Thermal Propagation Testing for Energy Storage Batteries: Standards, Methods, and Safety Evaluation

Thermal propagation testing is a critical safety evaluation for lithium-ion energy storage batteries, assessing whether thermal runaway initiated in a single cell can spread to adjacent cells and escalate into fire or explosion at the module, pack, or container level. This article provides a comprehensive technical overview of thermal propagation experiments, covering applicable standards such as GB/T 36276, UL 9540A, and IEC 62933-5-2, sample requirements, trigger methods, test procedures, and acceptance criteria. It serves as a practical reference for battery manufacturers, energy storage system integrators, and accredited third-party laboratories engaged in the safety assessment of stationary and grid-scale energy storage systems.

Technical Overview

Thermal propagation, also referred to as thermal runaway propagation, describes the sequence in which a lithium-ion cell enters thermal runaway and transfers sufficient heat—through conduction, convection, radiation, or ejecta—to neighboring cells, causing them to also undergo uncontrollable self-heating. In energy storage systems (ESS), where hundreds or thousands of cells are densely packed into modules, racks, and containers, a single-cell failure can cascade into a large-scale fire or explosion if propagation is not effectively suppressed.

A thermal propagation experiment is designed to deliberately initiate thermal runaway in one cell (the trigger cell) under controlled laboratory conditions and to observe, measure, and quantify how the failure develops across the battery assembly. The experiment records critical parameters such as cell surface temperatures, onset time of adjacent cell failures, venting gas composition, smoke release, jet flames, and pressure buildup. The results determine whether the battery design incorporates adequate propagation barriers, thermal insulation, spacing, or venting strategies to contain the failure.

Thermal propagation testing has become a mandatory safety gate for stationary energy storage products in major markets. Key reference standards include GB/T 36276 and GB 44240 for lithium batteries used in electrical energy storage, UL 9540A for evaluating thermal runaway fire propagation in battery energy storage systems, IEC 62933-5-2 for grid-integrated EES safety requirements, and IEC 62619 for industrial application lithium batteries. These standards define trigger methods, instrumentation requirements, observation periods, and pass criteria at the cell, module, and unit levels.

Testing Samples

Cell-Level Samples

Cell-level testing establishes the baseline thermal runaway characteristics of the battery chemistry and form factor. Typical samples include LFP (lithium iron phosphate) or NMC (nickel manganese cobalt) prismatic, pouch, or cylindrical cells in a fully charged state, usually at 100% state of charge (SOC). The experiment measures the cell's thermal runaway onset temperature, peak temperature, self-heating rate, and vent gas composition. These data are essential inputs for designing propagation mitigation at higher assembly levels.

Module-Level Samples

Module-level samples consist of a complete battery module with representative electrical connections, cell spacing, thermal barriers, and busbars as used in the final product. One cell within the module—typically a centrally located cell surrounded by multiple neighbors—is selected as the trigger cell. The module test evaluates whether thermal runaway propagates to adjacent cells and whether fire, explosion, or reignition occurs. This is the most common configuration referenced by GB/T 36276 and UL 9540A module-level tests.

Pack, Rack, and Unit-Level Samples

For large systems, testing may extend to complete battery packs, racks, or energy storage units installed within a representative enclosure or container. Unit-level tests, as described in UL 9540A, evaluate system-level behavior including deflagration risk, fire spread between racks, wall surface temperatures, and the performance of fire detection and suppression provisions. These tests support installation-level hazard analysis and permitting for grid-scale projects.

Testing Items

Thermal propagation experiments typically cover the following evaluation items:

• Thermal runaway trigger verification: confirmation that the selected trigger method successfully induces thermal runaway in the designated cell.
• Trigger cell characterization: onset temperature, peak temperature, temperature rise rate, and duration of the thermal runaway event.
• Propagation behavior assessment: whether adjacent cells undergo thermal runaway, the propagation sequence, and the time delay between successive cell failures.
• Temperature monitoring of adjacent cells: surface temperatures of neighboring cells recorded throughout the test to verify temperature limits defined by the standard.
• Fire and explosion observation: occurrence of open flame, jet fire, deflagration, or explosion during and after the test.
• Gas generation analysis: composition and volume of vented gases, including hydrogen, carbon monoxide, carbon dioxide, and hydrocarbons such as methane and ethylene.
• Smoke and ejecta characterization: quantity and trajectory of smoke, particulates, and ejected material.
• Housing integrity and pressure response: deformation, rupture, or pressure rise within the enclosure.
• Electrical behavior: voltage drop of the trigger cell and adjacent cells, short circuits, and insulation resistance after the event.
• Observation period verification: monitoring for reignition, delayed failures, or secondary events for a defined period (commonly 1 hour to 24 hours after the event).

Testing Methods

Trigger Methods

Several trigger methods are accepted by relevant standards, and the choice depends on the product design and the specific standard invoked:

• Nail penetration: a steel nail of specified diameter is driven into the trigger cell at a controlled speed to induce an internal short circuit and rapid localized heating.
• Overheating (thermal abuse): the trigger cell is heated by external heaters or an oven until thermal runaway onset, allowing precise control of the heating rate.
• Overcharge: the trigger cell is charged beyond its specified limits until forced overcharge leads to thermal runaway.
• External heating element: a film or block heater attached to the cell surface provides a defined thermal stimulus, commonly used in UL 9540A cell-level testing.

Test Setup and Instrumentation

The test specimen is installed in a ventilated, explosion-protected chamber equipped with fire suppression capability and gas exhaust treatment. Thermocouples are attached to the surfaces of the trigger cell, all adjacent cells, and representative distant cells, as well as to barrier materials and enclosure walls. High-speed and standard video cameras record the event from multiple angles. Gas analyzers or gas sampling bags connected to laboratory chromatography equipment quantify vent gas composition. Voltage monitoring channels track the electrical response of each cell group throughout the experiment.

Test Procedure

A typical thermal propagation test follows this sequence:

• Sample preparation: cells or modules are cycled as specified, charged to the required SOC, and stabilized at the specified ambient temperature.
• Sensor installation: thermocouples, voltage taps, and gas collection devices are installed and verified.
• Trigger initiation: the selected trigger method is applied to the designated cell, and data acquisition begins at high sampling frequency.
• Event monitoring: temperatures, voltages, gas concentrations, and visual phenomena are continuously recorded during the propagation event.
• Post-event observation: the specimen is monitored for the observation period specified by the standard to check for reignition or delayed propagation.
• Teardown and analysis: the specimen is disassembled to document damage patterns, and collected data are compiled into a comprehensive test report.

Acceptance Criteria

Acceptance criteria differ by standard and market. Common requirements include: no fire or explosion at the module or pack level following the trigger event, no thermal runaway propagation to adjacent cells, or—if propagation is permitted—no escape of flames or hazardous ejecta outside the enclosure, no deflagration hazard, and no failure of fire barriers within the specified observation period. Temperature thresholds on adjacent cells, such as limits on peak surface temperature or maximum temperature rise, are frequently specified as quantitative evidence of propagation containment.

Applications

Thermal propagation testing supports a wide range of engineering and commercial objectives across the energy storage industry:

• Product certification: demonstrating compliance with GB/T 36276, GB 44240, UL 9540A, IEC 62933-5-2, and related safety standards required for market access in China, North America, Europe, and other regions.
• Design optimization: comparing thermal barrier materials, cell spacing, cooling strategies, and venting structures to improve propagation resistance during research and development.
• Grid-scale project approval: providing the hazard analysis data required by authorities having jurisdiction, fire departments, and insurers for utility-scale ESS installations.
• Container and enclosure validation: verifying that battery compartments, deflagration vent panels, and fire suppression provisions perform as intended under worst-case single-cell failure.
• Quality assurance: investigating field failures and validating corrective actions for manufacturing defects or design changes.
• Transport and storage safety: supplementing abuse testing data required for the safe logistics of large-format lithium batteries.

FAQ

What is the difference between thermal runaway testing and thermal propagation testing? Thermal runaway testing evaluates the behavior of an individual cell when forced into failure, whereas thermal propagation testing evaluates whether that failure spreads to neighboring cells and larger assemblies. Propagation testing is performed at the cell, module, pack, or unit level and focuses on cascade behavior and containment.

Which standards govern thermal propagation experiments for energy storage batteries? The principal standards include GB/T 36276 and GB 44240 in China, UL 9540A in North America, and IEC 62933-5-2 and IEC 62619 in the international IEC framework. Each standard defines its own trigger methods, instrumentation, and pass criteria, so the applicable standard depends on the target market and product type.

What state of charge is used for testing? Most standards require testing at a high state of charge, commonly 100% SOC, because a fully charged cell stores maximum electrochemical energy and represents the worst-case condition for thermal runaway severity and propagation risk.

How long does a thermal propagation test take? The active test event may last from minutes to several hours depending on the design and trigger method, but a complete program—including sample preparation, instrumentation, trigger verification, post-event observation, and teardown—typically requires several days. Full multi-level test campaigns across cell, module, and unit levels can extend over several weeks.

What constitutes a passing result? Passing criteria are standard-specific. Common acceptance outcomes include no fire or explosion, no propagation to adjacent cells, surface temperatures of neighboring cells remaining below defined limits, no external flame or ejecta escaping the enclosure, and no reignition during the observation period. Detailed pass requirements must always be confirmed against the invoked standard.

Can thermal propagation testing be performed in-house? Manufacturers with specialized abuse test facilities can perform development-level testing, but certification results generally must be generated by accredited third-party laboratories with the required scope, explosion-rated chambers, gas analysis capability, and accreditation to ISO/IEC 17025.