UL9540A - Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems
UL9540A test determines the capability of battery technology to undergo thermal runaway and then evaluates the fire and explosion hazard characteristics of those battery energy storage systems (BESS) that have demonstrated a capability to undergo thermal runaway. Data generated is used to determine the fire and explosion protection required for an installation of BESS intended for installation, operation and maintenance in accordance to applied regulations. Four level of testing is conducted depending on the performance of the test and listed below in priority:
- Cell Level
- Module Level (not discussed here)
- Unit Level (not discussed here)
- Installation Level (not discussed here)
Cell level testing is conducted first and if thermal runaway cannot be induced in the cell and cell vent gas is nonflammable in air (according to ASTM E918), then no further testing is required, otherwise Module level test is required.
Module level testing is intended to force one or multiple cells to thermal runaway in order to get cell to cell propagation. In case the effects of thermal runaway are contained by a module design and cell vent gas is not nonflammable, no further testing is required, otherwise unit level testing is performed.
Belmont Scientific, Inc. (BSI) offer complete line of testing at Cell and Module level including:
- Thermal runaway test (Cell level)
- Thermal runaway propagation test (Module level)
- Volume of gas generation
- Gas composition, lower flammability limit (LFL) and burning velocity, explosion severity parameters (Pmax) through our partner lab
- External flaming and flying debris hazard
Test setup / Facilities:
BSI tested an exemplar cylindrical 18650 cell with thermal runaway initiated by flexible heater and a temperature controller. The temperature controller was set to heat the cell at an average rate of 5˚C/min to an end temperature of 200˚C and hold for 30 minutes.
During this time, if the cell vents and catches fire, it is considered thermal runaway is induced (Go). Another scenario where cells vented but did not catch fire, is considered thermal runaway could not be induced (No Go).
Test setup is shown in Figure 1 and 2. BSI has well-ventilated laboratory equipped with explosion chambers (high pressure vessel Figure 3) equipped heaters, electrical power supply, cell voltage and temperature measurement, pressure transducer and temperature controller.
Figure 1: Flexible heater and thermocouple location
Figure 2: Flexible heater and thermocouple location
Sample Test:
The cell temperature vs time plot is shown below with vent opening and thermal runaway temperature.
Figure 4: Cell temperature vs Time
Figure 3: High pressure vessel
Figure 4: Cell temperature vs Time
Table 1: Thermal Stability Test result
| Cell ID | Cell Capacity | Cell Weight | S.O.C. | Cell Voltage | Vent Open T | Thermal Runaway | Test Result |
|---|---|---|---|---|---|---|---|
| Typical | mAh | g | – | V | ˚C | ˚C | – |
| 18650 | 2500 | 44.683 | 100% | 4.182 | 139 | 181 | Vent opened and thermal runaway Induced |
An additional test (exactly same) was conducted in triplicate but inside a high-pressure vessel to measure the volume of spent gas generated and collected for gas analysis. The gas analysis is done using GC-MS and exemplar gas volume measured are summarized in Tabel 2 and gas composition summarize in Table 3 below:
Figure 3: High pressure vessel
Table 2: Thermal Runaway Vent Gas Volume Test Results
| Sample: INR21700, 4.5Ah | Weight loss(%) | Peak Vessel P (Psia) | Peak Cell T (C) | End Vessel P (Psia) | Total Volume (L) |
|---|---|---|---|---|---|
| Average | 61.4 | 179.1 | 543.5 | 54.1 | 9.4 |
| Average Deviation | 4.1 | 14.4 | 68.9 | 2.2 | 0.5 |
Table 3. Thermal Runaway Vent Gas Analysis Summary
| Component | Average | Mean Deviation | D.L |
|---|---|---|---|
| Gas Volume (L) | 9.4 | 0.5 | % v/v |
| Non-Hydrocarbon Gases | |||
| Nitrogen | 19.37 | 3.378 | 0.01 |
| Oxygen | 0.125 | 0.005 | 0.01 |
| Argon | 0.29 | 0.007 | 0.01 |
| Carbon Dioxide | 10.5 | 0.578 | 0.05 |
| Carbon Monoxide | 46 | 1.667 | 0.05 |
| Hydrogen | 19.6 | 0.978 | 0.05 |
| Hydrocarbon Gases | |||
| Methane | 3.587 | 0.309 | 0.001 |
| Ethylene | 0.17 | 0.043 | 0.001 |
| Acetylene | 0.01 | 0.002 | 0.001 |
| Ethane | 0.15 | 0.018 | 0.001 |
| Propylene | 0.044 | 0.016 | 0.001 |
| Propane | 0.007 | 0.002 | 0.001 |
| Isobutane | 0.001 | 0.0004 | 0.001 |
| n-Butane | 0.022 | 0.004 | 0.001 |
| Butene | 0.023 | 0.003 | 0.001 |
| Isopentane | nd | nd | 0.001 |
| n-Pentane | 0.003 | 0.000 | 0.001 |
| Pentenes | 0.009 | 0.003 | 0.001 |
| Hexanes + | 0.157 | 0.031 | 0.001 |
FAQ:
1. What is UL9540A?
UL 9540A is a test method developed by Underwriters Laboratories (UL) determines the capability of battery to undergo thermal runaway and then evaluates the fire and explosion hazard characteristics of battery energy storage systems (BESS). It is not a certification by itself but a testing protocol that provides data for safety evaluation. Manufacturers, integrators, and system owners use it to understand how a battery system behaves under abuse conditions and whether fire propagation is likely. The purpose is to assess the fire propagation risk of batteries at the cell, module, unit, and installation level.
2. What does UL9540A evaluate?
- How a single cell in thermal runaway might propagate to adjacent cells in a module of pack.
- Gas release, heat generation, and potential fire/explosion hazards.
- Whether the system design (venting, spacing, fire barriers, suppression systems) prevents cascading failures.
3. What is outcome of UL9540A?
Provides data-driven evidence to support safe installation of battery storage systems in residential, commercial, and utility-scale applications.
4. What is difference between UL9540 and UL9540A?
UL 9540A = Test method for thermal runaway & fire propagation risks
UL 9540 = Certification for the overall battery energy storage system
5. When Is UL 9540A Testing Required?
- To support UL 9540 certification
- To meet fire code requirements, such as NFPA 855, International Fire Code (IFC), or local AHJ (Authority Having Jurisdiction) rules
- For large-scale energy storage installations where fire propagation risks must be evaluated
6. What gases are measured during UL 9540A?
CO, CO₂, H₂, CH₄, and other flammable/ toxic gases released during thermal runaway
7. What if cells don’t undergo thermal runaway during Cell level testing?
If thermal runaway cannot be induced in the cell during cell level testing and cell vent gas is nonflammable in air (according to ASTM E918), then no further testing is required, otherwise Module level test is required.