Battery Energy Storage Systems (BESS) are becoming the backbone of grid stability and renewable integration. However, as systems grow denser and cell designs evolve, the explosion hazards associated with thermal runaway have become one of the most misunderstood risks in the industry. Too often, explosion safety is treated as an afterthought—secondary to fire safety or assumed to be automatically resolved by detection and ventilation. Advanced engineering, recent test data, and real-world incidents tell a different story: Explosion hazards are distinct from fire hazards and require system-specific protection solutions. They must be addressed early in the design phase to avoid unsafe installations, permitting bottlenecks, and costly retrofits.
Where the Hazard Begins: It’s Not Just Heat
Thermal runaway in lithium-ion batteries is not a purely thermal event; it is a failure that generates flammable gas. When a cell fails, it releases a flammable “vent gas cocktail” of hydrogen, carbon monoxide, hydrocarbons, and other components.
In containerized BESS, even a few failing cells can release hundreds to thousands of liters of flammable gas. If this gas finds an ignition source, the result is a deflagration (a premixed combustion wave that creates overpressure under confinement). Crucially, the entire container doesn’t need to be full of gas for a significant explosion to occur. Partial-Volume Deflagrations (PVD)—localized pockets of gas—can generate enough overpressure to blow doors off hinges and risk the integrity of the enclosure.
The NFPA 855 Framework: A Two-Pronged Approach
NFPA 855:2026, the leading standard for BESS safety, requires explosion control whenever flammable gas concentrations can exceed 25% of the Lower Flammable Limit (LFL). It categorizes safeguards into two buckets:
| Strategy | Common Methods | Goal | Nature |
| Prevention | NFPA 69 (Combustible Concentration Reduction – CCR); Controlled Ignition | Limit gas accumulation | Active: Depends on detection, controls, actuation and power. |
| Mitigation | NFPA 68 (Deflagration Venting) | Manage overpressure | Passive: Works even if the power is out. |
The gold standard is not choosing one; it is using both. NFPA 855 acknowledges that prevention systems (like mechanical ventilation) can fail or be overwhelmed by rapid gas release, making passive mitigation (deflagration venting) a mandatory safety layer.
Why Prevention Alone Falls Short: An Expert Perspective
In a recent episode of The Fire Science Show (Episode 231), Dr. Lorenz Boeck, REMBE® Chief Scientific Officer and WPI Adjunct Professor, discussed BESS explosion prevention and mitigation alongside Nick Bartlett, P.E., Atar Fire, Fire Protection Engineer, and experienced BESS safety consultant. The consensus is clear: BESS explosion hazards are complex and require system-specific analysis. Combined prevention and mitigation is essential, where prevention alone can leave critical gaps:
- Gas Pockets: Ventilation doesn’t always clear “dead zones” between battery racks, in narrow gaps between batteries and walls, or in corners.
- Speed of Event: Rapid thermal runaway can outpace the CFM (cubic feet per minute) capacity of a fan.
- System Failures: Detectors can fail to detect in time, louvers can fail to open, or power can be lost during the very emergency where they are needed most.
Modern BESS safety design is moving away from empirical “typical expected-failure” scenarios toward more well-defined credible worst-case scenarios. Authorities Having Jurisdiction (AHJs) are increasingly demanding thorough and conservative analysis to ensure that if a deflagration happens, the hazard is managed safely.
Engineering Best Practices: Future-Proofing Your Design
The energy storage industry moves faster than the code cycle. To be truly “future-proof,” stakeholders should consider these evolving best practices beyond codes and standards:
- Post-Fire Deflagrations: Traditional prevention systems (NFPA 69) often perish in a fire. However, deflagrations can occur even hours after a fire starts if oxygen enters a gas-heavy (ventilation-limited) enclosure. Only passive venting (NFPA 68) protects against this. Late deflagrations are often observed in large-scale BESS fire tests – the threat is real. Combining prevention and mitigation solves the issue.
- Functional Safety: Unlike traditional high-risk applications, BESS safety electronics, especially components used to detect gas and control active prevention systems, commonly lack Safety Integrity Level (SIL) ratings. Implementing Failure Mode and Effects Analysis (FMEA) combined with Layer-of-Protection Analysis (LOPA) is a hallmark of high-quality system integration.
- Full-Scale Deflagration Testing: While Hazard Mitigation Analysis (HMA) often relies heavily on numerical modeling, there is a growing trend toward validating safety designs through full-scale deflagration testing. Much like large-scale fire testing, full-scale deflagration tests provide the empirical evidence needed to verify deflagration vent sizing and placement as well as prevention system design and structural integrity and quantify the external effects of an event. These data provide a clear basis for system certification, AHJ approval, and insurance underwriting. REMBE® supports customers worldwide using our extensive experience in explosion testing and data analysis.
- Deflagration Vents Tailored to the Application: Traditional roof-mounted deflagration vents can struggle where snow loads and severe weather are credible and limit design flexibility – preventing, for example, the stacking of BESS enclosures. Innovation in side-mounted panels, like REMBE®’s patented BESS.TGV, allows for flame and pressure deflection upward, protecting personnel and property while maintaining IP66/67 and other critical ratings.
Advancing Global Competency Through Research, Standards Development and Training
As a leader in the international explosion safety community for over five decades, REMBE® has invested significantly in the science behind these recommendations. This includes large-scale testing campaigns at the REMBE® Research + Technology Center, where we validate design methods and certify novel prevention and mitigation solutions under real-world conditions, as well as extensive numerical simulation studies of BESS thermal runaway and deflagration scenarios.
Our scientific program actively contributes to the advancement of global safety through participation in standards committees such as NFPA 68, NFPA 69, and EN 14994. This scientific leadership has been recognized internationally, including the recent Best Presentation Award at the 2025 International Symposium on Lithium Battery Fire Safety in Hong Kong and speaking invitations to keynote events like the 2025 SFPE Engineering Solutions Symposium in Lisbon.
We believe that sharing this expertise is vital for industry growth. In collaboration with Atar Fire, we are providing technical training through our “BESS Safety Master Class—NFPA 855, 68, 69” in London (May 21-22, 2026). For all stakeholders, including engineers, AHJs, insurance companies, developers, and manufacturers, this training provides the “why” behind the standards, offering practical clarity on how to apply standards to complex, modern installations.
The Bottom Line
BESS explosion risk is a known, quantifiable hazard. It isn’t about “if” a system might fail, but ensuring that when a failure occurs, it is managed safely. By integrating both explosion prevention and mitigation, we can build resiliency and protect both the equipment and the communities it serves.