Battery thermal runaway is a rapid, uncontrollable rise in temperature inside a lithium-ion battery, where heat-generating chemical reactions feed on themselves faster than the heat can escape. Once it starts, the battery’s internal temperature can climb from normal operating range to over 400°C in seconds, potentially causing fires, explosions, and the release of toxic gases. It’s the root cause behind most lithium-ion battery fires in electric vehicles, phones, e-bikes, and grid-scale energy storage systems.
How Thermal Runaway Starts
A lithium-ion battery operates safely within a narrow temperature window, typically below about 60°C. Problems begin when something pushes the internal temperature past that range. The triggers vary: a manufacturing defect, physical damage (like a puncture or crush), overcharging, or an external heat source. Any of these can create a hot spot inside the cell, and once internal temperatures reach roughly 90 to 120°C, the first domino falls.
At that threshold, a thin protective film on the battery’s negative electrode, called the SEI layer, begins to decompose. This layer normally keeps the electrode stable. When it breaks down, the exposed electrode reacts directly with the surrounding electrolyte liquid, generating heat. That heat pushes the temperature higher, which triggers more reactions, which generate more heat. The process becomes self-sustaining.
The Chain Reaction, Stage by Stage
Thermal runaway unfolds in a predictable sequence, each stage hotter and more dangerous than the last.
Between 90°C and 130°C, the SEI layer decomposes and the electrode begins reacting with the electrolyte. These early reactions produce moderate heat, around 40 to 350 joules per gram of material depending on the specific chemistry involved. On their own, they might not be catastrophic, but they raise the temperature enough to trigger the next stage.
Around 130°C, the plastic separator that keeps the positive and negative electrodes apart begins to soften and shrink. If it fails completely, the electrodes can make direct contact, creating an internal short circuit that dumps electrical energy as heat almost instantly.
From roughly 180°C to 230°C, the electrolyte itself starts decomposing in highly energetic reactions, with peak heat output around 355 joules per gram. At roughly the same temperature range, the battery’s positive electrode material begins breaking down and releasing oxygen. This is the critical turning point: the battery is now generating its own oxygen supply internally, which means it can sustain combustion even without outside air.
Above 210°C, the binding materials that hold the electrodes together also start reacting, adding yet more heat. By this point, temperatures are climbing so fast that the entire process from first warning to full fire can take just a few seconds. The cell typically vents hot gas, sparks, or flames, and in a battery pack with many cells packed together, the heat can spread to neighboring cells and start the whole process over again. This cell-to-cell spread is called thermal runaway propagation.
Toxic Gases Released During Failure
The visible fire is only part of the danger. A battery in thermal runaway releases a cocktail of hazardous gases well before flames appear, and sometimes without any visible fire at all.
Carbon monoxide is one of the most concerning. In module-level testing, concentrations above 1,000 ppm have been measured repeatedly, with some tests recording levels as high as 5.6% of the surrounding air volume. For context, CO concentrations above 1,200 ppm can be life-threatening within minutes. Hydrogen fluoride, an extremely corrosive and toxic gas, has been measured at up to 76 ppm during battery failure tests. Even small amounts of HF can cause severe burns to the lungs and eyes.
The gas mixture also includes methane, ethylene, and other flammable hydrocarbons in the hundreds of ppm range, along with smaller amounts of hydrogen chloride and phosphorus-containing compounds. Alcohols like methanol and ethanol can reach over 1,200 ppm combined. These gases make battery fires particularly hazardous in enclosed spaces like garages, basements, or cargo holds, where ventilation is limited.
How Battery Chemistry Affects the Risk
Not all lithium-ion batteries behave the same way during thermal runaway. The cathode material, the positive electrode, plays a major role in determining how much heat is released and how dangerous the failure becomes.
Batteries using lithium cobalt oxide (the chemistry common in older consumer electronics) generate the most heat during thermal runaway, making them the most dangerous in overheating scenarios. Nickel-manganese-cobalt (NMC) batteries, widely used in electric vehicles, fall in the middle. Lithium iron phosphate (LFP) batteries produce less total heat during runaway and are generally considered the most thermally stable of the three. However, LFP cells can actually reach thermal runaway faster under certain conditions, even though the consequences are less severe once it occurs.
This is one reason the EV industry has been shifting toward LFP for mass-market vehicles and stationary energy storage, where the lower energy density is an acceptable trade-off for improved safety margins.
How Batteries Are Kept Cool
Preventing thermal runaway starts with keeping battery temperatures in a safe range during normal use. Electric vehicles and large battery systems use dedicated thermal management systems that fall into three broad categories: air-based, liquid-based, and passive material-based.
Air-based systems blow cooled air across the battery cells. They’re the simplest and cheapest but the least effective at removing heat quickly. Liquid-based systems circulate a coolant through channels running between cells, keeping maximum battery temperatures below 40°C when properly designed. They’re more effective but consume more energy to operate.
Passive systems use phase change materials, substances that absorb large amounts of heat as they melt, acting like a thermal buffer. In one comparative study of electric vehicles under real driving conditions, a phase-change-material system consumed nearly 15% less energy than an air-based system and almost 23% less than a liquid-based system. Battery degradation over 10 years was also slightly lower: about 6.95% capacity loss compared to 7.17% for air cooling and 7.26% for liquid cooling. Many modern designs combine approaches, using liquid cooling alongside phase change materials for redundancy.
Early Warning and Detection
Every lithium-ion battery pack in an EV or energy storage system includes a battery management system (BMS) that monitors voltage, current, and temperature at the cell level. When readings drift outside normal ranges, the BMS can reduce charging speed, limit power output, or shut the system down entirely.
Temperature monitoring alone has a limitation, though: by the time the outside of a cell feels hot, internal reactions may already be well underway. Newer detection approaches focus on catching the problem earlier. Gas sensors designed to detect the electrolyte vapors and other compounds vented in the earliest stages of decomposition can provide warnings minutes before a temperature spike becomes measurable. Pressure and strain sensors, including fiber-optic sensors embedded directly inside battery cells, can detect the subtle swelling that occurs as gas builds up internally. These approaches aim to catch thermal runaway during its first, still-controllable moments rather than after the chain reaction has taken hold.
Industry Safety Standards
The primary safety standard for large battery installations in the U.S. and Canada is UL 9540A, a test method that evaluates whether thermal runaway in one cell or module will propagate to the rest of the system. It’s the only national consensus standard specifically designed for fire safety testing of battery energy storage systems, and it’s referenced in the major building and fire codes, including NFPA 855 (the standard for stationary energy storage installation), the International Fire Code, and the International Residential Code.
UL 9540A testing works at multiple scales: individual cells, modules, units, and full installations. The goal isn’t to prevent thermal runaway from ever happening in a single cell (that can never be fully guaranteed) but to ensure that when it does, the system’s design contains the failure and prevents it from cascading into a large-scale fire. Manufacturers of home battery systems, grid-scale storage, and EV battery packs all use these test results to demonstrate that their products meet code requirements for safe installation.