1. Introduction

Battery energy storage systems (BESS) are at the core of modern renewable energy solutions, powering industries, balancing grids, and enabling distributed energy systems. However, as these systems scale up, their safety has become a critical focus. One of the most severe safety challenges is thermal runaway—a chain reaction within a battery triggered by internal or external factors that spirals out of control. So, what exactly happens inside the storage container when thermal runaway occurs? Let’s dive into the science behind this phenomenon and its cascading effects.

2. What Causes Thermal Runaway?

Thermal runaway is a complex process that typically starts with one or more internal or external triggers that destabilize a battery’s chemistry. These triggers can range from electrical faults to physical damage, ultimately resulting in excessive heat and uncontrollable chemical reactions.

Internal Factors: When the Battery Itself Fails

•   ●  Short Circuits:

An internal short circuit occurs when the battery separator (a thin layer that keeps the positive and negative electrodes apart) is damaged, causing direct contact between the electrodes. This unleashes a surge of electrons, rapidly generating heat that can trigger further material breakdown.

External short circuits can also cause excessive current to flow through the battery, producing heat that the system may fail to dissipate, leading to overheating and runaway.

•   ●  Overcharging or Over discharging:

Overcharging forces lithium to accumulate on the anode, forming sharp dendrites that can puncture the separator, creating a short circuit.

Overdischarging pushes the battery beyond its safe voltage range, causing the electrolyte to degrade and generate heat and gas, which compounds the problem.

•   ●  Manufacturing Defects:

Tiny imperfections—like uneven separators, impurities in the materials, or faulty electrode coatings—can create localized hotspots that may escalate into full-blown thermal runaway.

External Factors: The Environment Takes a Toll

•   ●  Physical Damage:

Impacts, punctures, or compression during transportation or installation can compromise the battery's structural integrity, exposing reactive components to each other or the external environment, sparking fires or explosions.

•   ●  Excessive Heat:

High ambient temperatures accelerate chemical reactions within the battery, pushing it closer to its thermal stability limits. This buildup of heat can eventually cause the electrolyte and electrodes to break down, releasing more heat and flammable gases.

3. What Happens Inside the Container During Thermal Runaway?

Thermal runaway doesn’t just impact a single battery cell—it often cascades across multiple cells, modules, and eventually the entire system. The internal dynamics can be broken down into the following key phases:

3.1 Temperature Escalation

The hallmark of thermal runaway is an uncontrollable temperature spike. A single cell can reach temperatures between 400°C and 1,000°C. This heat spreads through conduction (via metallic components) and radiation, igniting neighboring cells and creating a domino effect.

3.2 Gas Emissions

The extreme heat causes the electrolyte and other materials to decompose, releasing:

  ●  Flammable gases: Hydrogen, methane, and carbon monoxide mix with air, significantly increasing the risk of fire or explosion.

•   ●  Toxic gases: Hydrofluoric acid (HF), a byproduct of electrolyte breakdown, poses a severe risk to human health and the environment.

These gases rapidly increase the pressure inside the container, creating additional hazards.

3.3 Pressure Build-Up

The accumulation of gas leads to a rapid rise in pressure. Without effective venting or pressure relief mechanisms, the container could rupture or explode, causing catastrophic damage to the system and nearby infrastructure.

3.4 Fire and Explosion

As the pressure and heat intensify, the flammable gases are likely to ignite, leading to:

•   ●  Fires: High temperatures ignite the battery’s flammable components.

•   ●  Explosions: When flammable gases mix with air in the right proportions, they can detonate, causing widespread damage.

4. The Domino Effect: How Thermal Runaway Spreads

Thermal runaway rarely stops at the cell level. Its cascading effects can compromise the entire energy storage system. Here’s how:

4.1 Propagation Mechanisms

•   ●  Heat Conduction: Heat spreads through metal connections or the casing, triggering neighboring cells.

•   ●  Thermal Radiation: High temperatures emit infrared radiation that heats nearby components, accelerating the spread.

•   ●  Gas Jets: Released gases can escape under high pressure and ignite nearby cells, exacerbating the fire.

4.2 Effects on Battery Modules

•   ●  Complete Module Failure: Multiple cells overheating can cause the failure of the entire module, damaging its monitoring and control systems.

•   ●  Thermal Overload: Heat from multiple cells overwhelms the cooling system, making containment nearly impossible.

4.3 Impact on Internal Systems

•   ●  Cooling System Breakdown: Cooling systems designed to manage normal operating heat cannot handle the surge in temperature and pressure, leading to system failure.

•   ●  BMS (Battery Management System) Damage: The extreme conditions can disable the BMS, leaving the system without the ability to monitor or mitigate the situation.

•   ●  Electrical Components Malfunction: Inverters, circuit breakers, and other components may fail under extreme conditions, further escalating the problem.

4.4 Structural Damage to the Container

•   ●  Deformation and Rupture: The container may warp under intense heat and pressure, eventually failing if venting systems are inadequate.

•   ●  Fire Spread: Flames and gases escaping from the container can ignite external equipment or surroundings, leading to larger-scale incidents.

4.5 Environmental and External Hazards

•   ●  Toxic Emissions: Gases like HF can spread beyond the container, posing risks to personnel and the environment.

•   ●  Fire Propagation: Flames can spread to nearby systems or structures, compounding the damage.

5. The Consequences of Thermal Runaway

The effects of thermal runaway extend far beyond the container itself. Here are the potential outcomes:

•   ●  Direct Economic Losses: Replacing damaged equipment and repairing structural damage can be costly.

•   ●  Environmental Impact: Toxic emissions and water or soil contamination may result from the release of harmful chemicals.

•   ●  Grid Disruptions: Thermal runaway in a key energy storage system can lead to blackouts or energy supply interruptions.

6. Conclusion

Thermal runaway is a highly destructive chain reaction that starts with minor triggers and escalates into a system-wide crisis. The combination of heat, flammable gases, and pressure creates a hazardous environment within the storage container, with effects that can extend well beyond. Understanding the mechanisms of thermal runaway and its cascading effects is crucial for designing safer, more resilient energy storage systems that can support the transition to renewable energy without compromising safety.