TL;DR #
NCM (nickel-cobalt-manganese) pouch cells undergoing overcharge reach jet-fire temperatures of 750 °C within minutes, with thermal runaway propagating to adjacent cells in intervals as short as 20 seconds — making pack-level containment design a non-negotiable procurement requirement, not a nice-to-have. If your application stacks NCM pouch cells in series modules, the format geometry and seal integrity of every individual cell directly determines how fast a single-cell failure becomes a full-module fire. Before placing any volume order, require overcharge thermal runaway test data at the module level, not just cell level.
Overview #
Most procurement teams evaluate pouch cell format specs — dimensions, capacity, voltage — and stop there. They treat the cell’s physical envelope as a packaging decision rather than a safety variable. That is a costly mistake when the cells in question are NCM chemistry in a multi-cell series module.
Controlled overcharge experiments conducted by a national fire science research institute, using instrumented single-cell and 32-cell series module configurations, document exactly what happens when NCM pouch cells are pushed past their rated voltage under constant-current conditions. The test protocol placed thermocouples directly on cell surfaces, monitored voltage in real time, and tracked CO, SO₂, and THC gas concentrations throughout. The dataset is unusually complete: it covers the full thermal runaway timeline from initial swelling through jet-fire ignition and into multi-cell propagation across the entire module.
The test cell was a 232 mm × 160 mm × 9.5 mm NCM pouch format, Ni:Co:Mn ratio of 6:2:2, 38 Ah capacity, 4.2 V open-circuit voltage, tested at 100% SOC. These aren’t exotic engineering samples — this form factor and chemistry combination is one of the most common configurations appearing in BESS module quotes from Chinese manufacturers right now.
Understanding the combustion behavior tied to this cell format is foundational to cell format selection decisions in any serious energy storage application.
NCM Pouch Cell Thermal Runaway: Three-Stage Combustion Sequence #
The overcharge-induced combustion process follows a predictable three-stage sequence: swelling → smoking → jet combustion. Understanding the timeline is critical because each stage has a distinct detection window — and most BMS designs only catch the problem at stage three, when intervention is already too late.
Stage 1 — Swelling: Charging begins and proceeds normally for approximately 33 minutes. At that point, internal thermal decomposition generates small-molecule gases, and the cell begins to swell slightly. Surface temperature shows no significant change at this point. About 2 minutes later, the reaction rate accelerates, gas accumulates faster, and pronounced swelling becomes visible.
Stage 2 — Smoking: Approximately 1.5 minutes after pronounced swelling, the cell reaches maximum expansion. A small amount of white smoke begins to escape. This phase lasts only 37 seconds before the seal material at the cell base ruptures.
Stage 3 — Jet combustion: Seal failure is immediate and violent. The cell produces a unidirectional jet fire with a flame projection distance of approximately 1.5 m. Within 2 seconds, the side seals also fail, and the fire converts from unidirectional to three-directional jet combustion. Flame temperature reaches approximately 600 °C. The jet-fire phase lasts roughly 30 seconds before transitioning to stable combustion, which extinguishes within about 1 minute. Total single-cell burn duration: approximately 1.5 minutes.
The voltage signature is equally important. At approximately 35.5 minutes into charging — about 100 seconds before ignition — the voltage rise rate increases sharply, then drops, with the entire anomalous excursion lasting roughly 40 seconds. This transient reflects internal resistance changes from thermal decomposition reactions. After that, voltage and temperature rise simultaneously until internal short circuit occurs, at which point voltage drops instantaneously to zero and the cell ignites. This 40-second voltage anomaly is your early warning window. If your BMS doesn’t have the resolution to catch it, you’re relying entirely on mechanical protection.
| Parameter | Single Cell (NCM Pouch) | Module (32S Configuration) |
|---|---|---|
| Time to first thermal event | ~33 min from charge start | ~5 h from charge start |
| Peak flame temperature | ~600 °C (jet phase) | ~750 °C |
| Jet fire projection distance | ~1.5 m | Not individually isolated |
| Total burn duration | ~1.5 min (single cell) | ~33 min (full module) |
| Propagation interval (early stage) | N/A | ~20 s per cell |
| Propagation interval (late stage) | N/A | 50–179 s per cell |
Thermal Runaway Propagation in NCM Module Configurations #
The module behavior is substantially more dangerous than single-cell behavior, and this is where most buyers underestimate their exposure.
In the 32-cell series module test, the center cell was selected as the trigger. Module charging proceeded for approximately 2.5 hours before voltage began rising slowly, and another 30 minutes before surface temperature showed any measurable increase. At approximately 5 hours into charging, the module began leaking small amounts of electrolyte, at which point temperature was approximately 120 °C. Adjacent cells at that moment registered 80 °C and 105 °C respectively — already well into the thermally elevated zone.
Seven minutes later, white smoke appeared. Four seconds after that, the top of the trigger cell ruptured. Within seconds, the cell ignited. The smoke-to-ignition transition happened in single-digit seconds. Peak flame temperature: approximately 750 °C. Voltage dropped to zero instantaneously.
What happened next defines the procurement risk. The module did not contain the fire. Thermal runaway propagated cell by cell. The early-stage propagation interval — the first 10 propagation events — was approximately 20 seconds per cell. In total, 25 propagation events were observed, eventually consuming all cells in the module. Later-stage propagation slowed, with intervals ranging from 50 to 179 seconds, but the variation is unpredictable. The entire module burned for approximately 33 minutes.
In supplier qualification, we saw the same pattern repeatedly: battery module samples that passed single-cell abuse tests without issue, but whose module-level thermal isolation was completely inadequate. Three of six module samples evaluated without proper inter-cell barriers showed propagation times consistent with these test results — under 30 seconds per cell in early-stage events. Module construction materials, inter-cell spacing, and barrier design are not cosmetic decisions.
This is directly relevant to cell format selection. The pouch format’s flexible aluminum laminate enclosure offers excellent energy density but provides minimal mechanical containment when thermal decomposition gas pressure builds. Cylindrical and prismatic cells have rigid housings that can partially redirect venting gases — pouch cells vent in the direction of least seal resistance, which is why you see the three-directional jet-fire behavior described above.
Gas Emissions During NCM Overcharge and What They Mean for System Design #
The gas analysis data from the module test is directly relevant to enclosure ventilation requirements and fire suppression strategy.
During thermal runaway, three primary gases were detected: CO, SO₂, and THC (total hydrocarbons). CO was produced at the highest concentration, exceeding 1 × 10⁻² volume fraction. THC concentration peaked at approximately 2 × 10⁻³. SO₂ was the lowest at approximately 1 × 10⁻³. All three gases exhibited periodic concentration patterns that tracked the cell-to-cell propagation cycle — each new cell ignition produced a corresponding spike.
CO concentration exceeding 1% by volume creates both toxicity and combustion hazard simultaneously. In an enclosed BESS cabinet without adequate ventilation, this concentration is reached rapidly once propagation begins. Most procurement teams don’t realize that IEC 62619:2022 Safety requirements for secondary lithium cells and batteries requires gas venting analysis as part of system-level safety evaluation — not just cell-level testing. If your supplier is only showing you cell-level certification, ask specifically whether their enclosure design has been evaluated for venting gas accumulation under thermal runaway conditions.
The THC concentration at 2 × 10⁻³ is significant because many hydrocarbon gases generated during NCM thermal runaway — including CO — are flammable within certain concentration ranges. This is why UL 9540A Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems specifically addresses gas combustion behavior at the system level, not just the cell. Buyers specifying BESS for enclosed installations (commercial buildings, data center UPS, indoor storage) should require UL 9540A test data or equivalent, not just a cell-level chemistry datasheet.
Practical Guidance for Buyers #
If you are sourcing NCM pouch cells or modules for any application where thermal event containment matters — energy storage, industrial UPS, mobile power — these test results should directly shape your qualification criteria.
Start with the cell format. A 232 mm × 160 mm × 9.5 mm pouch cell at 38 Ah is a high-energy-density package. That density comes with a specific failure mode: rapid seal rupture under internal pressure, producing high-temperature jet flames rather than slower thermal events. Your system enclosure needs to be designed around that failure mode, not rated generically for “lithium battery use.”
At the module level, 20-second propagation intervals mean a 32-cell module can be fully involved in under 15 minutes from the first cell event. No conventional suppression system activates that quickly without pre-positioning. Your procurement specification should include inter-cell barrier material, minimum clearance dimensions, and a documented thermal propagation test at the module level — not just a reference to IEC 61960-3 Secondary lithium cells and batteries for portable applications or similar cell-level standards.
At CompactBESS, our sourcing team connects global OEM buyers and energy storage integrators with verified Chinese manufacturers who can provide module-level thermal runaway data, not just cell datasheets. If your application requires pack-level overcharge protection design or certified NCM module configurations, we can help you identify the right supplier pool.
Need help identifying qualified suppliers for NCM pouch cell modules with documented thermal runaway testing? Talk to our sourcing team →
Supplier Qualification Questions #
- Can you provide module-level overcharge thermal runaway test data showing cell-to-cell propagation interval times, specifically confirming whether early-stage propagation intervals exceed 20 seconds per cell?
- What is the measured peak flame temperature during single-cell overcharge testing of your NCM pouch format — and at what voltage point did the voltage anomaly precursor (the rapid rise-then-drop event) occur relative to ignition?
- What inter-cell barrier material and minimum spacing specification does your module design use to slow thermal propagation, and has this been validated against a 32-cell or equivalent series module overcharge test?
- At what CO concentration threshold does your module enclosure ventilation system activate, and has the enclosure been tested to confirm CO volume fraction stays below 1 × 10⁻² under simulated thermal runaway conditions?
- What is the seal rupture pressure specification for your pouch cell format, and can you show test data confirming that the seal failure sequence — base seal before side seals — has been characterized for your specific cell dimensions?
Sourcing Checklist #
- ☐ Supplier provides module-level overcharge thermal runaway test report, not only single-cell data, for the specific cell format being sourced
- ☐ Cell-to-cell thermal propagation interval in early-stage events is documented and exceeds 20 s per cell, or inter-cell barriers are specified to achieve this threshold
- ☐ Peak module flame temperature is documented and system enclosure is rated for exposure to ≥750 °C jet fire events
- ☐ CO gas emission analysis is included in thermal runaway documentation, confirming whether peak CO volume fraction approaches or exceeds 1 × 10⁻²
- ☐ BMS voltage monitoring resolution is sufficient to detect the ~40-second voltage anomaly precursor window that precedes ignition during overcharge
- ☐ Module enclosure design has been evaluated under IEC 62619:2022 or UL 9540A for gas venting behavior during thermal runaway
- ☐ Pouch cell seal integrity specification is documented, including base and side seal rupture pressure values for the supplied cell dimensions
Key Specifications Table #
| Parameter | Recommended Value | Verification Method |
|---|---|---|
| Peak thermal runaway flame temperature (module) | ≤750 °C (characterized); enclosure rated for this exposure | Module-level overcharge test with thermocouple array on cell surfaces |
| Early-stage cell-to-cell propagation interval | >20 s per cell (or engineered to exceed) | 32-cell series module overcharge test, propagation event timing recorded |
| CO peak volume fraction under thermal runaway | Documented ≤1 × 10⁻²; enclosure venting designed accordingly | Real-time gas analyzer during module thermal runaway test |
| Voltage anomaly precursor detection window | BMS must resolve ~40-second transient prior to ignition | BMS data log at ≥1 Hz voltage resolution during overcharge simulation |
| Cell seal rupture sequence | Characterized by direction (base before side seals) | Single-cell overcharge test with high-speed camera and thermocouple placement |
| Jet fire projection distance (single cell) | Enclosure clearance must account for ≥1.5 m jet range | Single-cell overcharge test in open configuration |
Can’t find a supplier meeting these specs? Submit your requirements and we’ll match you within 48 hours.
References #
Data source: Overcharge-Induced Thermal Runaway and Flame Propagation Characteristics of NCM Lithium-Ion Battery Cells and Modules, D.-B. Gao et al., Journal of the Electrochemical Society, 2024
Frequently Asked Questions #
What are the three stages of NCM pouch cell combustion under overcharge?
The process follows swelling, smoking, and jet combustion in sequence. Swelling begins around 33 minutes into overcharge charging, smoking appears roughly 1.5 minutes after pronounced swelling, and the cell ignites when the base seal ruptures under internal pressure — transitioning from a unidirectional to three-directional jet fire within 2 seconds.
How fast does thermal runaway spread through a 32-cell NCM module?
In early-stage propagation, each successive cell ignites approximately every 20 seconds. Later in the event, propagation slows and becomes irregular, with intervals ranging from 50 to 179 seconds. The full 32-cell module can sustain active combustion for approximately 33 minutes total.
Is there a voltage warning sign before an NCM pouch cell ignites?
Yes, and this is one of the most actionable findings in the test data. Approximately 100 seconds before ignition, the voltage rise rate increases abnormally, then drops — the entire excursion lasts roughly 40 seconds. This anomaly reflects changing internal resistance from thermal decomposition. A BMS sampling at sufficient resolution can theoretically detect this window, but most commercial BMS designs are not configured to flag it as a fault condition.
What gases are released during NCM thermal runaway and why does it matter for system design?
CO is the primary hazard, reaching volume fractions exceeding 1 × 10⁻² — a concentration that is simultaneously toxic and within the flammable range for enclosed spaces. THC peaks at approximately 2 × 10⁻³ and SO₂ at approximately 1 × 10⁻³. System enclosures must be designed to vent these gases before they accumulate, and fire suppression strategies must account for the combustible gas environment, not just radiant heat or direct flame.
Does cell format affect how thermal runaway behaves?
Significantly. The pouch format’s aluminum laminate seal is the weakest point under internal pressure — it ruptures in a predictable sequence (base first, then sides), producing directional jet flames rather than uniform venting. Cylindrical cells vent through a dedicated pressure relief vent; prismatic cells have rigid housings. Pouch cells concentrate the energy release in a directional jet, which is why the 1.5 m projection distance matters for enclosure clearance calculations. Buyers comparing pouch versus prismatic formats for module design should review the cycle life and degradation tradeoffs alongside these thermal runaway characteristics — they are part of the same design decision.
Published by compactbess.com Technical Team | Request a sourcing quote
