By Andres Blanco MEng MSc CEng MIET, Founder and Director, Blanboz Limited, and Elements Green.
Battery energy storage systems are one of the most important technologies in the global energy transition. They store surplus renewable power and release it when the grid needs it most, reducing reliance on gas-fired generation and making clean energy more reliable. The scale of deployment reflects that importance: in 2024, approximately 100GWh of utility-scale BESS capacity was operational globally, with growth accelerating rapidly.
The safety record associated with that deployment is stronger than many people realise. The Electric Power Research Institute, EPRI, tracks reported stationary battery storage failure incidents globally. These are safety-related events, such as fires, thermal runaway or other failures that create an increased risk to people or equipment, rather than routine faults where a system simply stops working. Its data shows that the normalised failure rate, measured against deployed capacity, fell by around 99% between 2018 and 2025.
UK evidence provides useful additional context. In June 2026, the UK Government reported that fires at grid-scale battery sites in the UK are rare. The latest available five-year annual average fire incidence rate was 0.7% between 2020/21 and 2024/25. The Government noted that this was lower than the equivalent rate for fires at non-domestic buildings from all causes.
These figures measure different things and should not be interpreted as the probability of a particular BESS project experiencing a fire. However, taken together, they show that reported incidents remain relatively uncommon as deployment increases, while technology, testing and safety practice continue to develop.
The chemistry underpinning modern utility-scale storage has changed significantly over the past decade. Approximately 85% of new grid-scale BESS capacity now uses lithium iron phosphate (LFP), chemistry, more specifically LiFePO4. LFP has a thermal runaway onset temperature of around 270°C, compared to 150 to 200°C for older nickel-based chemistries previously used in many large-scale systems. During a failure event, LFP produces significantly less heat than those older chemistries.
Over the past two decades, cell-level reliability has improved dramatically across the lithium-ion battery industry, driven by advances in manufacturing, chemistry and quality control. Modern equipment, designed and built to current standards, is significantly more robust than what was deployed ten years ago. The incidents that shaped early public perceptions of BESS fire risk involved different technology, different standards and a less mature industry.
The risks are real and deserve honest explanation
None of this means the risk is zero. It is not, and responsible developers should say so clearly.
The principal safety risk in lithium-ion battery systems is thermal runaway. Understanding it properly requires understanding what it actually is, because it behaves differently from a conventional fire.
Batteries are sensitive to what engineers call external abuse. This takes three forms:
- Electrical, such as overcharging or over-voltage;
- Mechanical, such as vibration or physical damage; and
- Thermal, meaning operation in extreme heat or cold.
Any of these can trigger internal short circuits between the cell’s anode and cathode, or between cells and surrounding equipment.
The mechanism itself is well characterised. The figures that follow describe the general behaviour of lithium-ion cells under ‘adiabatic’ test conditions: no heat is allowed to escape the cell. They illustrate the process rather than the performance of any particular utility-scale product.
Once triggered, a thermal runaway event moves through three escalating stages. In the first stage, chemical reactions begin within the battery cell at around 90°C, causing temperatures to rise gradually.
In the second, a reaction between the lithium ions and the electrolyte begins at around 130°C, with temperature now rising at approximately 0.40 degrees per second.
In the third stage, occurring above 200°C, the positive electrode material reacts with the electrolyte simultaneously. Temperature can increase at between 1.7 and 17 degrees per second. At that point the situation is moving very fast.
What makes battery fires particularly challenging for emergency services is that the reaction is exothermic: the battery generates its own heat. Unlike a conventional fire, where removing heat, fuel or oxygen can bring things under control, a battery in thermal runaway sustains its own reaction. In practical terms, thermal runaway cannot be tackled in the same way as a conventional fire. The response therefore focuses on containment, managing heat exposure to surrounding equipment and preventing propagation while the battery’s energy is dissipated.
The critical window
Safety is layered, not singular.
No single measure carries the safety case. A properly designed system places a sequence of independent barriers between a cell fault and a site-level incident:
- Cell and module design; qualified by test to UL 1973 and characterised under UL 9540A.
- The battery management system; monitoring cell voltage, current and temperature, and isolating the affected string before abuse conditions develop. This is the primary preventive control and it operates continuously.
- Off-gas and hydrogen detection, giving the earliest available warning that a cell has begun to vent.
- Ventilation and deflagration (a fast, explosive burn of built-up gas) venting, preventing a flammable atmosphere from accumulating and directing pressure away from where people are.
- Suppression, which addresses flame.
- Physical separation and containment, limiting consequence if the preceding layers are exceeded.
- An emergency response plan, agreed in advance with the local fire and rescue service.
Each layer is designed on the assumption that the layer before it may fail. That assumption, rather than confidence in any single component, is what makes the system safe.
The timing of a thermal runaway event is where modern BESS safety design focuses most of its effort.
According to analysis presented by Blanboz at the Institution of Mechanical Engineers webinar in August 2025, a representative thermal runaway scenario may involve an anomaly being detected at around three minutes.
By around minute 13, cells entering thermal runaway may begin to off-gas, releasing combustible gases into the battery container. Hydrogen dominates, alongside carbon monoxide, methane, ethylene and vaporised electrolyte.
In this representative scenario, ignition may occur approximately 10 to 13 minutes after off-gassing begins.
That window is short, but it is sufficient for a well-designed system to respond, provided the right sensors are in place and the emergency response plan is properly prepared. Hydrogen sensors are particularly important because hydrogen is released at the highest concentration during off-gassing, before smoke or visible fire appears. Early hydrogen detection is increasingly used as an early warning trigger because it can identify developing thermal runaway before temperatures escalate significantly.
In our experience, effective BESS safety is not just about batteries, containers and standards. It also depends on collaboration between developers, technical advisors and local fire and rescue services. Modern fire services are increasingly familiar with battery energy storage technology and work closely with project teams to understand site layouts, emergency procedures and the specific characteristics of lithium-ion battery systems long before a project becomes operational.
Explosion risk
In public discussions about BESS safety, fire tends to dominate. Explosion risk is equally important.
The explosion risk sits within the off-gassing window, before fire has started. Combustible gases accumulate inside the largely airtight battery container. Pressure builds quickly. Combined with combustible gases, this creates conditions for an explosion that can precede the fire itself.
Many modern BESS systems use condensed aerosol suppression as part of their fire protection strategy. This releases potassium compounds that interrupt the chemical chain reactions sustaining combustion. Forced ventilation and natural venting allow gases to escape rather than accumulate.
It is worth being precise about what suppression does. It addresses flame. It does not cool cells and it does not stop thermal runaway propagating. Cells can continue to off-gas after suppression has operated, which means a flammable atmosphere can keep building inside an enclosure where the visible fire appears to have been dealt with.
That is the central lesson of the 2019 incident at the McMicken facility in Arizona. The installed suppression system operated as designed. Propagation continued regardless, gas accumulated, and a deflagration occurred when responders opened the enclosure. Four were injured. The industry response to that event is the reason gas detection, ventilation and deflagration venting are now treated as primary controls rather than supplementary ones, and the reason no responder should open a battery enclosure without gas monitoring in place.
These measures are currently recommended rather than universally mandatory depending on local authority requirements. Proposed revisions to National Fire Protection Association NFPA 855 are expected to strengthen requirements around ventilation and associated safety analysis, although final provisions remain subject to the standards development process.
These measures may form part of a BESS safety strategy, depending on the technology, equipment design, applicable requirements and site-specific risk assessment. Standards and guidance continue to develop as the industry gains further operational experience, so the appropriate combination of controls should be determined for each project rather than treated as a single prescribed solution.
Physical separation is also an important safety design principle. Appropriate separation distances depend on factors including equipment design, system certification, fire testing, manufacturer requirements and site-specific safety assessments. The aim is to limit the potential for an incident in one part of a BESS facility to affect adjacent equipment, while ensuring that the overall design reflects the evidence available for the particular system being deployed.
The standards framework
A range of international standards, testing regimes and UK guidance can inform the design and assessment of utility-scale BESS projects. Which provisions apply will depend on the equipment, system design, location and applicable regulatory requirements. Relevant standards include:
- NFPA 855 is the master standard for fire and explosion safety in energy storage systems, covering design, installation and operation.
- NFPA 68 and 69 address aspects of explosion protection and prevention. Their relevance will depend on the system design and the safety approach adopted for the particular project.
- UL9540A tests thermal runaway propagation from cell level upward through modules, racks and the full container.
- UL1973 covers stationary battery applications.
- UN38.3 governs transportation safety testing.
In the United Kingdom these sit alongside a domestic framework. The National Fire Chiefs Council publishes planning guidance for grid-scale battery storage, updated in December 2025, which sets expectations on:
- separation from occupied buildings
- identifying sensitive receptors (people or places nearby that could be affected if something went wrong) around a site
- firefighting water provision and run-off management
- producing a battery safety management plan and an emergency response plan developed jointly with the local fire and rescue service.
The UK Government has published health and safety guidance for grid-scale electrical energy storage. IEC 62933-5-2 and BS EN IEC 62619 provide the international and European standards basis, and duties under DSEAR apply wherever an explosive atmosphere may occur.
Together, these standards and guidance provide a framework for assessing and managing BESS safety. The detailed measures adopted for an individual project depend on its technology, equipment, layout, planning context and project-specific risk assessments. Requirements and industry practice will also continue to develop as technology and operational experience evolve.
Applying safety principles in project development
For any BESS project, the detailed safety approach needs to reflect the equipment ultimately selected, the site layout, testing evidence, applicable standards and project-specific risk assessments. These details are developed and refined as a project progresses through design, procurement, planning and preparation for operation.
At Elements Green, individual BESS projects are assessed according to their own design, site context, equipment and applicable requirements. Detailed safety measures are refined as projects progress and the relevant technical information becomes available. This allows the final approach to reflect the selected technology, supporting test evidence and engagement with specialist advisors and the relevant authorities, including local fire and rescue services.
This project-specific approach is important because BESS technology and safety practice continue to evolve. Measures such as equipment selection, separation, detection, ventilation, explosion protection, fire response and emergency planning need to work together as part of the overall safety strategy, rather than relying on any one measure in isolation.
For communities, planning authorities and investors, the key consideration is how a developer identifies, manages and mitigates safety risks. This includes project design, equipment specification, operational procedures and adherence to recognised industry standards. In my view, modern BESS technology can be deployed safely when these factors are treated as fundamental design requirements from the outset rather than added later as compliance measures.
Andres Blanco MEng MSc CEng MIET is Founder and Director of Blanboz Limited, a specialist BESS project consultancy. He presented the technical material underpinning this article at the Institution of Mechanical Engineers webinar on 5 August 2025. Blanboz Limited provides technical advisory services to Elements Green. www.blanboz.com
Full presentation source: Blanboz and IMechE (UK) – Overview of Battery Energy Storage Systems – YouTube
Post-publication footnote 22/09/2026: Elements Green also recognises the importance of engaging insurers early in the development process. As with safety, testing and emergency planning considerations, understanding insurance requirements at an early stage can help inform project design, reduce the risk of late-stage layout changes and support a more efficient development process.
