Battery Thermal Runaway in BESS: Causes, Prevention, and System-Level Design

As battery energy storage systems (BESS) continue to scale in size and energy density, safety has become one of the most critical concerns—particularly the risk of thermal runaway.

While thermal runaway is often described as a cell-level failure, in real-world energy storage systems it is fundamentally a system-level challenge. A single cell failure can propagate across modules, racks, and even entire containers if not properly controlled.

What makes this issue more complex is that thermal runaway is not caused by a single factor. It results from interacting electrical, thermal, and mechanical stresses—often amplified by temperature imbalance and insufficient heat dissipation.

In this article, we focus specifically on the causes of thermal runaway, how it propagates through battery systems, and the engineering strategies used to prevent it.

What Is Thermal Runaway in Lithium Batteries

Thermal runaway refers to an uncontrollable increase in temperature within a battery cell, triggered by internal reactions that generate heat faster than it can be dissipated.

Once initiated, this process can lead to rapid temperature escalation, gas release, fire, or explosion. In BESS, thermal runaway is not isolated—it can propagate from one cell to adjacent cells, eventually affecting entire modules or system-level structures.

What Causes Thermal Runaway in BESS Systems

Thermal runaway in energy storage systems is typically the result of multiple interacting factors.

Electrical abuse, such as overcharging or short circuits, can generate excessive internal heat. Mechanical damage—including compression, vibration, or puncture—may compromise cell integrity and initiate failure.

A critical but often overlooked factor is thermal accumulation. In high-density systems, heat generated during operation may not dissipate effectively, especially under continuous or high-rate cycling. This leads to a gradual increase in baseline temperature over time.

From a material perspective, lithium iron phosphate (LFP) batteries begin to experience accelerated degradation above approximately 60°C, where internal protective layers become unstable. As temperatures rise to 80–100°C, internal reactions intensify. Beyond 120°C, exothermic reactions may trigger thermal runaway.

In large-scale BESS deployments, thermal accumulation significantly increases the likelihood of reaching these critical thresholds.

How Thermal Runaway Propagates in Battery Systems

Understanding propagation is essential, as it determines whether a localized failure becomes a system-level incident.

In BESS architectures, thermal runaway typically spreads through a hierarchical structure:

• Cell → adjacent cells
• Cells → module
• Module → rack or pack
• Pack → container-level system

When a cell fails, it releases heat and flammable gases. These trigger multiple heat transfer mechanisms:

• Conduction through structural components such as frames and busbars
• Convection via hot gas movement between cells
• Radiation accelerating heat transfer across nearby surfaces

These combined pathways create a chain reaction. Without effective thermal isolation, a single-cell failure can escalate rapidly into a larger system event.

Why Temperature Difference (ΔT) Accelerates Thermal Runaway

Temperature uniformity plays a critical role in preventing failure.

Cells operating at higher temperatures degrade faster and are more likely to reach critical thresholds earlier than others. These localized “hot spots” act as initiation points for failure.

In practical systems:

• Liquid cooling can maintain temperature variation within ±2–3°C
• Air cooling may result in ±8–15°C variation

Maintaining a stable battery operating temperature range helps reduce these risks.

Temperature imbalance does not just affect performance—it increases the probability of thermal runaway initiation and propagation.

For a deeper understanding of how thermal design impacts performance and system reliability, see our guide on 

battery thermal management in energy storage systems.

How to Prevent Thermal Runaway in BESS

Preventing thermal runaway requires a layered safety approach across the entire system.

Cell-Level Measures

Cell chemistry and internal design influence inherent thermal stability. LFP batteries offer improved resistance to thermal failure compared to other lithium chemistries.

Module-Level Design

At the module level, thermal isolation and structural design help prevent heat from spreading between cells. Proper spacing and insulation materials are essential for limiting propagation.

System-Level Protection

At the system level, multiple protection mechanisms must work together.

Battery management systems (BMS) monitor temperature, voltage, and current to detect abnormalities early. Hardware protection devices—such as fuses and manual service disconnects (MSDs)—enable rapid fault isolation.

These systems must operate in coordination to detect, contain, and mitigate abnormal thermal events before they escalate.

Understanding how operating conditions influence battery behavior—particularly through battery state of charge (SOC)—is also essential for maintaining safe operation.

The Role of Liquid Cooling in Thermal Runaway Prevention

Cooling plays a critical role in controlling thermal runaway risk.

Unlike air cooling, liquid cooling enables more efficient and targeted heat removal, allowing tighter control of cell temperature.

By maintaining temperature variation within a narrow range, liquid cooling reduces the formation of hotspots that can trigger failure.

More importantly, it can slow the propagation process itself. By continuously removing heat, it delays temperature escalation and extends the time required for thermal runaway to spread. This additional response time is critical for system-level protection mechanisms to activate and contain the event.

For a detailed comparison, see: Liquid Cooling vs Air Cooling in Battery Energy Storage Systems

Safety Standards and Testing (UL9540A and Beyond)

Thermal runaway behavior must be validated through standardized testing.

UL9540A evaluates how thermal events propagate at different system levels, including cell, module, rack, and full container configurations. This testing provides essential data for system design and safety planning.

Additional engineering methods, such as IEC standards and DFMEA, are used to identify and mitigate risks during development.

These validation processes are critical for ensuring safe deployment in commercial and industrial energy storage systems.

Engineering Approach to Thermal Runaway Safety

In advanced energy storage systems, preventing thermal runaway is not addressed through a single component, but through coordinated system design.

This includes:

- Cell selection based on thermal stability  

- Module-level structural isolation to limit propagation  

- System-level integration of BMS monitoring and fault response  

- Thermal management strategies designed to control temperature distribution  

In practice, this type of system-level engineering approach is increasingly adopted in high-performance BESS solutions, where safety, reliability, and lifecycle performance must be addressed together rather than independently.

Battery system developers such as ACE Battery apply this integrated approach in real-world projects, combining thermal design, intelligent control, and structural safety to ensure stable operation under demanding conditions.

Design Considerations for BESS Projects

In real-world applications, preventing thermal runaway depends on system design rather than individual components alone.

High-load environments—such as EV charging, data centers, and industrial systems—place continuous thermal stress on battery systems. As energy density increases, especially with large-format cells like 314Ah, the importance of precise thermal control becomes even greater.

System-level coordination between battery design, protection mechanisms, and thermal control is essential to ensure safe operation.

Conclusion — Thermal Runaway Is a System Design Challenge

Thermal runaway is not simply a battery failure—it is a system-level challenge that requires coordinated design across materials, monitoring systems, structural engineering, and thermal control.

As energy storage systems continue to scale, the ability to control temperature, limit propagation, and respond effectively to abnormal conditions will define long-term system safety and reliability.

For project developers and system integrators, evaluating how thermal design, system architecture, and safety mechanisms work together is essential when selecting a battery solution.

Working with experienced battery system developers can help ensure that safety considerations are addressed early in the design process—rather than after deployment challenges arise.

ACE Battery develops energy storage systems with integrated thermal management and safety-focused design to support demanding commercial and industrial applications.