The Off-Gas Trade-Off for Lithium Battery Safety
The study of a lithium-ion battery (LIB) system safety risks often centers on fire potential as the paramount concern, yet the benchmark testing method of the day, UL 9540A, is keen to place fire risk as one among at least three risks, alongside off-gas and explosion. In this blog, we’ll shift some focus towards off-gas and explosion risks to understand which stakeholders care most about off-gas and explosion potential and why.
The Context of UL9540A
Underwriter Laboratories (UL) released its 4th and current edition of UL9540A “Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems Standard” in 2019. It is the current safety standard to which so many important other codes and standards — like the International Fire Code, California Fire Code, NFPA’s 855 “Standard for the Installation of Stationary Energy Storage Systems” — point. UL 9540A is especially relevant when a lithium-ion battery (LIB) system project aims for tighter spacing between units/groups or larger energy capacities of battery units/groups.
UL 9540A outlines and limits how a recognized UL9540A test laboratory is to observe and quantify the initiation and propagation of a fire condition, called thermal runaway (TR), which can be defined as the “self-heating of an electrochemical system in an uncontrollable fashion… [that] progresses when the [batery] cell’s heat generation is at a higher rate than it can dissipate, potentially leading to off-gassing, fire, or explosion” (NEC 2023, 2020).
The finer-grained details and data we can capture about the thermal conditions at which TR begins in a given product’s cell, the better. Each test level— cell, module, unit, installation— establishes and carries over important quantifiables. The more data we can repeatably collect regarding the “when”, “at which temperatures,” and “under which thermal ramp rates” the propagation of TR occurs at each test-level executed for a given LIB product under test, the better we can assess, design for, and manage TR risk of that LIB product.
Looking for an explanation of how UL 9540A test method relates to and differs from the UL 9540 listing? We’ve got you covered.
Off-Gassing begins before and continues through TR Onset
One common misconception about LIB technology is that there is nothing to worry about if we avoid the onset of TR. Offgassing begins during degradation phase, prior to a given TR onset temperature. The progression of a LIB cell from safe operation to degradation to uncontrollable TR can be visualized by plotting cell temperature over time, as shown below.
How much flammable off-gas is too much at this cell vent? UL 9540A establishes a quantifiable threshold for unsafe levels of flammable off-gassing (see Section 7.4 of UL 9540A entitled “Cell vent gas composition test”). Here, a cell is driven into TR within a pressurized vessel to collect component gases at venting. Using gas chromatography, the gas composition and mixture are determined.
This mix of flammable gases is then synthesized in a new test protocol and the Lower Flammability Limit (LFL) for the synthetic gas mixture is determined, both at ambient temperature and at the cell vent temperature. LFL is often reported as a percentage of air volume, indicating the concentration at which a volume of air containing the flammable gas can be ignited.
As a safety precaution, the current UL 9540A stipulates that whatever the LFL observed at the cell-level for a given mixture of off-gas, the performance goals at the unit-level portion of the test include a threshold of a quarter of that LFL by the total volume of relevant space. This threshold is restated in the below example 9540A report from TUV, as a unit-level performance criterion: “The concentration of flammable gas does not exceed 25% LFL in air for the smallest specified room installation size.”
A history of LIB’s involved in fires, regardless of “primary” cause
A brief review of battery fires reveals that TR alone may not be the main or even sufficient cause for many large BESS fires. While there is limited data about the root causes of battery fires, a few publicly available databases exist such as this one from EPRI. The limited data that is available reveals that most BESS fires have stemmed from external causes or broader system design issues such as external shorts or water ingress, rather than from thermal runaway propagation.
As mentioned above, it’s important to remember that LIB technology will produce flammable vent gases prior to reaching thermal runaway. These vent gases may then become a fuel source in the event of an external failure, which can then lead to ignition, explosion, or deflagration.
All lithium-ion chemistries to date carry inherent flammable off-gassing risk.
It is less common, but not uncommon, to hear proponents of LIB products claim that TR is not of concern for some LIB chemistries. Unfortunately, this isn’t an accurate depiction.
“Since all commercial Lithium Ion cell technologies available at time of this writing are able to be driven into thermal runaway (and emit flammable gas in that process), Lithium Ion cells commercially available today do not meet the CELL and MODULE level performance criteria. As a result, Lithium Ion BESS must always complete at least three test levels (CELL, MODULE, and UNIT LEVEL).”
The Sustainable Energy Action Committee, Informational Bulletin on the UL 9540 Safety Standard and UL 9054A Test Method (June 2024)
Lithium iron phosphate (LiFePO4) batteries carry higher TR onset temperatures than many others named for various cathode materials. This is, indeed, an advantageous cathode choice that offers a wider thermal range of operation before TR onset. But that doesn’t preclude LFP batteries from being involved in fires.
To state it again: the flammable off-gassing that occurs and propagates at the module level in all commercially available LIB to date, including LFP technologies, oftentimes becomes a first fuel source for many fire or deflagration events caused by failures external to the battery unit like water ingress, external shorts, etc. We who are actively working for more and more energy storage deployment to modernize the grid and accelerate the green energy transition are better served with accurate and candid communication, even around a potentially worrisome set of topics like fire and explosion risk management.
That’s not to say that lithium-ion batteries are prohibitively dangerous. All energy resources pose some inherent safety risk, and few established categories are as young and quickly evolving as LIB. With our wider community of stakeholders’ perspectives in tow, we will continue to improve how we measure, evaluate, and account for those risks in our product and system design.
Protect those who protect us under duress.
So where do we stand: more lithium BESS installs? A resounding “yes, and.” More energy storage is vital to the energy transition ahead of us, but we also must do so as safely as possible for all stakeholders, of which there are many. In discussion with several former fire professionals and current AHJ representatives explosion risk is a major concern, and oftentimes a larger one than fire.
From the perspective of fire service professional training protocols, only two states of operation a lithium battery: normally operational or actively on fire. Because everything in between poses a risk for explosion. Some LIB manufacturers have devised engineering solutions that include actively lighting off-gasses at LFL, sacrificing the enclosure, and burning all the batteries up to avoid fastly worsening catastrophic explosion hazard potential.
One SME offered the motivation shared by fire professionals and the AHJ’s that advocate for them as follows: “From a first responder perspective we need to know what that safe level of engagement is and distance to be able to even put an incident command post. Once the unit or system is on fire, at least we know it’s not going to blow up, which sounds like a strange win.”
“We’d rather have a known ongoing battery fire than an unmonitored off-gassing event ripe for deflagration or detonation.”
NFPA explosion terminology check:
To discuss managing off-gassing risk management for lithium-ion batteries in greater detail and to meet some key stakeholders around UL9540A, join us in Austin, TX on October 2nd and 3rd for our first inaugural Education Summit. Register today!
