When a lithium-ion cell begins to fail, it does not burst into flames without warning. It first starts to break down chemically, and one of the earliest measurable signs of that breakdown is the release of volatile organic compounds. For battery engineers, this makes the VOC detector one of the most valuable safety tools available, and one of the most underused.
What VOCs reveal about a failing cell
The liquid electrolyte in most lithium-ion cells is a blend of organic carbonate solvents with a lithium salt. When a cell is damaged, overcharged, overheated or suffers an internal short circuit, the electrolyte begins to decompose. Gases build up inside the cell until the pressure opens its safety vent. What escapes into the pack enclosure is a mixture dominated by vaporised solvents and their decomposition products, along with carbon dioxide, carbon monoxide and hydrogen.
Many of these compounds are VOCs, and they are released at the point of venting, often well before surface temperatures on neighbouring cells rise far enough to trigger a conventional temperature alarm. A VOC detector positioned inside the pack is therefore reading the chemistry of failure directly, rather than waiting for its thermal consequences.
Why temperature and voltage are not enough
Battery management systems are excellent at what they were designed to do: monitor cell voltages, estimate state of charge and keep cells within their thermal window. But they have limitations as a thermal runaway warning system. Temperature sensors are usually placed at a limited number of points, so a failing cell may be some distance from the nearest one. Voltage can be misleading, because healthy cells in parallel can hold up the voltage of a degrading cell and disguise the problem in the short term.
By the time temperature-based detection responds, the failing cell may already be at an advanced stage. A VOC detector closes that gap by providing a signal at the very start of the event.
Independent validation
The strongest evidence for VOC-based detection comes from independent testing. Sandia National Laboratories in the United States evaluated Metis Engineering’s Cell Guard alongside two other detection products and published the results in the Journal of the Electrochemical Society. Cell Guard, which uses VOC detection as a central part of its sensing strategy, detected the thermal event in under 60 seconds. A competing hydrogen-only sensor took seven minutes longer to register the same event, by which time thermal runaway was well advanced.
In an electric vehicle, those minutes decide the outcome. They can give occupants time to stop safely and leave the vehicle, and give the control system time to isolate the pack.
Inside a battery-grade VOC detector
Cell Guard’s VOC sensor uses metal oxide semiconductor technology optimised for detecting electrolyte decomposition products, with a measurement range from 0 to 6,553.5 ppm. Metis describes the approach as chemistry agnostic, working consistently across NMC, LFP and LMFP cells without calibration changes for different battery types.
The VOC channel is not used in isolation. Cell Guard also measures absolute pressure with a resolution of 0.0001 bar, air temperature, relative and absolute humidity and dew point, with an optional triple-axis accelerometer that records shocks up to ±24g. When a cell vents, the pack typically sees a VOC spike together with a pressure step. Seeing both at once gives the control system far greater confidence that a real event is under way, which reduces the risk of false alarms.
Placement and integration
A VOC detector can only warn of what reaches it, so placement matters. Metis recommends one sensor per battery pack, positioned near the breather or vent path where gases released anywhere in the enclosure will pass. For large energy storage systems, more than 100 Cell Guard units can be daisy chained on a single CAN network.
Integration is designed to be straightforward. Cell Guard communicates over CAN bus with configurable address and baud rate and is supplied with a DBC file, so its data can be decoded by the BMS, vehicle control unit or data logger immediately. It has been developed in line with ISO 26262 processes, tested to ISO 7637-2:2011, ISO 16750-2:2012 and ISO 16750-4:2010, and is used by OEMs and Tier 1 suppliers in ASIL B applications. It has been deployed in more than 500 systems.
When background VOCs are high
Some environments contain high background VOC levels that could mask or mimic a venting event. Energy storage systems assembled with large amounts of adhesive are a common example. For these applications, Metis offers hydrogen-capable variants, including Cell Guard with Hydrogen and H Guard, which add hydrogen sensing from 0 to 20% and are designed to detect thermal events reliably even where ambient VOCs remain elevated.
Meeting tougher expectations
Regulators are raising the bar. China’s GB 38031-2025, which applies to new type approvals from 1 July 2026, requires that a traction battery produces no fire or explosion for at least two hours after thermal runaway, and that a warning is raised within five minutes. Other markets are also tightening requirements. Early, reliable detection underpins every containment and mitigation strategy designed to meet them.
From warning to action
A VOC detector cannot stop a cell failing. It buys time, and time is what engineers need to protect people and assets. Fed over CAN into the BMS, a VOC alarm can trigger contactor opening, cooling changes, driver warnings or fire suppression. Logged over the life of the pack, it contributes to a record of battery health that supports warranty, insurance and second-life decisions.
To discuss integrating VOC-based thermal runaway detection into your battery system, contact Metis Engineering at info@metisengineering.com.
