Thermal runaway is a fast-escalating failure in a battery cell where heat triggers more heat, potentially ending in fire. The earliest reliable warning comes from off-gassing (VOCs) inside the pack—often minutes before excessive heat causes temperatures to spike. Metis Engineering’s Cell Guard detects VOCs (plus dew point, humidity, hydrogen and shock) and publishes data over CAN, creating a vital window to isolate faults, cool the system and, with luck, prevent propagation and full blown thermal runaway, AKA, fire.
What is thermal runaway?
Thermal runaway is a self-accelerating chain reaction in which rising cell temperature speeds up chemical reactions inside lithium ion batteries. As electrolyte and electrodes decompose, they release heat and flammable gases. Left unchecked, the cell can vent, ignite, and trigger neighbouring cells—turning a small defect into a pack-level event, increasing the risk of thermal runaway – a fire.
Typical pathway for different battery types :
- Initiation: Overcharge, internal short, physical damage or external heat compromises separator/SEI.
- Decomposition: Electrolyte breaks down; pressure and flammable gases build.
- Thermal runaway & propagation: Temperatures accelerate; adjacent cells are driven into the same failure.
Why lithium ion battery safety is also a performance issue
Safety measures do more than prevent rare catastrophes—they protect performance and uptime day-to-day:
- Asset protection: Avoiding a single pack incident can save equipment, facilities and reputations.
- Higher availability: Early warnings enable controlled shutdowns rather than extended outages.
- Longer life: Catching localised faults such as moisture ingress early reduces heat stress and slows degradation.
- Better data: Clean, time-aligned health signals improve diagnostics and predictive maintenance.
The detection gap in conventional monitoring
A Battery Management System (BMS) must watch the entire battery system’s temperature, voltage, current and SoC/SoH. These are essential, but they often remain within normal limits until very late in the failure sequence. By the time a temperature excursion becomes obvious, the opportunity to act may have narrowed to seconds.
To “move left” on the timeline, operators need an earlier, chemistry-specific precursor—a signal that appears before temperature spikes to prevent thermal runaway .
VOCs: the earliest practical warning
One of the first signs of cell distress is volatile organic compound (VOC) release as the electrolyte begins to decompose. This off-gassing happens inside the enclosure and can precede temperature rises by valuable minutes. Monitoring VOC concentration within the pack provides a direct, actionable indicator that something is going wrong with individual cells , such as an internal short circuit —early enough to interrupt the sequence.
Meet Cell Guard: early warning at the source
Metis Engineering’s Cell Guard is a compact, rugged sensor designed for installation inside lithium ion battery packs and enclosures.
What it monitors during normal operation
- VOCs: Early indicator of electrolyte decomposition/off-gassing
- Hydrogen: Useful for certain chemistries/environments
- Humidity & dew point: Flags moisture ingress and condensation risk
- Shock/acceleration: Correlates impacts with subsequent faults
How it integrates
- CAN interface: Publishes measurements and diagnostic flags as standard CAN frames
- Real-time: Low-latency data for BMS/ECU use, logging and alarms
- Configurable: Thresholds and IDs tailored to your DBC/data model
Why it matters
By detecting precursor gases rather than waiting for a thermal spike, Cell Guard helps create an intervention window—often several minutes—so the system and operators can act decisively, safeguarding the energy stored .
Turning early warnings into action
With a VOC-first alert, a BMS or safety PLC can execute graded responses:
- Unload electrically: Stop charge/discharge to cut internal heating.
- Isolate modules/strings: Contain the issue and prevent propagation.
- Boost cooling: Increase fans/pumps or coolant flow.
- Pre-emptive suppression: Trigger aerosol, inert gas or liquid agents before ignition risk peaks.
- Notify & log: Alert operators and record high-fidelity data for root-cause analysis.
The result: fewer false alarms, more controlled interventions and a meaningful reduction in risk associated with maximum temperatures .
Safety, compliance and insurability
Early fault detection supports compliance with major frameworks (e.g., automotive and stationary lithium ion battery safety standards) by demonstrating proactive hazard identification, mitigation, and adherence to fire safety standards. Insurers increasingly recognise the value of early detection in reducing claim severity and improving overall risk profiles.
Use cases across sectors
- EVs & buses: Adds pack-internal context to BMS decisions; supports safe pull-over strategies.
- Energy storage (ESS): Helps prevent container-level incidents and costly downtime.
- Aerospace & marine: Provides redundancy where evacuation or firefighting access is limited.
- Second-life lithium ion batteries: Flags inconsistent modules early for selective removal.
Implementation best practice
- Placement: Mount sensors near probable vent paths or within module plenums to capture earliest gas signatures.
- DBC/ID planning: Define CAN IDs, scaling, units and diagnostics; separate fast safety frames from slower telemetry.
- Thresholds: Set graded VOC/hydrogen thresholds with timing hysteresis to balance sensitivity and nuisance alarms.
- System drills: Test the entire chain—from alert to isolation/suppression—and document SOPs.
- Data hygiene: Log events to refine thresholds and correlate with temperature, current and shock data.
FAQs
What is thermal runaway in simple terms?
A self-heating loop where cell reactions generate more heat, accelerating until venting, ignition or propagation occurs.
Why not rely on temperature sensors alone?
Temperature often lags behind early chemical reactions, which also produce heat and changes. VOCs can appear before temperature spikes, offering a longer head start.
Will Cell Guard work with my BMS?
Yes—Cell Guard publishes data over CAN, so it integrates with BMS/ECUs, loggers and gateways that support standard CAN messaging.
Can it help with moisture-related faults?
Yes—humidity and dew point measurements help detect ingress/condensation that can cause shorts and corrosion.
Does it replace a Battery Management System ?
No. It complements the BMS, adding an early-warning layer and richer context for safer, smarter decisions in energy storage systems .
Does Cell Guard work with all battery systems and chemistries?
Yes – Cell Guard works with LFP, LMFP, pouch cells, prismatic cells and cylindrical cells of all form factors.
Key takeaways
- Thermal runaway is fast and dangerous—but rarely surprise-fast if you monitor the right precursor.
- VOCs are a practical early indicator of electrolyte decomposition inside the pack.
- Cell Guard combines VOC, hydrogen, humidity/dew point and shock sensing with CAN integration to create a usable intervention window.
- Early detection protects people and assets, improves uptime, and supports compliance and insurability.
Next step: Want earlier, cleaner battery safety signals for electric vehicles ? Explore how Metis Engineering’s Cell Guard can integrate into your pack architecture and BMS to reduce risk, avoid thermal runaway, and enhance lithium ion battery pack performance.
