Early detection of Thermal Runaway in Lithium-Ion batteries using Metis Engineering’s Cell Guard sensor
Thermal runaway (TR) is one of the most critical safety challenges facing lithium-ion (Li-ion) battery systems today. It refers to a self-accelerating exothermic reaction in which, as thermal runaway occurs, a rising internal temperature triggers further chemical reactions, heat generation, gas release, and, in severe cases, fire or explosion. This sequence can escalate rapidly, leading to the production of flammable gases and ving little time for intervention. As lithium-ion batteries continue to power electric vehicles (EVs), grid-scale energy storage, marine vessels, and aerospace systems, the ability to detect and respond to the early stages of thermal runaway is becoming essential for safety, reliability, and regulatory compliance.
Metis Engineering’s Cell Guard VOC battery safety sensor offers a highly effective early-warning solution. By detecting the volatile organic compounds (VOCs) emitted during the earliest stages of cell failure caused by manufacturing defects , it provides a valuable intervention window. This enables operators to take corrective action to prevent thermal runaway before it fully develops, potentially preventing catastrophic damage, loss of assets, and the release of toxic gases.
Understanding Thermal Runaway and its precursor events
Thermal runaway in a lithium-ion battery cell typically follows a chain reaction sequence. Internal or external abuse, such as overcharging, overheating, short circuits, or mechanical damage, can compromise the solid electrolyte interphase (SEI) or the separator. This degradation, often resulting from physical damage, initiates electrolyte decomposition, which can produce heat and gases inside the cell. If unchecked, the accumulation of heat and pressure can push the cell into full-scale thermal runaway, often causing adjacent cells to fail in a rapid propagation event.
Once propagation begins, the risk to the pack and its surroundings increases exponentially. In EVs, this could result in a fire spreading throughout the vehicle; in stationary storage, it could cause large-scale facility damage. Preventing this sequence requires detecting the earliest, most subtle signs of failure—long before conventional warning signs appear.
Why conventional monitoring often fails
Traditional battery management systems (BMS) focus on monitoring temperature, voltage, and state of charge. While these are essential parameters for various battery types , they often remain stable until moments before or during catastrophic failure. This means that by the time a BMS detects a rise in battery temperature or voltage drop, the opportunity for preventative action has already narrowed to seconds, not minutes.
This gap in detection capability is a key reason why battery fires in electric vehicles or storage facilities can escalate quickly. Closing this gap requires identifying a precursor signal that emerges earlier in the failure sequence, something that provides a clear and measurable warning for fire safety .
VOCs: The earliest detectable warning sign
Studies, including research published in the Journal of The Electrochemical Society and by Sandia National Laboratories, confirm that one of the earliest detectable signs of impending cell failure is the release of VOCs inside the battery pack. These flammable gases—such as ethylene carbonate and diethyl carbonate—are generated when the electrolyte begins to break down during cell venting, often resulting in excessive heat and several minutes before the temperature spike associated with thermal runaway.
Because VOCs emerge during the gas venting stage, detecting them offers a unique opportunity to act before the internal temperature exceeds the critical threshold of 150–200°C, which can lead to extremely high temperatures where runaway is almost inevitable.
The Metis Engineering Cell Guard solution
Metis Engineering’s Cell Guard sensor is specifically engineered to detect these trace VOC emissions at concentrations as low as parts per billion. Installed inside the battery pack enclosure, the sensor continuously samples the internal atmosphere, analysing it in real time. The system communicates via a CAN interface, allowing seamless integration with the BMS and enabling immediate automated responses.
Beyond VOC detection, Cell Guard also monitors dew point, humidity, hydrogen levels, and shock loads. This multi-parameter capability provides a comprehensive view of pack health, supporting both safety and predictive maintenance strategies.
Creating a critical intervention window
Pack-level tests show that Cell Guard can detect VOC emissions 7–17 minutes before the onset of thermal runaway. This early warning window is critical as it can prevent situations where thermal runaway starts . Integrating this with the battery management system allows for a range of interventions, including:
- Cutting the electrical load to the affected pack, halting charge and discharge cycles so the cells can cool passively.
- Isolating the failing module from the rest of the pack to prevent thermal propagation.
- Activating thermal management systems, such as fans or coolant loops, to dissipate heat.
- Triggering fire suppression systems—such as aerosol, inert gas, or liquid agents—before ignition.
By taking these actions during the gas venting stage, operators can stall or even completely prevent the thermal runaway sequence.
How Cell Guard compares to other diagnostic tools
While electrochemical impedance spectroscopy (EIS) can also detect early cell degradation, it is more complex and less suited to in-situ, real-time monitoring, especially in large, high-capacity packs. EIS measurements can be affected by pack architecture and high temperatures, while internal short circuit conditions in lithium ion cells require controlled testing conditions. VOC sensors like Cell Guard, in contrast, are not impacted by pack size or complexity and deliver a fast, reliable signal that can be acted upon immediately.
Other gas sensors may lack the sensitivity to detect VOCs at such low concentrations or may not be designed for the harsh internal environment of a lithium ion pack. Cell Guard’s small, robust form factor allows installation close to the cells for maximum accuracy, ensuring safe operation without compromising durability.
Regulatory and industry implications
Global safety standards such as ISO 6469, UN ECE R100, and UL 2580 are placing greater emphasis on early fault detection in lithium-ion battery systems. Incorporating early-warning VOC detection in energy storage systems directly supports compliance with these standards. For manufacturers, it also offers a competitive advantage by demonstrating proactive safety measures to regulators, insurers, and customers.
In addition, the insurance industry increasingly recognises that early detection systems can reduce claims related to thermal runaway incidents. Implementing VOC detection may help operators secure lower premiums and more favorable terms, as it can also help avoid thermal runaway.
Applications Beyond EVs
The benefits of early thermal runaway detection extend far beyond electric vehicles. In stationary storage, detecting a failing cell before ignition can prevent multi-megawatt lithium ion battery fires, especially as the risk of thermal runaway increases . In aerospace applications, where evacuation is not always possible, VOC detection provides vital redundancy. Marine operators can integrate the system for offshore platforms or vessels where access to firefighting resources is limited.
Even in second-life battery applications, where ageing cells present unpredictable risks, Cell Guard can monitor VOC levels in real time in a single battery pack, enabling operators to remove suspect modules before they compromise system safety.
Building a Safer Battery Future
As most lithium ion batteries become larger and more energy-dense, the consequences of thermal runaway events, including self destruction, grow more severe. Traditional monitoring methods alone cannot guarantee sufficient warning. Metis Engineering’s Cell Guard fills this critical safety gap by detecting VOC emissions early enough to take effective action.
By providing a multi-minute intervention window, Cell Guard allows systems to shut down, cool, or isolate failing battery cells, even preventing the escalation into full thermal runaway – fire. This protects assets, extends battery life, improves compliance, and enhances public trust in electrified transport and energy storage.
In the race to make batteries safer, smarter, and more resilient, early thermal runaway detection is not just a technological advantage—it’s a necessity.
