What gases are released
When a lithium-ion cell overheats or is damaged, its electrolyte can vent as vapour. As failure progresses, the released gases may include hydrogen, carbon monoxide, carbon dioxide, various hydrocarbons and hydrogen fluoride. Some of these are flammable, some are toxic and some are both, which means an accumulation of off-gas can present an explosion hazard as well as a serious health risk to anyone nearby.
The exact composition depends on chemistry, state of charge and the way the cell fails. Because venting can begin before temperatures rise dramatically, detecting these gases can provide a warning before thermal runaway propagates from one cell to its neighbours. That early window is valuable, because it may allow automatic systems or trained staff to intervene while the event is still confined.
| Gas | Hazard | Detection consideration |
|---|---|---|
| Electrolyte vapour | Flammable, early indicator | Specialised off-gas sensors |
| Hydrogen | Highly flammable, rises | Sensors at high level |
| Carbon monoxide | Toxic and flammable | Standard CO detectors |
| Hydrogen fluoride | Toxic and corrosive | Specialist sensors and PPE planning |
Designing detection
Sensor selection should reflect the gases expected and the purpose of the detection system. Off-gas sensors designed to respond to electrolyte vapour can give very early warning of cell failure, while hydrogen and carbon monoxide detectors support broader life safety and explosion protection. Placement matters because gases behave differently: hydrogen rises towards the ceiling, while heavier vapours may collect at lower levels.
Detection should trigger alarms and, where appropriate, automatic actions such as increasing ventilation, isolating chargers or shutting down equipment. Sensors need regular calibration and functional testing, following the manufacturer's recommendations, to remain reliable. A detector that has drifted out of calibration can give false reassurance, which is arguably worse than having no detector at all. Maintenance records should therefore be kept and reviewed alongside alarm logs.
- Choose sensors for the expected gas mix
- Place sensors according to gas behaviour
- Link detection to alarms and automatic responses
- Calibrate and test on a fixed schedule
Ventilation and explosion prevention
Ventilation dilutes flammable gases and reduces the likelihood of dangerous concentrations forming. Enclosures and storage rooms may need exhaust systems specifically designed for battery off-gas, with fans that start automatically when gas is detected and continue running until concentrations fall. Supply air paths matter as much as extraction, because poorly planned airflow can leave pockets where gas accumulates. Airflow modelling can reveal those pockets before construction.
In enclosed spaces, explosion prevention or deflagration venting may also be required. Guidance such as NFPA 855, NFPA 68 and NFPA 69 is frequently referenced for energy storage installations, and local codes may impose their own requirements. Designers should consider these measures together with fire protection from the outset, rather than adding them after the layout and structure have already been fixed.