Battery Energy Storage Systems (BESS) thermal runaway is a dangerous chain reaction where lithium-ion battery cells generate excessive heat, leading to fire, explosion, and toxic gas release. This uncontrolled process occurs when battery temperatures exceed safe operating limits, triggering chemical reactions that produce more heat and spread to adjacent cells. Understanding thermal runaway is crucial for protecting BESS investments and ensuring operational safety.

What is BESS thermal runaway and how does it occur?

BESS thermal runaway is an uncontrolled chemical chain reaction in lithium-ion battery cells where heat generation exceeds heat dissipation, causing temperatures to rise rapidly beyond safe limits. This process becomes self-sustaining as elevated temperatures accelerate chemical reactions, producing even more heat and creating a dangerous cycle.

The thermal runaway process begins when a battery cell reaches its onset temperature, which typically ranges from above 130°C to over 200°C, depending on the specific lithium-ion chemistry, state of charge, and cell design. At these temperatures, the electrolyte starts decomposing, releasing flammable gases and generating additional heat. The separator between the positive and negative electrodes begins to break down, often at temperatures above $150^\circ\text{C}$, leading to its melting and subsequent direct contact between electrodes, thereby creating internal short circuits.

Once initiated, thermal runaway progresses through several stages. The cell’s internal pressure builds as gases form, potentially causing the cell casing to rupture. The chemical reactions become increasingly violent, with temperatures potentially reaching 800°C or higher. This extreme heat can ignite the flammable gases, creating fire and releasing highly toxic substances including hydrogen fluoride (HF) and carbon monoxide (CO), as well as flammable gases such as hydrogen and methane, and other toxic compounds like hydrogen cyanide (HCN) and formaldehyde.

The conditions that trigger thermal runaway include:

• Exceeding safe temperature ranges
• Internal or external short circuits
• Physical damage to cell structure
• Overcharging beyond the cell’s capacity limits
• Manufacturing defects such as metal particles inside cells or faulty separators

What happens to battery systems during thermal runaway events?

During thermal runaway events, battery systems experience rapid temperature escalation, typically from normal operating temperatures around 25°C–40°C to over 800°C within minutes. The affected cells release large volumes of toxic and flammable gases whilst the system’s structural integrity deteriorates rapidly, creating immediate fire and explosion risks.

The physical changes during thermal runaway events are dramatic and destructive. Battery cells swell as internal pressure builds from gas generation, often causing the protective casing to crack or rupture. The electrolyte boils and vaporises, whilst the plastic separator melts, allowing direct electrode contact. Cell terminals may melt or deform, disrupting electrical connections throughout the system.

Gas generation is particularly dangerous, with each cell potentially releasing several litres of gas, which is a mix of highly flammable and toxic substances. Flammable gases often include hydrogen, carbon monoxide (CO), and various hydrocarbons such as methane (CH₄) and ethylene (C₂H₄), while highly toxic emissions include hydrogen fluoride (HF). These gases create both immediate health hazards and explosive atmospheres that can ignite from any spark or heat source.

The cascading effect represents one of the most serious aspects of BESS thermal runaway. Heat from the initial failing cell conducts to neighbouring cells, potentially triggering thermal runaway in adjacent units. This cell-to-cell propagation can spread throughout entire battery modules or even complete systems if not properly controlled. The Battery Management System (BMS) may lose communication with affected areas, reducing the system’s ability to respond or provide early warnings.

System components beyond the batteries also suffer damage. Cooling systems become overwhelmed and may fail, electrical connections melt or corrode from the toxic gases, and structural elements like housing and mounting systems can be compromised by extreme temperatures and corrosive environments.

What causes thermal runaway in BESS installations?

Several factors can trigger thermal runaway events in BESS installations:

• Manufacturing defects represent a primary cause, including:
• Internal foreign material contamination (such as metal particles) contaminating cells during production
• Faulty separators that allow internal short circuits
• Inconsistent cell chemistry that creates weak points
• Physical damage from impacts, vibration, or improper handling can also compromise cell integrity and trigger dangerous reactions.

Overcharging occurs when the BMS fails to properly regulate charging voltage or current, forcing cells beyond their safe capacity limits. This can happen due to software errors, sensor failures, or communication breakdowns between system components. Even brief periods of overcharging can initiate the chemical processes leading to thermal runaway.

Extreme temperatures pose significant risks to BESS installations. Prolonged exposure to high ambient temperatures, inadequate cooling system performance, or direct sunlight on battery enclosures can push cells beyond their safe operating range. Conversely, extremely cold conditions followed by rapid heating can create thermal stress and cell damage.

System design flaws contribute to thermal runaway risks through inadequate spacing between cells, insufficient cooling capacity, or poor heat dissipation pathways. Electrical faults such as loose connections, corroded terminals, or damaged wiring can create resistance heating or short circuits that initiate thermal runaway.

Environmental factors including moisture ingress, dust accumulation, and exposure to corrosive substances can degrade cell performance and create failure conditions. Poor installation practices, such as mixing battery cells of different ages or capacities, can create imbalances that stress individual cells beyond their limits.

Age-related degradation also increases thermal runaway susceptibility as cells lose capacity and develop internal resistance over time, making them more vulnerable to the conditions that trigger dangerous reactions.

How can thermal runaway be prevented in BESS installations?

Proper Battery Management Systems represent the first line of defence, continuously monitoring cell voltage, current, and temperature whilst maintaining safe operating parameters through precise charge control and load balancing. Advanced BMS units can detect early warning signs and automatically disconnect problematic cells before thermal runaway occurs.

Thermal monitoring and management systems are essential for prevention:

• Installing multiple temperature sensors throughout battery modules
• Implementing active cooling systems with adequate capacity
• Designing proper ventilation to remove heat and prevent gas accumulation
• Installing thermal barriers between cells to slow heat propagation

Quality control measures during procurement and installation significantly reduce thermal runaway risks. This involves sourcing batteries from reputable manufacturers with proper certifications, conducting incoming inspections to verify cell quality, and ensuring proper handling procedures throughout the installation process.

System design considerations include maintaining adequate spacing between battery modules, implementing fire-resistant enclosures, and designing electrical systems with appropriate protection devices such as fuses and circuit breakers. Proper grounding and surge protection help prevent electrical faults that could trigger thermal runaway.

Regular maintenance protocols should include periodic inspection of connections, cleaning of ventilation systems, verification of BMS functionality, and thermal imaging to identify hot spots before they become dangerous. Keeping detailed maintenance records helps identify patterns that might indicate developing problems.

Environmental controls such as maintaining stable ambient temperatures, controlling humidity levels, and protecting systems from physical damage through appropriate enclosures and security measures also contribute significantly to thermal runaway prevention.

What safety measures should be in place for BESS thermal runaway events?

Fire suppression systems specifically designed for lithium-ion battery fires are essential, including clean agent systems, water mist installations, or specialised battery fire extinguishing agents. Traditional water-based suppression systems can worsen battery fires due to electrical conductivity and the potential for spreading electrolytes.

Emergency response procedures must be established and regularly practised:

• Clear evacuation plans
• Communication protocols with local fire services
• Specific procedures for isolating affected battery systems
• Specialised training for emergency responders dealing with battery fires and toxic gas releases

Detection systems should provide early warning of thermal runaway conditions through smoke detection, gas monitoring for hydrogen fluoride (HF) and carbon monoxide (CO), and thermal monitoring that can identify rapid temperature increases before full thermal runaway occurs.

Containment strategies include:

• Designing battery enclosures with explosion venting
• Implementing physical barriers to prevent fire spread between modules
• Ensuring adequate separation distances from other equipment and structures
• Installing proper ventilation systems to remove toxic gases and prevent explosive gas accumulation

Personal protective equipment appropriate for battery emergencies should be readily available, including respiratory protection against toxic gases, thermal protective clothing, and specialised tools for safely disconnecting electrical systems during emergencies.

Communication systems must remain functional during emergencies, with backup power supplies for critical safety systems and clear protocols for notifying relevant authorities and personnel. Regular safety training ensures all personnel understand the risks and proper response procedures for thermal runaway events.

Who needs BESS thermal runaway coverage and protection?

Multiple stakeholders in the BESS industry require comprehensive thermal runaway protection and insurance coverage:

Developers and project owners face direct liability for thermal runaway events that cause property damage, environmental contamination, or business interruption. Standard general liability policies typically exclude pollution-related claims, creating coverage gaps that environmental liability insurance must fill.

Operators and asset managers need protection against operational risks, including cleanup/remediation costs, regulatory compliance costs, and defense costs associated with thermal runaway events. This protection is vital because standard general liability policies typically exclude coverage for pollution-related incidents; specialized environmental liability coverage is necessary to fill this critical gap.

Investors and lenders require assurance that their investments are protected against catastrophic losses from thermal runaway events, including soil/groundwater contamination, firefighting runoff damage, and chemical leakage incidents.

EPC contractors need coverage during construction and commissioning phases when installation errors or defective components could lead to thermal runaway events during the warranty period.

Why is comprehensive thermal runaway protection essential?

BESS thermal runaway events present complex risks that extend beyond immediate fire and explosion hazards. The release of toxic gases including hydrogen fluoride (HF) and carbon monoxide (CO), combined with potential soil/groundwater contamination and firefighting runoff, creates environmental liabilities that standard insurance policies exclude through pollution exclusions.

The financial consequences of thermal runaway events can be severe, encompassing emergency response costs, environmental cleanup/remediation costs, business interruption losses, and regulatory compliance costs. Without proper environmental liability coverage to address these pollution exclusions, stakeholders face significant uninsured exposures.

As BESS technology continues expanding, understanding thermal runaway risks and implementing comprehensive prevention and protection strategies becomes increasingly critical for project success and financial security. Proper insurance coverage, combined with robust safety systems and maintenance protocols, provides the foundation for sustainable BESS operations and investment protection.

Take Action to Protect Your BESS Investment Today

Don’t wait until a thermal runaway event threatens your BESS project and financial security. The complex risks associated with battery energy storage systems require specialized environmental liability coverage that goes beyond standard insurance policies. Contact our BESS insurance experts today to assess your thermal runaway exposure, identify coverage gaps in your current policies, and secure comprehensive protection that safeguards your investment against catastrophic losses. Schedule your free consultation now to ensure your project has the robust insurance foundation it needs for long-term success.

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