UL 9540A
Immersion Cooling Test Results
EticaAG has completed the first ever UL 9540A test for an Immersion Cooled Battery Energy Storage System.
The Problem was... They couldn't get it to burn.
What is UL 9540A?
UL 9540A is the test method used to evaluate thermal runaway behavior in battery energy storage systems (BESS). It intentionally forces a battery cell into thermal runaway and measures how the system responds.
The test evaluates:
Whether thermal runaway propagates from cell to cell or module to module
Peak and total heat release rate
Flammable and total gas generation
Smoke production and fire behavior
External surface temperatures
The resulting data is used by Authorities Having Jurisdiction (AHJs), fire officials, insurers, engineers, and project developers to support permitting decisions. UL 9540A testing is required for most BESS installations in the United States and plays a critical role in system siting and approval.
Why Thermal Runaway Propagation Matters
Thermal runaway begins with a single cell failure.
If the heat generated by the initiating cell transfers to adjacent cells, it can trigger a chain reaction known as propagation. When propagation occurs, multiple cells enter thermal runaway in sequence, significantly increasing total heat release, gas generation, and fire intensity.
Propagation Behavior Impacts:
Site layout, footprint, and spacing requirements
AHJ approvals and permitting timelines
Overall project risk profile
A single-cell event is fundamentally different from a cascading multi-cell failure. Systems that limit or prevent propagation reduce peak heat release, minimize gas production, and simplify siting requirements.
WITH & WITHOUT IMMERSION FLUID
UL 9540A Pre-Test Setup
A pre-test using UL 9540A setup and conditions was conducted at a Nationally Recognized Testing Laboratory (NRTL) to document how immersion cooling impacted thermal runaway data.
- One designated initiating cell was fitted with external film heaters (Cell 13)
- Heater ramped at 6°C per minute
- Minimum 2 charge/discharge cycles and 100% SOC at test start
- Measured temperatures, gas, smoke, and heat release rate.
Two Configurations Evaluated
Each configuration used the same forced initiation approach to evaluate thermal runaway behavior and whether propagation could occur.
Module Not Filled with Fluid
Module Filled with Circulating Immersion Fluid
Test Results
Without Immersion
Thermal runaway was initiated in the module using external film heaters on the designated initiating cell. Thermal runaway occurred at approximately 70 minutes, shown by a rapid temperature spike in the initiating cell and a corresponding rise in nearby cell temperatures.
With Circulating Immersion
The module filled with circulating dielectric immersion fluid was tested using the same forced initiation method. In this configuration, the heater was unable to induce thermal runaway. Even after 115 minutes of heating, the initiating cell did not vent.
The temperature curves remained comparatively stable. Circulating immersion fluid absorbed and transported heat away from the initiating cell, preventing escalation to thermal runaway. Without thermal runaway, propagation could not occur.
Without Immersion
Initiating Cell Temperatures
With Immersion
Initiating Cell Temperatures
Without Immersion
Surrounding Cell Temperatures
Thermal Runaway @ 70min
With Immersion
Surrounding Cell Temperatures
Thermal Runaway Not Observed
Why Was a Test Conducted Without Immersion Fluid?
Immersion Prevented the Propagation Event Required for UL 9540A
UL 9540A requires a propagation event so heat release, gas generation, and fire behavior can be measured during a cell-to-cell failure. With circulating immersion fluid, thermal runaway could not be induced, so the required propagation event could not be generated in that configuration.
To complete UL 9540A testing, the module was tested without immersion fluid. This modified test configuration does not represent the final commercial product, which operates with cells submerged in circulating dielectric immersion fluid.
The certified test documentation states:
The [EticaAG] battery module is equipped with immersion cooling technology that all cells are immersed in the dielectric liquid to prevent cell-to-cell thermal runaway propagation. In order to meet the test criteria in UL 9540A, the module level immersion cooling liquid needed to be removed. The tested configuration is therefore not representative of the final product configuration. This artificial test condition was done in order to achieve the UL 9540A requirement of creating cell-to-cell propagation.
Official UL 9540A Test Results (No Immersion)
EticaAG also completed official UL 9540A testing using the same core test setup and initiation approach used in the earlier pre-test. The official UL 9540A test was conducted without immersion fluid to establish thermal runaway and enable propagation measurement in accordance with the standard’s requirements. Results are summarized below for both the module-level and unit-level tests.
Propagation was only achievable when immersion cooling fluid was removed.
UL 9540A Module Level
Module After Test
After testing, the EticaAG module showed minimal external damage
Initiating Cell Temperature
Initiating Cell 13 spiked to over 500°C and entered thermal runaway
Adjacent Cell Temperature
Propagation was limited to a single cell (Cell 8)
UL 9540A Unit Level
Testing Cabinet Setup
Two cabinets were setup and measured
Initiating Cell Temperature
Initiating Cell 13 spiked to over 600°C and entered thermal runaway
Adjacent Cell Temperature
Propagation was limited to a single cell (Cell 8)
UL 9540A Testing Note:
This data represents a modified non-commercial configuraiton, where immersion cooling fluid was removed specifically to allow thermal runaway propagation to occur and be measured under UL 9540A requirements.
What This Means for Energy Storage Safety & Siting
The UL 9540A data demonstrates a clear difference in system behavior depending on whether immersion cooling is active.
When the module was tested with circulating immersion fluid, thermal runaway could not be induced. Without a thermal runaway event, propagation could not occur.
For Authorities Having Jurisdiction and project developers, the takeaway is straightforward:
Active immersion cooling prevents thermal runaway escalation
Circulating fluid limits heat transfer between cells
Propagation cannot be achieved with immersion cooling
UL 9540A evaluates worst-case failure conditions. The test results show that when immersion cooling is active, the system resists thermal runaway and prevents failure from spreading from one cell to another.
That performance is built into the design of every EticaAG module.
