning, installation spacing requirements, and emergency response protocols.

Testing Items

Thermal runaway testing programs typically include the following evaluation items:

• Thermal runaway trigger testing using heating, nail penetration, or overcharge methods
• Thermal runaway characteristic measurement, including trigger temperature, maximum temperature, and temperature rise rate
• Thermal propagation testing between cells, modules, and pack enclosures
• Vent gas analysis for composition, volume, flammability, and toxicity
• Smoke, fire, and explosion behavior observation
• Pressure buildup and enclosure integrity assessment
• Thermal event warning time validation for BMS alarm functions
• Post-test examination of cell rupture, electrolyte leakage, and residue analysis

Detection criteria for confirming thermal runaway typically combine a critical temperature rise rate (for example, 1 degree Celsius per second sustained), an abrupt voltage drop, and observable venting or fire. Most standards require at least one temperature criterion plus one voltage criterion to be satisfied simultaneously.

Testing Methods

External Heating Method

The most widely accepted trigger method uses an external heat source, such as a flexible film heater or heating plate, attached to the cell surface. The heater raises the cell temperature at a controlled rate until thermal runaway occurs or a specified upper limit is reached. This method is the preferred approach in GB 38031 for electric vehicle battery packs and in UL 9540A for energy storage cells, because it is controllable, repeatable, and representative of real-world overheating scenarios.

Nail Penetration Method

A steel nail of specified diameter is driven into the cell at a controlled speed to create an internal short circuit. Nail penetration directly simulates internal fault conditions, though results can vary with nail diameter, penetration depth, and speed. It is frequently used in research and development comparisons and referenced in standards such as IEC 62660-3 and SAE J2464.

Overcharge Trigger Method

The cell is charged beyond its specified voltage limit at a defined current until thermal runaway occurs or the safety limit is reached. Overcharge testing evaluates the robustness of cell chemistry and current interrupt devices under charging system failures.

External Short Circuit Method

The cell terminals are connected through a low-resistance circuit to induce high current discharge and internal heating. This method evaluates the effectiveness of protection devices such as fuses, current interrupt devices, and positive temperature coefficient elements.

Thermal Propagation Test Procedure

In propagation testing per GB 38031 and UL 9540A, one cell in a module or pack is deliberately driven into thermal runaway using an approved trigger method. Instrumentation monitors temperatures across adjacent cells, cell voltages, gas concentrations, and enclosure conditions. The test evaluates whether propagation occurs, how quickly it spreads, and whether the system provides adequate warning before hazardous conditions develop. For BESS installations, UL 9540A defines a four-level test sequence progressing from cell characterization through module, unit, and installation-level testing.

Instrumentation and Data Acquisition

Tests employ thermocouples and infrared imaging for temperature mapping, high-speed cameras for event documentation, gas chromatography and mass spectrometry for vent gas composition analysis, and pressure transducers for enclosure loading. All data are recorded at high sampling rates to capture the rapid dynamics of thermal runaway events.

Applications

Thermal runaway testing serves multiple purposes across the new energy value chain:

• Regulatory compliance and market access for electric vehicle batteries under GB 38031, UN ECE R100, and IEC 62660
• Safety certification of stationary energy storage systems under UL 9540A, UL 1973, IEC 62619, and GB/T 36276
• Research and development optimization of cell chemistry, separator materials, and electrolyte additives
• Design validation of thermal barriers, aerogel insulation, and venting structures in pack engineering
• Fire safety planning, installation classification, and emergency response protocols for BESS facilities
• Quality control and batch consistency verification in battery manufacturing
• Failure analysis following field incidents or warranty claims

Automotive OEMs, battery manufacturers, energy storage integrators, and aerospace and marine battery developers all rely on thermal runaway test data to demonstrate product safety, secure certifications, and refine thermal management designs.

FAQ

What is thermal runaway in a battery?

Thermal runaway is an uncontrolled, self-accelerating exothermic reaction inside a battery cell. When internal temperatures exceed a critical threshold, the separator melts, electrodes decompose, and the electrolyte reacts exothermically, producing heat, flammable gases, and pressure faster than the cell can dissipate them, potentially resulting in fire or explosion.

Which standards govern thermal runaway testing?

Key standards include GB 38031 for electric vehicle battery safety in China, UL 9540A for thermal runaway fire propagation in energy storage systems, IEC 62660-3 for propulsion cells, IEC 62619 for industrial cells, SAE J2464 for abuse testing, UN ECE R100 for vehicle approval, and GB/T 36276 for energy storage batteries. The applicable standard depends on the target market and application.

What trigger methods are accepted by major standards?

External heating is the most universally accepted method due to its controllability and repeatability. Nail penetration and overcharge are accepted as alternative triggers in several standards. GB 38031 permits heating, penetration, or overcharge, while UL 9540A primarily specifies heating-based initiation with procedure flexibility based on cell characterization results.

What are typical pass criteria for thermal propagation tests?

For electric vehicle batteries under GB 38031, the system must either prevent thermal runaway propagation entirely or provide a thermal event warning at least five minutes before fire or explosion hazards endanger occupants. For BESS under UL 9540A, criteria include surface temperature limits, flaming duration limits, and re-ignition behavior at the unit level.

How many samples are required for thermal runaway testing?

Sample requirements vary by standard and test level. Cell characterization under UL 9540A typically requires multiple cells to establish trigger conditions statistically, while propagation tests may require complete modules or packs. Accredited third-party laboratories confirm exact sample quantities based on the applicable standard and product configuration.

Can aged or cycled batteries be tested for thermal runaway?

Yes, and it is increasingly recommended. Aged cells often exhibit lower trigger thresholds and different gas compositions compared to fresh cells. Evaluating cells after representative cycle aging provides more realistic safety data for in-service conditions and supports second-life battery applications.

Why is vent gas analysis important?

Vent gas analysis identifies the composition and volume of gases released during thermal runaway, including hydrogen, carbon dioxide, carbon monoxide, and hydrocarbons. This data is essential for explosion hazard assessment, ventilation system design, fire suppression selection, and emergency responder safety planning.