BESS Thermal Runaway: Fire Risk & Mitigation Strategies

Fires in electric vehicles (EVs) and Battery Energy Storage Systems (BESS) have continued to make headlines in recent years. In January 2025, a massive BESS thermal runaway event at the Moss Landing Energy Storage Facility in California led to the evacuation of 1,500 residents. The fire, which was allowed to burn out due to the risk of explosion, highlighted the growing concerns over large-scale BESS safety.

Meanwhile, multiple EV battery fires led to further recalls across the industry. GM’s earlier $1.9 billion recall of Bolt batteries due to thermal runaway remains one of the most costly, but Hyundai, Ford, and BMW have also faced significant recalls linked to battery fires.

Although thermal runaway in BESS can result from manufacturer defects, clean energy technicians must implement advanced thermal management strategies. What is thermal runaway? It is a chain reaction where a battery cell overheats uncontrollably, leading to fire or explosion.

What Is Thermal Runaway?

Thermal runaway is a hazardous process where an uncontrolled rise in temperature triggers a feedback loop, releasing more energy and causing further temperature spikes. If left unchecked, it can lead to catastrophic failures in lithium-ion batteries, potentially resulting in fires or explosions.

This phenomenon is not limited to battery storage—it also occurs in chemical engineering (exothermic reactions), electrical engineering (power dissipation issues), civil engineering (concrete curing heat), and astrophysics (runaway nuclear fusion in stars).

What Is Thermal Runaway In BESS?

BESS thermal runaway occurs when a damaged lithium-ion battery cell releases flammable or toxic gases, triggering a chain reaction that spreads to adjacent cells. As this process accelerates, extreme heat and pressure build up, significantly increasing the risk of fires or explosions.

Preventing thermal runaway in BESS requires robust thermal management systems, early fault detection, and improved battery designs. With BESS thermal runaway incidents on the rise, the industry must prioritize safety measures to mitigate risks and ensure the reliability of energy storage systems.

How BESS Thermal Runaway Occurs

BESS thermal runaway happens when heat accumulates faster than it can dissipate, leading to a dangerous chain reaction. A key component in lithium-ion batteries is the separator, a porous membrane that keeps the anode and cathode apart while allowing ion transfer.

If the separator degrades due to excessive heat, the battery short-circuits, triggering thermal runaway. This process can escalate rapidly, potentially causing fires or explosions.

Risks of Thermal Runaway in BESS

Thermal runaway in BESS poses severe risks to property, human safety, and the environment. If not properly managed, it can lead to uncontrollable fires, explosions, and the release of toxic gases. Once BESS thermal runaway begins, it is extremely difficult to stop, making proactive safety measures essential. Proper thermal management, system monitoring, and rapid-response strategies are critical to mitigating these hazards.

Factors Contributing to BESS Thermal Runaway

Several factors can initiate thermal runaway in BESS, including overcharging, overheating, manufacturing defects, battery misuse, and short circuits. In many cases, heat accumulates within the battery faster than it can be expelled. This causes the electrolyte to transition from a liquid to a gas, increasing internal pressure beyond safe limits. If venting fails, the pressure buildup can lead to catastrophic failure.

Preventing BESS Thermal Runaway

Intervening early in the process can prevent BESS thermal runaway, especially if the underlying abuse factor is addressed. However, if the battery separator is compromised, the positive and negative electrodes can mix, generating excessive heat and smoke—clear warning signs of imminent failure. At this stage, BESS thermal runaway is nearly unavoidable, reinforcing the need for robust safety systems and early fault detection in energy storage solutions.

Early Detection: The Key to Stopping Thermal Events

Preventing a battery thermal event is far more effective than attempting to contain one. The goal is to intervene early—ideally before the battery cell separator is compromised—when an underlying fault (like overcharging or excessive heat buildup) is first detected.

Monitoring Indicators to Prevent Failure

Detecting internal temperature spikes and the subsequent release of gases is critical. Off-gassing is often the earliest warning sign of imminent cell failure.

Specialized monitoring equipment is essential for this. Conventional gas detectors may not be sensitive enough to identify low concentrations of flammable gases like methane or ethylene. Industry leaders are increasingly relying on dedicated Carbon Monoxide detection to identify the beginning of cell failure, as a primary product of the electrolyte decomposition process that precedes smoke and fire.

Corrective System Shutdowns

The Battery Management System (BMS) is the first line of defense, similar to the safety circuit in a smartphone. A robust BESS safety system continuously monitors performance data. If the BMS detects excess gases or critical temperature and voltage abnormalities, it must execute a rapid and controlled shutdown of the failing battery unit.

This swift action helps allow internal gas levels to dissipate and prevents the chain reaction from spreading. While the affected cell will likely be permanently damaged and require replacement, an effective shutdown prevents catastrophic system failure. For this reason, many Authorities Having Jurisdiction (AHJs) and insurance providers now mandate advanced off-gas monitoring as a core safety requirement.

Physical Containment and Mitigation Strategies

Once a cascading thermal event has begun, lithium-ion battery fires become extremely difficult to suppress. The fire at the Arizona Public Service (APS) facility, for example, highlighted that even with suppression systems functioning as designed, they could not prevent the chemical chain reaction. This makes physical design and specialized training critical.

Facility Design for Safety

Building BESS facilities with safety in mind helps mitigate fire risks:

• Non-Combustible Materials: Utilizing fire-resistant and non-combustible materials for the facility structure prevents the fire from spreading to the building itself.

• Venting and Airflow: Proper ventilation and adequate ceiling height prevent the accumulation of highly flammable and toxic gases released during cell failure. Systems must incorporate blast panels or dedicated exhaust paths to safely vent these gases outside, preventing the internal pressure buildup that can lead to an explosion.

  
  

Lessons in Emergency Response

The APS incident also taught a crucial lesson: the gases released during the chain reaction are highly flammable. When firefighters opened the door to the container, the sudden influx of oxygen ignited the trapped gases, causing an explosion.

This underscores the need for BESS-specific firefighter training. Traditional firefighting tactics, which focus on immediate water application, can sometimes worsen a thermal event. First responders and system operators must be trained in proper lockout/tagout procedures and gas monitoring protocols to manage the unique hazards of lithium-ion fires.

Long-Term Prevention, Best Practices, and Compliance

Prevention begins long before the system is operational—it starts with manufacturing, installation, and rigorous testing standards.

Lessons from Past Incidents

Investigations into past BESS thermal events have identified several key preventable contributors:

• Manufacturing Flaws: Latent cell defects that can cause internal chemical reactions and overheating under normal operating conditions.

• Damage During Construction: Physical damage to battery cells during the handling and installation phase can compromise their integrity and lead to internal shorts later. A structured BESS commissioning plan is essential to prevent this.

• Operating Outside Parameters: Deviating from manufacturer guidelines, such as improper charge rates, temperature limits, or overloading the system, causes excessive heat and stress on the cells. Comprehensive BESS safety training ensures operators adhere to safe operating conditions.

Industry Standards and UL 9540A

To meet safety requirements and assess risk, lithium-ion battery modules and racks undergo UL 9540A testing. This is a mandatory standard that evaluates the potential for a thermal event, documents critical thresholds (like the temperature at which the chain reaction begins), and measures how the event propagates.

The results of the UL 9540A test are vital for:

• AHJs and Building Owners: Assessing the required fire separation distances.

• Fire Departments: Planning appropriate response tactics.

• Engineers: Designing robust containment and ventilation systems.

Be Mindful of BESS Thermal Runaway Risks

High-profile incidents in both electric vehicles and BESS facilities have raised valid concerns about lithium-ion safety. As a clean energy professional, understanding thermal runaway prevention and implementing best practices is essential for building public trust and ensuring a lasting commitment to safety across the energy storage industry.

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Frequently Asked Questions on BESS Thermal Runaway

This section addresses key questions regarding the risks, causes, and prevention strategies for thermal runaway in Battery Energy Storage Systems (BESS).

What is Thermal Runaway?

Thermal runaway is a hazardous, uncontrolled chain reaction where a lithium-ion battery cell overheats, creating a feedback loop that releases more energy and causes further temperature spikes. If left unchecked, this process leads to fire or an explosion.

What causes Thermal Runaway in a BESS?

Several factors can initiate the event, including overcharging, overheating, short circuits, manufacturing defects, or physical damage sustained during installation. The root cause is when heat accumulates within the battery cell faster than it can dissipate, leading to degradation of the internal separator.

What is the earliest warning sign of imminent cell failure?

The earliest and most critical warning sign is off-gassing, which is the release of flammable or toxic gases from the cell as the electrolyte begins to decompose. Specialized systems, often utilizing dedicated Carbon Monoxide detection, are necessary to detect these low concentrations of gases before smoke or fire is visible.

How does the Battery Management System (BMS) prevent catastrophic failure?

The BMS acts as the first line of defense. It continuously monitors performance data. If it detects excess gases or critical temperature/voltage abnormalities, a robust system will execute a rapid and controlled shutdown of the failing battery unit. This action is crucial to prevent the chain reaction from spreading to adjacent cells.

Why are lithium-ion battery fires so difficult to suppress once they begin?

Once a cascading thermal event has begun, the chemical chain reaction within the cells generates its own oxygen, making traditional suppression methods ineffective at stopping the reaction. Furthermore, the event releases highly flammable and toxic gases. As was learned during the APS incident, the sudden introduction of oxygen (e.g., by opening a container door) can ignite these trapped gases, causing an explosion.

What role does UL 9540A testing play in BESS safety?

UL 9540A testing is a mandatory industry standard that rigorously evaluates the potential for a thermal event in battery modules and racks. The results are vital for Authorities Having Jurisdiction (AHJs), engineers, and fire departments, as they use the data to design facility structures, determine required fire separation distances, and plan appropriate emergency response tactics.