A battery does not have to be burning to become an explosion hazard. That is one of the most important—and often misunderstood—facts about lithium-ion Battery Energy Storage Systems. During thermal runaway, a failing cell can release a mixture of gases and vapours. If those gases accumulate inside a container or other enclosed space, reach a flammable concentration, and encounter an ignition source, the event can shift from a battery fire hazard to an explosion or deflagration hazard. That distinction changes the entire safety strategy. Our previous articles examined thermal runaway, BESS Fire Detection and BESS Fire Suppression. But there is another layer that cannot be treated as an extension of those problems: BESS explosion risk.
A suppression system may control flames without eliminating accumulated flammable gas. A BMS may detect abnormal battery behaviour without measuring the atmosphere inside the enclosure. Ventilation may dilute gases under one operating condition but become inadequate during a rapidly developing event.
And an enclosure designed to contain a fire can become a pressure hazard if gases accumulate and ignite inside it.
The industry’s experience—from McMicken to Moss Landing—has demonstrated why this distinction matters. Current standards and testing methodologies are now increasingly focused on understanding the behaviour of accumulated battery gases, deflagration and enclosure response rather than treating every BESS incident simply as a fire.
For India, this is particularly relevant as the country’s new BESS safety framework introduces requirements covering explosion protection, ventilation, gas detection and fire safety for applicable installations.
The question is no longer whether a BESS can catch fire.
It is:
What happens when the fire has not yet started—but the gases capable of creating one have already accumulated?
How Does a Battery Create an Explosion Hazard?
The pathway begins with cell failure.
When a lithium-ion cell experiences severe electrical, thermal or mechanical abuse, it can enter thermal runaway. The cell’s temperature rises rapidly, internal reactions accelerate and gases are generated.
Those gases can include combustible components such as hydrogen, carbon monoxide and hydrocarbons, alongside other decomposition products.
The exact composition is not universal. It depends on factors including:
- cell chemistry;
- state of charge;
- cell design;
- electrolyte formulation;
- temperature;
- failure mechanism; and
- the stage of thermal runaway.
UL Solutions’ UL 9540A methodology specifically evaluates the composition and flammability of gases at cell level, and at larger scales examines heat and gas release and the potential for ignition, deflagration and re-ignition.
The critical sequence is therefore:
Cell failure
↓
Thermal runaway
↓
Gas generation
↓
Gas accumulation
↓
Flammable concentration
↓
Ignition
↓
Deflagration / explosion
Not every battery failure reaches the final stage. But when these conditions align, the hazard can change dramatically.
Fire and Explosion Are Not the Same BESS Hazard
This distinction deserves much more attention. A fire is primarily a combustion event.
An explosion or deflagration involves rapid combustion and pressure development within a confined or partially confined volume.
In a BESS enclosure, the difference is critical.
If gases escape gradually into the atmosphere and burn at a controlled rate, the consequences can be very different from a situation where gases accumulate inside a sealed container and ignite almost simultaneously.
The latter can produce:
- rapid pressure rise;
- flame projection;
- structural damage;
- flying debris;
- secondary fires; and
- danger to emergency responders.
UL Solutions’ large-scale deflagration research describes vented BESS deflagrations as capable of producing flaming, pressure waves and projectiles around the enclosure.
That is why BESS explosion risk needs its own engineering assessment.
A project can have a sophisticated fire suppression system and still have an explosion hazard if combustible gases are allowed to accumulate.
The McMicken Lesson: The Fire Was Not the Only Danger
The 2019 McMicken incident in Arizona remains one of the industry’s most important case studies.
The facility was a relatively small 2 MW/2 MWh lithium-ion BESS, commissioned in 2017.
On 19 April 2019, smoke was reported from the building. First responders arrived and investigated the incident. Several hours later, when the enclosure door was opened, a catastrophic failure occurred and firefighters were injured. APS subsequently commissioned a detailed technical investigation.
The DNV GL investigation concluded that the event involved a cascading thermal-runaway process followed by an explosion. APS’s published material identifies the incident as a critical source of lessons for future BESS safety.
Importantly, there was disagreement over aspects of the initiating mechanism: LG Chem subsequently disputed DNV GL’s proposed root cause.
That uncertainty itself is worth noting.
But there is less ambiguity about the larger safety lesson:
A BESS enclosure can become an explosion hazard when gases generated during battery failure accumulate and subsequently ignite.
That means emergency responders need to know what is happening inside the container before opening doors or entering the enclosure.
This is one reason gas detection, remote assessment and controlled response are becoming increasingly important components of BESS explosion risk management.
Why Opening the Door Can Become a Dangerous Moment
Imagine a container containing damaged batteries. There may be no large visible flame. There may be smoke. There may be an unusual smell. Inside, however, combustible gases may be accumulating. If the enclosure remains closed, the gas mixture may stay within the container. Opening a door can suddenly alter the pressure, introduce fresh oxygen and potentially expose the mixture to an ignition source. This does not mean that opening every BESS container inevitably causes an explosion. It means that emergency procedures must recognise the possibility of accumulated combustible gases.
McMicken demonstrated exactly why this matters.
The explosion occurred several hours after the initial smoke report and shortly after responders opened the enclosure. Four firefighters were injured in the event.
That changed how the industry thinks about BESS emergency response.
The question is not simply:
“Is there a fire?”
It becomes:
“What atmosphere is inside the enclosure?”
Off-Gassing Is the Bridge Between Fire and Explosion
This is where our earlier BESS Fire Detection article connects directly with this one.
During thermal runaway, cells can release gases before a large visible fire develops.
That creates a potential window for detection.
But gas detection has to be designed for the right purpose.
A detector intended to identify a high concentration of combustible gas is not necessarily equivalent to a detector designed to identify early battery off-gassing.
Likewise, a carbon-monoxide detector may provide useful information but should not automatically be treated as a universal indicator of every battery failure mode.
NFPA technical work has specifically discussed combustible-gas concentration reduction using ventilation and monitoring, while noting the role of CO detection in identifying gases produced during battery thermal runaway.
This leads to an important engineering principle:
Detecting the gas is only the first step.
The system must then determine:
- How much gas?
- Where is it?
- How quickly is the concentration changing?
- What ventilation is available?
- Has the concentration entered a dangerous range?
- What should the system do next?
Ventilation Is Not Simply “More Air”
Ventilation is often discussed as though its purpose is straightforward:
Gas detected → increase airflow → problem solved.
Real BESS systems are more complicated.
Ventilation must be designed around:
- enclosure volume;
- gas-generation rate;
- gas composition;
- gas density;
- equipment layout;
- rack geometry;
- airflow paths;
- vent location;
- ambient conditions;
- HVAC operation;
- electrical equipment;
- ignition sources.
And the objective matters.
There is a difference between:
normal ventilation
and
emergency combustible-gas concentration reduction.
A system designed only to keep equipment within normal operating temperatures may not be adequate to control a rapidly developing gas-release event.
NFPA technical proposals have explored increased ventilation rates following detection of combustible gas or carbon monoxide, explicitly treating ventilation and detection as part of a critical safety system.
But ventilation itself can create another question:
What happens to the gas once it leaves the container?
The gas cannot simply be moved from a dangerous location to another dangerous location.
Discharge points, nearby equipment and ignition sources all matter.
Why a Sealed Container Can Become a Pressure Problem
Containers are attractive for BESS deployment because they provide modularity, weather protection and a controlled environment.
But enclosure design creates an important trade-off.
The container must provide sufficient protection against external conditions while also managing internal hazards.
If combustible gases accumulate and ignite, pressure can rise extremely rapidly.
An enclosure that is not designed for that pressure may suffer structural failure.
That is why explosion protection can involve deflagration venting, explosion prevention or other engineered measures, depending on the hazard assessment and applicable standards.
NFPA guidance identifies NFPA 68 for deflagration venting and NFPA 69 for explosion prevention systems.
The design objective is not necessarily to make an enclosure completely explosion-proof.
It may instead be to:
prevent ignition, reduce the probability of a flammable atmosphere, safely relieve pressure, or control the consequences of deflagration.
Those are different strategies.
Explosion Venting Is Not a Simple Hole in the Container
This is another area where simplistic BESS design can become dangerous.
A vent has to be engineered.
The size, location, pressure characteristics and interaction with internal equipment all matter.
Current NFPA technical work specifically identifies parameters such as:
- gas explosion characteristics;
- vent opening pressure;
- allowable reduced pressure;
- effective gas volume;
- distance between battery modules; and
- distance between modules and vent locations.
The same technical work warns that dense battery-module arrangements can influence turbulence and explosion propagation and that vents placed without accounting for internal geometry may not adequately control overpressure.
This is significant.
A 20-foot shipping-container shell may look like a standardised product.
But the inside is not standardised.
Rack arrangement, module density, cable routing, HVAC equipment and other obstructions can influence how gases move and how pressure develops.
So explosion protection has to be designed for the actual enclosure.
UL 9540A Is Becoming More Important for Explosion Risk
UL 9540A has historically been associated strongly with thermal-runaway fire propagation. But the methodology increasingly addresses the explosion side of the problem too. At the cell level, testing examines gas composition and flammability. At module and unit levels, it examines gas release and the potential for ignition or deflagration.
The sixth edition, published in March 2026, introduces a more demanding installation-level scenario that includes ignition of vented gases and a post-deflagration condition for assessing enclosure design and related fire-protection performance.
This represents a major shift in the way BESS safety is being evaluated.
The industry is moving from:
“Does thermal runaway propagate?”
toward:
“What happens when the gases produced by that thermal runaway interact with the complete installation?”
That is a much harder question.
2026: The Industry Is Testing the Explosion, Not Just Talking About It
The move toward large-scale explosion testing is particularly important.
UL Solutions has conducted large-scale deflagration experiments at Sandia National Laboratories using a modified 20-foot intermodal container.
The testing examined severe-case deflagrations and measured the explosion dynamics relevant to BESS enclosure safety. UL Solutions says the work is intended to support improved methodologies for evaluating pressure, venting and other explosion hazards.
And in June 2026, UL Solutions launched evaluation services specifically aimed at helping BESS manufacturers and developers address fire and deflagration hazards under applicable NFPA requirements.
This is an important market signal.
Explosion protection is moving away from generic industrial assumptions and toward BESS-specific evidence.
Moss Landing Shows the Hazard Does Not End When Flames Disappear
The January 2025 Moss Landing incident adds another dimension.
The facility’s Moss 300 system had approximately 100,000 lithium-ion batteries, and EPA reported that about 55% were damaged during the fire.
The fire began on January 16, went out on January 18 and experienced a flare-up on February 18.
EPA also monitored for hydrogen fluoride and particulate matter during the emergency response because lithium-ion battery fires can produce hazardous gases.
The continuing response demonstrates something important about BESS explosion risk:
A damaged battery can remain a hazard after the primary fire has ended.
EPA continues to oversee the safe removal of damaged batteries because batteries affected by the fire may be unstable and could catch fire again during subsequent work. Thermal cameras are being used to identify temperature rises that could indicate renewed thermal runaway.
The incident therefore reinforces a broader principle:
Post-fire battery management is still battery hazard management.
Why Fire Suppression Cannot Solve Explosion Risk Alone
BESS Fire Suppression can be valuable for controlling flames and heat.
But suppression does not automatically remove the gases already released by damaged cells.
A container could theoretically have:
Excellent fire suppression + dangerous gas accumulation
at the same time.
That is why the safety architecture needs separate consideration of:
- Fire
- Can flames be controlled?
- Thermal runaway
- Can cell-to-cell propagation be limited?
- Gas
- Can combustible gases be detected and managed?
- Explosion
- What happens if those gases ignite?
- Pressure
- Can the enclosure safely manage the resulting event?
A system that answers only the first question is incomplete.
India’s BESS Safety Framework Is Now Addressing Explosion Protection
This issue is becoming particularly relevant for India.
The Central Electricity Authority’s 2026 amendment to the Measures Relating to Safety and Electric Supply Regulations creates a dedicated BESS safety framework that comes into force on 1 April 2027.
Among other provisions, the framework addresses:
- explosion protection;
- forced ventilation;
- thermal management;
- BMS monitoring;
- smoke, gas, heat and flame detection;
- automatic fire suppression;
- emergency shutdown; and
- fire-safety auditing.
That combination is important.
It recognises that a battery container cannot be treated simply as an electrical room with a fire extinguisher.
The final framework requires explosion protection for applicable battery containers and addresses ventilation as part of the safety architecture.
For developers, that means the BESS explosion risk assessment needs to become a front-end engineering question, rather than something added after the battery containers have already been selected.
What Should Developers Ask About BESS Explosion Risk?
Before approving a BESS design, developers should be asking much more than:
“Does the container have an explosion vent?”
Ask:
- What gases does the battery generate during thermal runaway?
- And what evidence supports that answer?
- At what concentration does the safety system respond?
Where are the gas sensors installed? - Why there?
- What happens when gas is detected?
- Alarm?
- Shutdown?
- Ventilation?
- Isolation?
- What happens if ventilation fails?
- Is there a secondary protection strategy?
- What happens if power is lost?
- Does the explosion-protection system remain functional?
- Where does vented gas go?
- Could it reach an ignition source?
- Where does a deflagration vent discharge?
- Could it expose personnel or neighbouring equipment?
- Has the actual enclosure configuration been tested?
Not merely the battery cell.
- What does the UL 9540A evidence actually demonstrate?
- Cell?
- Module?
- Unit?
- Installation?
- What happens after an event?
- How are damaged batteries isolated, monitored and removed?
These questions turn BESS explosion risk from a compliance checkbox into an engineering discipline.
The Danger of Treating Every BESS as the Same
A major challenge for the industry is that “lithium-ion BESS” is not one uniform technology.
Two systems can both use lithium-ion batteries and still have very different:
- cell chemistry;
- cell format;
- module design;
- rack arrangement;
- cooling systems;
- enclosure volumes;
- gas-release characteristics;
- ventilation systems.
Therefore, explosion data from one BESS cannot automatically be transferred to another.
This is one reason system-specific testing matters.
UL Solutions’ work on large-scale deflagration testing specifically considers enclosure geometry and internal rack obstructions because those physical characteristics can influence explosion behaviour.
For India’s rapidly expanding BESS market, this could become a major procurement issue.
A certificate may demonstrate that a product or system has been evaluated.
It does not automatically mean that every possible project configuration behaves identically.
What Does a Good Explosion-Protection Strategy Look Like?
There is no single universal design.
But a robust architecture typically combines several layers.
1. Prevent
Good cell manufacturing, thermal management, BMS protection and electrical controls reduce the probability of initiating failure.
2. Detect
BMS, temperature and gas detection can identify abnormal conditions.
3. Isolate
Affected battery sections can be electrically isolated where the system design permits.
4. Control the atmosphere
Ventilation or other engineered measures can reduce the probability of reaching dangerous combustible-gas concentrations.
5. Prevent ignition
Potential ignition sources need to be considered and controlled.
6. Protect the enclosure
Explosion prevention or deflagration venting can be designed according to the hazard.
7. Contain the event
Spacing and passive protection can limit consequences for neighbouring systems.
8. Protect responders
Emergency procedures must account for the possibility of gas accumulation and delayed ignition.
That is a very different philosophy from simply installing a fire alarm.
The Five-Minute Problem Becomes a Gas Problem
Our previous article asked what happens during the first five minutes after abnormal battery behaviour is detected.
For explosion risk, another question should be added:
How much combustible gas can accumulate before the system acts?
The answer depends on the battery, enclosure and ventilation.
This is why response time alone is not enough.
Suppose a detector responds quickly but the ventilation system cannot remove the gas quickly enough.
The system may still approach a dangerous condition.
Conversely, a ventilation system may have enormous capacity but activate too late because detection was inadequate.
The safety system therefore needs to consider:
Gas generation rate + detection threshold + response time + ventilation rate + enclosure volume + ignition probability
That is the real BESS explosion equation.
The Next BESS Safety Benchmark May Be Pressure, Not Flames
The industry has spent years asking:
How far can a BESS fire spread?
That remains important.
But increasingly, another question is emerging:
What pressure does an enclosure experience if accumulated gases ignite?
That is why large-scale deflagration research matters.
UL Solutions’ Sandia testing is aimed at measuring precisely these explosion dynamics, while emerging standards work is considering BESS-specific parameters for deflagration venting.
This could eventually influence:
- container design;
- vent sizing;
- site separation;
- equipment layout;
- responder exclusion zones;
- gas-detection strategies;
- emergency procedures.
The industry’s safety vocabulary is expanding.
Fire spread is no longer enough.
Explosion consequences have to be understood too.
So, How Serious Is BESS Explosion Risk?
The wrong answer would be:
“Every BESS is an explosion waiting to happen.”
That is neither technically accurate nor useful.
The opposite answer—“modern BESS cannot explode”—would be equally irresponsible.
The actual risk depends on the system.
A credible BESS design identifies how gases can be generated, how they can accumulate, how they can be detected, what prevents ignition, how ventilation works and what happens if ignition nevertheless occurs.
The industry’s incident history proves why this matters.
McMicken demonstrated the consequences of accumulated gas and an enclosure deflagration. The Victorian Big Battery showed how thermal runaway can remain contained without an explosion, highlighting the value of system design and separation. Moss Landing demonstrated the scale and persistence of hazards associated with a major battery incident and the need for extended monitoring and controlled recovery.
These are not arguments against BESS.
They are arguments for better BESS engineering.
The Real BESS Explosion Risk Is Not the Battery Alone
A lithium-ion cell does not automatically become an explosion hazard simply because it undergoes thermal runaway.
The more dangerous chain is:
Thermal runaway
→ gas generation
→ gas accumulation
→ flammable mixture
→ ignition
→ rapid combustion
→ pressure
The engineering objective is therefore to break that chain at as many points as possible.
- Prevent the failure.
- Detect the failure.
- Detect the gas.
- Control the atmosphere.
- Prevent ignition.
- Relieve or contain pressure.
- Protect neighbouring systems.
- Protect emergency responders.
- And monitor the system after the event.
- That is what a mature BESS explosion risk strategy looks like.
India Has the Opportunity to Get This Right Before BESS Scales Further
India is still building its utility-scale BESS ecosystem.
That creates a window.
The country does not have to repeat every mistake made by earlier markets before strengthening its standards.
The CEA’s 2026 BESS safety framework is an important step because it explicitly brings explosion protection, ventilation, gas detection, fire suppression and emergency shutdown into the same safety architecture.
But regulation alone will not determine whether projects are safe.
The real test will be in project specifications, independent engineering, system-level testing, commissioning and emergency planning.
Developers should demand evidence.
Integrators should understand the actual gas and thermal behaviour of their battery systems.
EPC companies should treat ventilation and explosion protection as engineered safety systems—not container accessories.
And regulators, insurers and lenders should increasingly ask for evidence that the complete BESS installation, not just its individual components, has been evaluated for credible fire and explosion scenarios.
Because a BESS can be designed to prevent an explosion.
But that requires recognising the hazard before the flames appear.
The Question India Should Be Asking Now
The next generation of Indian BESS projects will be larger, denser and increasingly important to grid reliability.
As deployment grows, the industry’s safety conversation needs to mature with it.
The question should no longer be:
“Does this BESS have fire protection?”
It should be:
“What happens when a cell fails?”
Then:
- “What gases are released?”
- “Where do those gases go?”
- “How quickly are they detected?”
- “What prevents them from reaching a flammable concentration?”
- “What happens if they ignite?”
- “Where does the pressure go?”
- “Can neighbouring containers remain safe?”
And finally:
“Can the system protect people even when the battery behaves in the worst credible way?”
That is the standard the BESS industry should be moving toward. Because BESS explosion risk is not simply the risk that a battery catches fire. It is the risk that a battery failure creates an atmosphere capable of turning a fire into a pressure event. And once that distinction is understood, the safety strategy changes completely. The safest BESS is not the one that promises an explosion will never happen. It is the one engineered so that when a cell fails, the failure does not get the opportunity to become an explosion.
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