Lithium-ion batteries have become synonymous with electric vehicles, smartphones and other portable electronics. But increasingly, some of the world’s largest concentrations of these batteries are not moving at all.
Battery energy storage systems (BESS) are becoming an important part of modern electricity networks. They can absorb electricity when generation is high, store it, and return that power to the grid when it is needed. As countries integrate greater quantities of intermittent renewable energy such as wind and solar, the ability to store electricity at scale is becoming increasingly valuable.
Lithium-ion technology currently dominates new electrochemical energy storage deployments. Sandia National Laboratories has reported that lithium-ion batteries account for more than 90% of new electrochemical energy storage, while UK government guidance identifies lithium-ion systems as a major technology within grid-scale electrical energy storage.
Yet concentrating large numbers of lithium-ion cells into modules, racks and containers introduces a distinctive fire-safety challenge. Individual failures are uncommon, but when they occur, thermal runaway can generate intense heat, toxic and flammable gases, fire and, under certain conditions, explosions.
As BESS installations grow in both number and capacity, the question is therefore not simply how much energy they can store. It is how safely that energy can be contained.
Why battery storage is becoming critical to the energy transition
Renewable electricity presents a fundamental challenge for grid operators: generation and demand do not always coincide.
Solar panels can generate significant electricity in the middle of the day, while demand may peak several hours later. Wind farms can produce large amounts of power overnight or during periods of relatively low consumption. Without sufficient flexibility in the electricity system, some renewable generation may have to be curtailed.
BESS can help bridge that gap. Batteries charge when electricity is available and discharge when demand rises, while also providing services that can help balance and stabilise the electricity network.
This role means energy storage is increasingly being deployed at significant scale. Individual installations can contain many thousands of cells organised into battery modules, racks and larger enclosures.
That scale is precisely what makes BESS fire safety different from the problem presented by a single consumer battery.
From one failing cell to a system-wide problem
At the centre of the lithium-ion fire hazard is thermal runaway.
A cell can enter thermal runaway when internal heat generation becomes greater than its ability to dissipate that heat. The process can be initiated by several forms of abuse or failure, including internal short circuits, electrical faults, excessive temperatures and physical damage.
Once initiated, a series of exothermic reactions can cause temperatures inside the cell to rise rapidly.
In an isolated battery, that is already dangerous. Inside a densely packed lithium-ion battery storage system, however, the consequences can become considerably more complex.
The National Fire Protection Association explains that thermal runaway in one cell can heat neighbouring cells, potentially producing a chain reaction. If sufficient heat is transferred from one cell to another, failure can propagate through a module and potentially beyond it.
This makes propagation one of the defining challenges of BESS safety.
A modern system must therefore do more than try to prevent the first cell from failing. It must also be engineered so that, if failure does occur, the event is detected and contained before it develops into a much larger incident.
Fire is not the only danger
One of the most important characteristics of lithium-ion battery failure is that visible flames tell only part of the story.
During thermal runaway, cells can vent gases as their internal components decompose. The UK’s National Fire Chiefs Council (NFCC) warns that the process can rapidly produce large volumes of toxic and explosive vapours and can lead to fire.
In an enclosed BESS container, gas accumulation presents an additional hazard.
If sufficient flammable gas collects and subsequently encounters an ignition source, the result can be a deflagration or explosion. This means fire protection strategies have to consider not only heat and flames but also gas detection, ventilation, enclosure design and the possibility of delayed ignition.
The issue fundamentally changes the way a battery storage incident must be assessed. A container that is not visibly burning cannot automatically be assumed to be safe.
For emergency responders, understanding what is happening inside the enclosure can therefore be as important as observing what is happening outside it.
Why extinguishing a BESS fire is so difficult
Traditional firefighting relies heavily on removing one or more elements necessary for combustion. Lithium-ion battery incidents complicate that approach because the underlying electrochemical reactions occurring during thermal runaway can continue to generate substantial heat.
Cooling is consequently crucial.
But accessing the cells that need cooling can be difficult. Battery packs and modules are deliberately housed inside protective structures, while large BESS installations may contain tightly packed racks within containers or buildings.
There is another complication: opening an enclosure can alter its internal environment. Where flammable gases have accumulated, responder actions need to account for the possibility of ignition and explosion.
This is why emergency planning increasingly forms part of BESS design rather than being considered only after construction.
The NFCC recommends that safety and risk-mitigation measures for grid-scale projects are developed in collaboration with local fire and rescue services. Its planning guidance addresses issues including access, water supplies, fire detection, containment and emergency response arrangements.
The objective is increasingly to prevent an individual failure from escalating rather than assuming that conventional firefighting alone can resolve an incident once it is underway.
Moss Landing showed what is at stake
The potential consequences of large-scale battery fires were brought into sharp focus by the January 2025 fire at the Moss Landing energy storage facility in California.
The fire affected a large lithium-ion battery installation operated by Vistra and prompted evacuation orders in the surrounding area. Reuters reported at the time that approximately 40% of the battery facility had been affected, while firefighters allowed the blaze to burn because of the particular hazards surrounding the incident. No injuries were reported.
The event attracted international attention partly because of the scale of the facility, but it also highlighted an important distinction within the BESS sector.
Battery storage is not a single, static technology. System architecture is evolving. Newer installations increasingly incorporate modular designs, separation, monitoring and other safeguards intended to prevent a fault in one part of a facility from becoming a site-wide event.
That distinction matters when assessing BESS safety. High-profile incidents demonstrate what can happen when failures escalate, but they should not be treated as evidence that every battery storage facility presents an equivalent risk.
The challenge is understanding why some incidents remain contained while others propagate.
Building barriers against thermal runaway
Modern BESS safety therefore relies on multiple layers of protection.
The first is prevention. Battery management systems continually monitor parameters such as voltage, current and temperature, helping identify abnormal operating conditions and keeping cells within defined limits.
Thermal management is equally important. Cooling systems are designed to maintain batteries within suitable operating temperatures and reduce the development of localised hotspots.
Detection provides another layer. Depending on system design, sensors can identify heat, smoke and gases associated with developing battery failure. Earlier detection potentially provides more time for a system to isolate affected equipment and initiate protective measures.
Physical design then becomes critical if a cell does enter thermal runaway. Separation between cells and modules, fire-resistant barriers and careful enclosure design can reduce the probability that heat from one failure will trigger another.
UK fire and rescue guidance also points towards a layered approach involving automatic fire detection, suppression, redundancy and construction designed to restrict fire spread.
The important principle is that no single intervention can eliminate the hazard. Effective BESS safety depends on preventing failure where possible while assuming that failure can nevertheless occur.
Testing what happens when prevention fails
One of the most significant developments in battery storage safety has been the increasing emphasis on testing systems under failure conditions.
UL 9540A, for example, provides a test method specifically designed to evaluate thermal runaway fire propagation in battery energy storage systems. Testing can progress through different scales, generating information about how cells, modules and larger installations behave when thermal runaway is deliberately initiated.
This type of testing is important because laboratory measurements of an individual cell cannot necessarily predict everything that will happen when thousands of cells are installed together.
System geometry, spacing, ventilation, enclosure construction and thermal management can all influence how an incident develops.
Testing therefore asks a more practical question: if something goes wrong despite the preventative systems, what happens next?
For developers, regulators and fire services, those results can inform decisions about separation distances, fire protection, ventilation and emergency response.
International standards are evolving alongside the technology. UK government guidance identifies a range of standards relevant to electrical energy storage, including IEC 62933-5-4, which specifically addresses safety test methods and procedures for grid-integrated lithium-ion battery storage systems.
Early detection could transform battery fire safety
Another important area of development is identifying thermal runaway before temperatures begin to rise uncontrollably.
Conventional monitoring can detect abnormal temperature or electrical behaviour, but researchers are exploring whether combining multiple signals could provide earlier warning.
A 2026 study, for example, investigated a physics-guided approach combining temperature, voltage, mechanical force, deformation and state-of-charge information to identify developing thermal runaway under controlled mechanical-abuse conditions. The experimental work suggests that mechanical signals can contain useful precursors before rapid heating becomes apparent.
Other research is examining physics-informed machine learning to forecast temperature evolution and abnormal battery behaviour using several operating parameters simultaneously.
These approaches remain areas of active research rather than universal solutions, but they illustrate where BESS safety could be heading.
Instead of waiting for a smoke detector or temperature sensor to indicate that failure is already developing, future systems could potentially identify patterns associated with impending failure and isolate affected batteries earlier.
For an installation containing thousands of cells, those additional seconds or minutes could prove valuable.
Managing risk without slowing energy storage
Lithium-ion battery storage presents a difficult balancing act.
Grid-scale batteries can support electricity networks increasingly dependent on variable renewable generation. But deploying much larger quantities of stored electrochemical energy inevitably requires careful management of the hazards associated with that technology.
Those hazards should neither be dismissed nor exaggerated.
BESS incidents remain relatively uncommon, and the NFCC states that properly managed lithium-ion systems can be operated safely. At the same time, it recognises that failures can present significant hazards because of thermal runaway, toxic and explosive vapours, fire and explosion.
The challenge for the industry is therefore not to demonstrate that lithium-ion batteries can never fail. No complex energy technology can meet that standard.
Instead, the objective is to make sure that when an individual cell does fail, the consequences stop there.
That means better battery chemistry, more sophisticated monitoring, earlier warning systems, effective thermal management, robust physical barriers, appropriate testing and emergency plans designed around the unusual behaviour of lithium-ion batteries.
As battery storage expands, fire safety will increasingly become part of the infrastructure of the energy transition itself.
The lithium-ion battery revolution is moving far beyond the electric vehicle. The next test is whether battery safety can scale just as quickly as the systems it is being asked to protect.
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