Gas Sensing - Battery Design

Gas sensing within a battery pack is one way of understanding when a cell has failed and a possibility of thermal runaway if there is no intervention.

Electrolyte Detection

During a thermal runaway event, various gases are released. In the early stage, the cell vents a high concentration of electrolyte vapor. This includes gases such as Ethylene carbonate (EC), Diethyl carbonate (DEC), Ethyl Methyl carbonate (EMC) and Dimethyl carbonate (DMC) etc.
As the event progresses to an explosion, there is an increase in H2 and CO.
Honeywell

What Gases Can Be Sensed

Thermal runaway does not just produce heat. As a cell degrades and eventually vents, it releases a specific mix of gases that can be picked up well before temperature or voltage sensors show anything unusual. Research on lithium-ion off-gassing consistently points to a handful of species: carbon dioxide (CO2), typically the earliest and most abundant gas released and often the majority of the vented gas volume; hydrogen (H2) and carbon monoxide (CO), produced as electrolyte decomposition progresses further into the event; light hydrocarbons such as ethylene (C2H4) and methane (CH4); and the volatile electrolyte solvents themselves, ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and dimethyl carbonate, which the Electrolyte Detection page on this site covers in more depth. Different studies emphasise different gases depending on cell chemistry and the exact abuse scenario tested, which is part of why production sensors increasingly target more than one gas at once rather than relying on a single species [1] [2].

Examples of Gas Sensing in Production Packs

Two approaches show up repeatedly in commercial products. Honeywell’s BES (Battery Safety Electrolyte Sensor) series uses Li-ion Tamer electrolyte gas detection technology to catch the electrolyte vapour released at first vent, ahead of a full thermal runaway event, with Honeywell reporting alarms 5 to 20 minutes ahead of a fire under test conditions [3]. SGX Sensortech’s TRDU5 takes a different route, monitoring hydrogen and CO2 alongside pack pressure, temperature and relative humidity from a single point in the pack. It is claimed to detect cell venting within seconds regardless of cell size, chemistry or pack configuration, with a demonstrated sensor life of up to 10 years [4]. Both sit inside, or alongside, the battery management system rather than replacing it, feeding an early warning into whatever contactor opening or evacuation response the pack is designed to trigger.

Advantages of Gas Sensing

The main advantage is lead time. Voltage and temperature, the two signals a conventional BMS already watches, tend to change slowly in the earliest stage of a cell failure, whereas gas release can begin as soon as the cell’s internal pressure forces a vent, often before there is any meaningful change in temperature at the pack level [5]. That gap matters in practice: Honeywell’s own testing found the BES sensor alarmed around 7 minutes before a pouch cell reached thermal runaway during an overcharge test, while H2, CO, CO2 and CH4 sensors only registered a response once the cell was already exploding [3]. Gas sensing is also largely independent of cell chemistry, format and pack design, since it detects a byproduct of failure rather than monitoring the cell itself, which makes it easier to retrofit or standardise across different battery platforms.

Disadvantages and Challenges

The main problems are false alarms and long-term reliability. Ambient humidity, dust and other airborne contaminants can trigger a response in some sensor types, and several of the cheaper sensing technologies, including some chemical CO2 sensors and polymer-based sensors, are known to drift or degrade over time, needing recalibration or replacement that adds to total cost of ownership [2]. Cross-sensitivity is a related issue: a sensor tuned to respond to CO2 or hydrocarbons can also respond to unrelated gases in the surrounding environment, so distinguishing a genuine thermal event from background noise usually requires reference sensors monitoring the ambient air alongside the pack sensors, not just a single detector [6]. Placement is also non-trivial, since airflow inside a sealed pack is uneven, so a sensor positioned in the wrong spot can miss a vent event from a cell on the far side of the pack, or respond too slowly to be useful.

None of this makes gas sensing a replacement for a well designed BMS. It is best understood as an additional, faster layer of detection sitting alongside voltage and temperature monitoring, not instead of it.