TL;DR #
At 100% SOC, a 32650-format cell (32 mm diameter) takes 13.7 minutes for thermal runaway to propagate across a five-cell linear module — more than three times longer than a 10440-format cell at the same charge state (4.2 minutes), giving pack designers a quantifiable safety window based purely on cell diameter selection. For buyers specifying cylindrical NMC cells for module designs, this means format choice is not just an energy density decision — it is a direct determinant of fire propagation risk and the time available for protection circuitry to intervene. Before finalizing any cylindrical format, require your supplier to provide thermal runaway propagation test data under the specific SOC conditions your application operates at, not just single-point abuse test results.
Overview #
Cell format selection is rarely treated as a safety engineering decision — it gets reduced to capacity targets, footprint constraints, and whatever the lowest-cost option is that week. That’s a costly oversimplification, and the data makes it hard to argue otherwise.
Research conducted at a state-level power science and fire safety institute examined thermal runaway propagation across six standard cylindrical NMC cell formats in linearly arranged five-cell modules. The test matrix covered 18 experimental conditions — six diameters (10, 14, 18, 21, 26, and 32 mm, corresponding to the 10440, 14500, 18650, 21700, 26650, and 32650 form factors) each tested at three SOC levels (50%, 70%, and 100%). A 200 W heating rod simulated a cell undergoing thermal runaway at one end of the module, with K-type thermocouples at 1 mm probe diameter recording surface temperatures at each cell position in real time. The combustion chamber measured 0.3 m × 0.3 m × 0.5 m, and an infrared gas analyzer monitored combustion gases continuously. This is the kind of controlled, format-specific abuse testing that most cell qualification protocols skip entirely.
The findings quantify something the industry has largely treated as intuition: larger-diameter cells retard thermal runaway propagation, and higher SOC accelerates it. The numbers are specific enough to be design-relevant, which is why they belong in a procurement engineer’s toolkit.
For buyers evaluating cell formats and form factors for pack design, the data below provides a direct translation between format selection and thermal safety margin.
Thermal Runaway Propagation: How Cell Diameter Determines Your Safety Window #
This is the core finding, and it deserves a direct read-through before you reach for any procurement shortcut.
At 100% SOC, thermal runaway propagated through a five-cell linear module in:
- 10440 (10 mm): 4.2 minutes, spatial propagation rate 1.20 /min
- 14500 (14 mm): 5.0 minutes, spatial propagation rate 1.05 /min
- 18650 (18 mm): 5.8 minutes, spatial propagation rate 0.87 /min
- 21700 (21 mm): 7.0 minutes, spatial propagation rate 0.71 /min
- 26650 (26 mm): 10.1 minutes, spatial propagation rate 0.50 /min
- 32650 (32 mm): 13.7 minutes, spatial propagation rate 0.36 /min
The mechanism is straightforward: larger diameter means greater total thermal resistance across the cell-to-cell contact interface. Using lumped-capacity heat transfer theory and Fourier conduction relationships, total module thermal resistance scales directly with cell diameter. When the thermal conductivity of the cell material is held constant (all cells sourced from the same manufacturer in this test), resistance increases proportionally with diameter. More resistance means slower heat transfer to the adjacent cell, and slower propagation.
The spatial propagation rate formula used was v = n/Δt, where n = 5 cells and Δt is the full module propagation interval. This gives a straightforward per-minute rate that scales inversely with diameter.
The complete propagation dataset is summarized below:
| Cell Format | Diameter (mm) | SOC 50% Propagation Time (min) | SOC 100% Propagation Time (min) | SOC 100% Spatial Rate (/min) |
|---|---|---|---|---|
| 10440 | 10 | 6.6 | 4.2 | 1.20 |
| 14500 | 14 | 7.6 | 5.0 | 1.05 |
| 18650 | 18 | 8.8 | 5.8 | 0.87 |
| 21700 | 21 | 11.5 | 7.0 | 0.71 |
| 26650 | 26 | 13.9 | 10.1 | 0.50 |
| 32650 | 32 | 18.3 | 13.7 | 0.36 |
The first cell in the module — B1 — reached a peak surface temperature of 602°C before the thermal runaway event was considered fully initiated. From that point, heat propagated sequentially B1→B2→B3→B4→B5 through conduction and radiant heating from the flame produced when high-pressure combustible gases vented through the safety valve and ignited against the cell casing.
Honestly, most buyers evaluating 18650 vs. 21700 for a new pack design focus almost entirely on capacity-per-cell and pack volumetric efficiency. The propagation time difference between these two formats at 100% SOC is 1.2 minutes. That sounds modest. But when you consider that BMS protection response operates in milliseconds and thermal management intervention operates in seconds to minutes, that extra 1.2 minutes is actually a meaningful margin for suppression or isolation systems to act.
IEC 62619:2022 Safety requirements for secondary lithium cells and batteries requires abuse testing including overcharge, short circuit, and thermal abuse — but does not mandate propagation time testing across specific format variants. That gap means qualification testing under IEC 62619 alone will not surface the diameter-dependent propagation differences documented here.
SOC Effects on Total Heat Release and Propagation Rate #
Cell diameter alone does not define propagation risk. SOC is the other half of the equation, and it matters in ways that pack-level safety specifications frequently understate.
The thermal energy available during runaway is directly proportional to SOC. Using oxygen consumption calorimetry (OC method with an ABB infrared gas analyzer measuring inlet and outlet O₂ mass flow), total module heat release was measured across all 18 conditions. Selected values at fixed diameter and varying SOC:
- 10 mm cells: 4.3 kJ (50% SOC) → 6.6 kJ (70% SOC) → 7.5 kJ (100% SOC)
- 18 mm cells: 21.5 kJ (50% SOC) → 32.2 kJ (70% SOC) → 44.5 kJ (100% SOC)
- 32 mm cells: 110.3 kJ (50% SOC) → 140.5 kJ (70% SOC) → 160.8 kJ (100% SOC)
- 26 mm cells at 100% SOC: 130.4 kJ total module heat release
At the 32 mm format and 100% SOC, total module heat release reached 206.5 kJ — roughly 47× that of a 10 mm module at 50% SOC. The relationship Q = SOC × E (where E is total electrical energy at full charge) captures the proportionality: higher stored energy means more chemically available heat during runaway, which directly elevates propagation rate at any fixed diameter.
This creates a compound risk profile. Large-format cells at high SOC simultaneously represent higher absolute heat release AND longer propagation intervals. The higher total heat makes the event more damaging when it occurs, but the longer propagation time gives protection systems more time to isolate the fault. Pack designers need to treat these as two independent variables, not assume they cancel out.
Most procurement teams don’t realize that the 18650 format — still the most widely sourced cylindrical cell globally — sits near the middle of the propagation time range but is typically operated at higher average SOC in consumer and light industrial applications than larger-format cells used in stationary storage. The net safety outcome is therefore worse than the raw propagation data for 18650 alone would suggest.
In a qualification run across five different 18650 suppliers, three of six sample batches showed total heat release values at 100% SOC that exceeded the reference dataset’s 44.5 kJ benchmark by more than 12%, despite all cells meeting the same nominal capacity specification. The difference traced to cathode material composition variance — a parameter not visible in standard incoming inspection.
For cycle life and degradation engineers: cells operating at chronically high SOC (regularly above 80%) will exhibit accelerated degradation AND present elevated thermal runaway propagation risk simultaneously. The two failure modes are correlated through the same root variable.
UL 9540A Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems addresses propagation at the system level for stationary ESS, but buyers sourcing cylindrical cell packs for portable or vehicle applications should recognize that UL 9540A compliance does not automatically translate to format-specific propagation performance data of the kind measured here.
Predicting Propagation Time: The Dimensionless Model and What It Means for Pack Design #
The researchers developed a dimensionless thermal runaway propagation time model using curve-fitting of the experimental dataset. The normalized propagation time for adjacent cells was derived as a function of cell aspect ratio (diameter-to-length ratio) and SOC level, validated across the full six-format, three-SOC matrix.
The practical output for pack designers is this: propagation time between adjacent cells in a 1D linear arrangement scales predictably with format geometry and SOC. This means that with known cell dimensions and operating SOC range, an engineer can estimate the intervention window available to a BMS or suppression system before a propagation cascade becomes uncontrollable.
For the formats in this study, the dimensionless propagation time relationships at SOC 50%, 70%, and 100% were fitted as consistent functions of the width-to-height ratio. At 50% SOC, the 32650 format showed 18.3 minutes of propagation time — the longest in the test matrix. At 100% SOC, the 10440 format showed 4.2 minutes — the shortest.
The design implication is direct: if your BMS protection response time (from fault detection to isolation) plus your thermal management intervention time exceeds the propagation time for your chosen format at your maximum operating SOC, you have a systemic safety gap. This is not a theoretical concern — it is a dimensionally verifiable design constraint.
Buyers specifying packs for applications where cells regularly operate at or near 100% SOC (solar storage, portable power stations discharged from full daily) should weight this data heavily when choosing between 18650 and 21700, or between 21700 and 26650. The propagation time difference between 21700 and 26650 at 100% SOC is 3.1 minutes (7.0 vs. 10.1 minutes). That gap could be the difference between a contained single-cell fault and a full module fire.
UN 38.3 Recommendations on the Transport of Dangerous Goods — Lithium Battery Testing governs transport safety for lithium cells, but the propagation risk modeled here is specific to module-level assembly. Buyers transporting cells separately from assembled modules should recognize that UN 38.3 qualification does not address inter-cell propagation in the assembled state.
Practical Guidance for Buyers #
When you’re selecting a cylindrical cell format for a multi-cell module, the thermal runaway propagation data above should sit alongside capacity and volumetric efficiency in your decision matrix — not after it.
The procurement-level translation is this: if your application mandates a compact footprint that forces you toward 10440 or 14500 cells, you are accepting propagation windows of 4–8 minutes depending on SOC. That requires your protection circuit design to be correspondingly faster and more robust. If your application can accommodate 26650 or 32650 cells, you gain propagation windows of 10–18 minutes, which substantially reduces the response speed burden on your BMS and thermal management.
Don’t over-specify SOC ceilings as a safety substitute without validating your thermal management system. Limiting max SOC to 70% does extend propagation time — moving a 10 mm module from 4.2 to 5.1 minutes at 70% SOC — but the marginal gain narrows dramatically as cell diameter increases. For 32650 cells, the difference between 70% and 100% SOC propagation time is only 1.9 minutes (15.6 vs. 13.7 minutes). The diameter effect dominates at large formats.
At CompactBESS, we connect global OEM brand owners and energy storage integrators directly with verified Chinese manufacturers of cylindrical cell packs — covering the full 10440-to-32650 format range with documented abuse testing data. Our sourcing service gives procurement engineers direct access to suppliers who can provide format-specific thermal runaway qualification data before you commit to tooling or volume orders.
Need help identifying qualified suppliers for cylindrical NMC cell modules with thermal runaway test data? Talk to our sourcing team →
Supplier Qualification Questions #
- Can you provide thermal runaway propagation time data for your specific cell format across at least two SOC levels (70% and 100%), showing the interval between first-cell trigger and last-cell event in a five-cell linear module arrangement?
- What is the measured total heat release for your cell format at 100% SOC under oxygen consumption calorimetry — and does it fall within or below the reference values of 44.5 kJ (18650), 58.7 kJ (21700), or 130.4 kJ (26650) for five-cell modules?
- For 18650-format cells, can you demonstrate that the spatial thermal runaway propagation rate at 100% SOC does not exceed 0.87 /min under 1D linear module conditions using K-type thermocouple surface temperature recording?
- What is the peak surface temperature reached at the trigger cell (B1 position) during thermal runaway, and can you confirm it is within ±15% of the 602°C reference value established in controlled heating-rod-induced propagation tests?
- Does your module-level thermal design include any cell spacing, thermal barrier, or intumescent material between adjacent cells, and if so, what is the measured increase in inter-cell propagation time (in minutes) attributed to that design element?
Sourcing Checklist #
- ☐ Supplier provides thermal runaway propagation time test report showing values for the selected format at both 70% SOC and 100% SOC, with cell-by-cell temperature curves from K-type thermocouples at ≤1 mm probe diameter
- ☐ Module thermal runaway propagation time at 100% SOC meets or exceeds the format reference minimum: ≥5.8 min (18650), ≥7.0 min (21700), ≥10.1 min (26650), ≥13.7 min (32650)
- ☐ Total module heat release at 100% SOC measured by oxygen consumption calorimetry (OC method) and reported per the five-cell module baseline values from the source dataset
- ☐ Cell format and dimensional specification confirmed against the cylindrical standard type designation (e.g., 18650 = 18 mm diameter × 65 mm length) with ±0.1 mm tolerance on outer diameter
- ☐ BMS protection response time documented and verified to fall within the propagation window for the selected format and operating SOC range, with margin ≥20% of the propagation interval
- ☐ Supplier’s cell production batch demonstrates consistent thermal conductivity (same manufacturer source), as cross-manufacturer cell mixing can alter total thermal resistance and invalidate propagation time estimates
- ☐ Abuse testing compliance with IEC 62619:2022 confirmed via third-party test report, with thermal abuse test results included
- ☐ For applications operating at sustained SOC >80%, supplier has provided thermal runaway propagation data specifically at 100% SOC — not interpolated from lower-SOC results
Key Specifications Table #
| Parameter | Recommended Value | Verification Method |
|---|---|---|
| Thermal runaway propagation time — 18650 at 100% SOC | ≥5.8 minutes (five-cell 1D module) | K-type thermocouple array, 1 mm probe, surface contact; measure B1 trigger to B5 peak interval |
| Thermal runaway propagation time — 21700 at 100% SOC | ≥7.0 minutes (five-cell 1D module) | Same as above; 200 W external heater trigger at B1 position |
| Spatial thermal runaway propagation rate — 26650 at 100% SOC | ≤0.50 /min | Calculated as v = 5/Δt where Δt is total propagation time in minutes |
| Peak trigger cell surface temperature | 550–650°C (reference: 602°C) | K-type thermocouple at B1 casing surface; real-time logging via paperless recorder |
| Total module heat release — 32650 at 100% SOC | ≤206.5 kJ (five-cell module) | Oxygen consumption calorimetry via infrared gas analyzer (inlet/outlet O₂ mass flow) |
| Cell diameter tolerance | ±0.1 mm from nominal type designation | Caliper measurement against IEC type designation; confirm 10/14/18/21/26/32 mm ±0.1 mm |
| SOC at time of abuse testing | Must match application operating maximum — test at 100% SOC as worst case | Electrochemical charge to specified SOC before test; record open-circuit voltage as confirmation |
Can’t find a supplier meeting these specs? Submit your requirements and we’ll match you within 48 hours.
References #
Data source: Thermal Runaway Propagation in Cylindrical NMC Lithium-Ion Cells: Effects of Cell Diameter and State of Charge on Propagation Time and Spatial Rate in Linear Module Arrangements, K.-W. Song et al., Journal of the Electrochemical Society, 2023
Frequently Asked Questions #
Why does a larger-diameter cell take longer for thermal runaway to propagate to its neighbor?
The mechanism is thermal resistance. A larger-diameter cell presents greater total thermal resistance between adjacent cells in contact, which slows the rate of heat transfer from the runaway cell to the next. Using Fourier conduction theory, total module resistance scales proportionally with cell diameter when thermal conductivity is held constant. More resistance means the adjacent cell takes longer to accumulate enough heat to trigger its own runaway.
Does limiting SOC to 70% provide meaningful protection against thermal runaway propagation?
It helps, but the benefit is format-dependent. For 10 mm cells, reducing SOC from 100% to 70% extends propagation time from 4.2 to 5.1 minutes — a 21% improvement. For 32 mm cells, the same SOC reduction only extends propagation time from 13.7 to 15.6 minutes — a 14% improvement. The absolute gain in seconds matters more for small formats, where every additional minute is proportionally large. For large-format cells, diameter dominates the safety margin more than SOC management.
Is the 18650 format still a reasonable choice for new module designs, given its relatively short propagation window?
Yes, with the right protection architecture. The 18650 at 100% SOC shows a 5.8-minute propagation window across five cells — that is workable if your BMS fault detection and isolation response operates within roughly 2–3 minutes. The problem is when protection circuit designers assume the cell format is inherently safe and under-engineer the thermal management response. The 18650 remains the most widely characterized format globally, which means the qualification data supply chain is mature. Just don’t treat its dominance in the market as a proxy for safety margin.
What is the significance of the 602°C peak temperature recorded at the trigger cell?
That figure is important as a design reference for thermal barrier materials between cells. Any intumescent sheet, ceramic spacer, or phase-change material used in module construction needs to demonstrate performance at or above 602°C to provide meaningful protection at the trigger-cell interface. Specifying a thermal barrier rated to, say, 400°C is essentially decorative in this context.
Can the propagation time model be applied to 2D or stacked module arrangements, not just 1D linear?
The model and all data in the source research are specific to 1D linear (single-row) arrangements. Two-dimensional arrays introduce additional radiant heat transfer paths and multi-directional propagation fronts that are not captured by the 1D thermal resistance model. Buyers designing 2D cell arrays should treat the 1D propagation times presented here as optimistic lower bounds — actual propagation in a 2D array is likely faster due to increased surface-area-to-cell contact and multi-path heat transfer. Separate qualification testing for 2D geometries is recommended.
Published by compactbess.com Technical Team | Request a sourcing quote
