Every lithium-ion cell has a safety vent. When gas builds up inside the cell during a fault, the vent opens to release it before the casing ruptures. That moment, cell venting, is one of the clearest early warnings that a cell is heading towards thermal runaway. It also produces a physical signal that is surprisingly easy to measure: a change in pressure inside the battery enclosure. This is why pressure sensor battery pack monitoring has become a key part of modern battery safety strategies.
What happens when a cell vents
Thermal runaway starts with chemistry. An internal short, separator damage, overcharge or external heat causes the electrolyte to decompose, generating gases such as carbon dioxide, carbon monoxide, hydrogen and vaporised solvents. Pressure builds inside the cell until the vent opens and the gas escapes into the pack.
In a sealed or semi-sealed enclosure, that sudden release of gas raises the internal pressure. Even in a large pack, the step change from a single venting cell can be detected by a sufficiently sensitive sensor. This happens at the start of the failure sequence, often before neighbouring cells heat up enough for conventional temperature sensors to react.
Why the BMS alone can miss it
Battery management systems monitor cell voltages and a limited number of temperature points. They are essential for day-to-day control, but they are not designed to detect venting directly. Temperature sensors may be too far from the failing cell to see a rapid local rise, and cells connected in parallel can prop up the voltage of a weakening cell, masking the fault.
With pressure sensor battery pack monitoring, a single sensor inside the enclosure sees the whole volume of the pack at once. It does not matter where the venting cell is located; the pressure change reaches the sensor quickly.
The value of absolute pressure measurement
Not all pressure sensors are equally suited to this job. A gauge sensor measures relative to the surrounding atmosphere, which means its readings shift with weather and altitude. An absolute sensor measures relative to a vacuum reference, giving a stable baseline regardless of where the vehicle is or what the weather is doing. For a vehicle that may climb a mountain pass or an aircraft ground support vehicle operating at altitude, that stability is important for distinguishing a genuine venting event from environmental change.
Metis Engineering’s Cell Guard uses absolute pressure sensing across a range of 0.3 to 1.2 bar, with a resolution of 0.0001 bar, accuracy of 0.0005 bar between 0.3 and 1.1 bar, and an update rate of up to 50 Hz. That combination of fine resolution and fast updates allows it to pick up the subtle, rapid pressure change associated with venting.
Pressure plus chemistry: a stronger signal
On its own, a pressure change can have other causes, such as a rapid change in coolant temperature, a door slam on a sealed enclosure or a breather valve operating. That is why Cell Guard does not rely on pressure alone. It also measures volatile organic compounds from 0 to 6,553.5 ppm, air temperature, relative and absolute humidity and dew point, with an optional triple-axis accelerometer recording shock loads up to ±24g.
When a cell vents, the sensor typically sees a pressure step and a VOC spike together. Cross-checking the two gives the BMS or vehicle controller high confidence that a real event is occurring, allowing it to act decisively while avoiding nuisance alarms.
Proven detection speed
Sandia National Laboratories in the United States tested Cell Guard alongside two competing products and published the results in the Journal of the Electrochemical Society. Cell Guard detected the thermal event in under 60 seconds. A competing hydrogen-only sensor took seven minutes longer, by which point thermal runaway had reached an advanced stage.
Integration inside the pack
Pressure sensor battery pack integration has to respect the realities of pack design: limited space, harsh conditions and an existing network. Cell Guard measures 11.5 by 50 by 54.5 mm and communicates over CAN bus with configurable address and baud rate. It is supplied with a DBC file, so readings can be decoded immediately by the BMS or data logger. Metis recommends one sensor per pack, positioned near the breather where gas will pass. For large energy storage installations, more than 100 units can be daisy chained.
Cell Guard has been developed in line with ISO 26262 processes and tested to ISO 7637-2:2011, ISO 16750-2:2012 and ISO 16750-4:2010. It is used by OEMs and Tier 1 suppliers in ASIL B applications, works across NMC, LFP and LMFP chemistries and has been deployed in more than 500 systems.
Acting on the data
Once the controller receives a venting alert, it can open the pack contactors, change the cooling strategy, warn the driver to stop and leave the vehicle, or trigger fire suppression in a stationary system. In low power mode, the sensor can continue to monitor the pack while the vehicle is parked and wake the system if a threshold is crossed, using its 500 mA low-side drive output.
Setting the standard for tomorrow
Regulation is moving in the same direction. China’s GB 38031-2025, applying to new type approvals from 1 July 2026, requires no fire or explosion for at least two hours after thermal runaway and an alarm within five minutes. Early, reliable detection of venting, through pressure and gas sensing, is central to meeting requirements of this kind.
To discuss adding pressure-based venting detection to your battery pack, contact Metis Engineering at info@metisengineering.com.
