Single Battery Cells and Module Cells’ Swelling Performance Correlation Analysis

1. Preface

With the rapid advancement of the new energy sector, lithium-ion batteries are now extensively used in electric vehicles (EVs). As a result, the safety performance of these batteries has become a major focal point. During long-term cycling, battery module cell configurations — composed of multiple cells in series or parallel — commonly experience swelling due to lithium intercalation/deintercalation and gas generation. This swelling can compromise the structural integrity of the module casing and even lead to safety hazards. It is therefore essential to monitor battery cell swelling throughout its lifecycle and design effective EV battery cell swelling compensation strategies — including swelling support pads, cushioning pads for cell swelling, and pouch cell swelling pads — at the module cell and pack level.

Different module cell designs involve varying numbers of cells, electrical configurations, and pre-tightening forces, each influencing swelling behavior. Experimental analysis of these factors helps reveal how swelling evolves from single cells to full modules. Combining physical tests with simulation allows for better module cell design and safer EV batteries. This study provides fundamental data on the correlation between swelling thickness and swelling force in single cells and multi-cell modules, directly supporting predictive modeling for EV battery cell swelling compensation and swelling support pad design.

Figure 1. Single cell vs module cell configuration — the basis for scaling EV battery cell swelling compensation design

2. Experimental Equipment and Test Methods

2.1 Experimental Equipment

Tests were conducted using the In-situ Swelling Analyzer, model SWE2110 (IEST) (Figure 2). This system enables high-precision, real-time measurement of both thickness change and swelling force during battery cycling — the two parameters essential for designing swelling support pads for EV batteries and validating pouch cell swelling pad compliance specifications.

Figure 2. IEST SWE2110 In-Situ Cell Swelling Testing System — real-time thickness and force measurement for cell swelling management systems

2.2 Test Procedure

Charge-discharge protocol:

• Temperature: 25°C

• Rest: 60 min

• Charge: 0.5C CC to 4.35V, then CV until current ≤ 0.05C

• Rest: 30 min

• Discharge: 1.0C to 3.0V

2.3 Cell Thickness Swelling Test

Each cell or module cell was placed in the SWE2110 chamber. Using MISS software, parameters such as cell ID and sampling frequency were configured. The system automatically recorded real-time thickness change, swelling force, temperature, current, voltage, and capacity throughout cycling.

3. Experimental Process and Data Analysis

Battery swelling can be evaluated under three test modes (Figure 3), each directly relevant to a different aspect of swelling support pad and cell swelling management system design:

• Free swelling: The cell expands without mechanical constraint; useful for fundamental material expansion characterization, but does not reflect real module/pack conditions.

• Constant preload: A pre-tightening force (F₀) approximates module clamping conditions — the typical mounting condition when swelling support pads for EV batteries are sized to provide a baseline compressive preload — and allows observation of how this preload redistributes during cycling.

• Constant gap: Simulates rigid module housings with fixed clearance; cell expansion is mechanically resisted, producing internal forces that the pack structure and any pouch cell swelling pad must accommodate or compensate.

Each mode imposes different boundary conditions and mechanical interactions. In module cell configurations, the presence of a pouch cell swelling pad or other interstitial cushioning material adds complexity to force distribution. Under constant-pressure or constant-gap conditions, the role of the pouch cell swelling pad becomes critical in managing mechanical stress and supporting effective EV battery cell swelling compensation.

Figure 3. Three test modes for cell and module swelling — free swelling, constant preload, and constant gap — each relevant to different cell swelling management system designs

3.1 Correlation Between Single-Cell and Module Swelling Thickness

To simulate inter-cell layers in a module — the position where a pouch cell swelling pad or cushioning pad for cell swelling would typically be placed — a PET film was applied to individual cells (Figure 4). Cells were then stacked and tested under a 200 kg constant-pressure mode. Figure 5 shows the swelling thickness of individual cells and stacked modules during cycling.

Both single and multi-cell setups exhibited swelling during charging and contraction during discharging, consistent with Li-ion intercalation and deintercalation in graphite and ternary cathode materials. The measured module swelling thickness (solid line) closely matched the arithmetic sum of individual cell swelling (dotted line), though minor deviations occurred at end-of-cycle points, likely due to cell-to-cell inconsistency. This additive relationship is good news for EV battery cell swelling compensation design under constant-pressure conditions: module-level thickness accommodation can be reasonably predicted by summing single-cell swelling data, simplifying swelling support pad sizing calculations.

Figure 4. PET film interlayer between stacked cells (simulating pouch cell swelling pad position) and cell superposition schematic

Figure 5. Swelling thickness curves — single cells and stacked module under 200 kg constant pressure. Module swelling thickness closely tracks the arithmetic sum of single-cell values.

3.2 Correlation Between Single-Cell and Module Cell Swelling Force

Under constant-gap mode, swelling force was measured for single cells and stacked modules (Figure 6). As the number of cells increased, total module swelling force also increased — but not linearly. The total force was consistently less than the arithmetic sum of individual cell forces. This suggests that constraining the gap alters the internal mechanical and electrochemical state of the cells, affecting their swelling behavior in ways that simple additive models cannot capture. Further studies incorporating capacity measurements before and after grouping may clarify this phenomenon.

This non-linear force scaling is the single most important design consideration for cell swelling management systems and pouch cell swelling pad sizing under rigid-gap (fixed clearance) module designs: extrapolating single-cell force data linearly to predict module-level force will overestimate the actual force, potentially leading to over-engineered, unnecessarily stiff swelling support pads for EV batteries. The non-linear force increase may also indicate that beyond a certain stacking pressure, cell components such as electrodes and separators undergo microstructural compression, influencing overall electrochemical performance.

Figure 6. Swelling force variation — single cells vs stacked module under constant-gap mode. Module force scales non-linearly, less than the arithmetic sum of individual cells.

These findings indicate that in a fixed module or pack, the design and material of inter-cell pads — such as a pouch cell swelling pad — significantly influence overall swelling and force distribution. For example, CATL’s Kirin battery incorporates a multi-functional elastic interlayer that accommodates cell swelling and contraction, thereby enhancing cycle life and safety — a leading industrial example of EV battery cell swelling compensation engineering.

Figure 8. Multi-functional elastic interlayer in CATL‘s Kirin battery — an industrial pouch cell swelling pad and EV battery cell swelling compensation solution

4. Implications for Swelling Support Pad and Module Design

1. Preload design matters: Under preload (constant pressure) conditions, thickness behavior is predictable and additive — making swelling support pad sizing and engineering allowances easier to calculate. However, the preload level itself affects how cells share load and thus their electrochemical behavior, so preload should not be set arbitrarily.

2. Gap-limited designs require caution: Rigid module designs that rely on a fixed gap to control expansion can create uneven stress distributions and non-intuitive force accumulation — potentially shortening cell life or damaging the module casing if cell swelling management systems are not sized using actual module-level (not single-cell extrapolated) force data.

3. Consistency is critical: As stack size grows, single-cell consistency becomes increasingly important for accurate EV battery cell swelling compensation prediction. Small variations in cell capacity, internal resistance, or initial thickness amplify module-level discrepancies between predicted and actual swelling.

4. Compensation strategies: Effective EV battery cell swelling compensation should combine mechanical design (elastic interlayers, compliant gaskets, cushioning pads for cell swelling, multi-functional pouch cell swelling pads) with thermal and electrochemical management. Solutions such as CATL’s Kirin elastic interlayer — integrating structural beams, thermal plates, and breathing-friendly micro-bridges — illustrate industrial approaches to accommodating “breathing” cells.

5. Measurement best practice: Use in-situ swelling analyzers with multi-channel capability to obtain spatially resolved force and thickness data during cycling, validating swelling support pad performance under realistic constant-pressure and constant-gap boundary conditions rather than relying solely on single-cell free-swelling data.

5. Conclusion

This study used an in-situ swelling analyzer (SWE2110) to evaluate swelling thickness and force in single cells and multi-cell modules. Under constant pressure, module swelling thickness closely followed the arithmetic sum of individual cell swelling — validating additive thickness models for EV battery cell swelling compensation. However, under constant-gap conditions, swelling force did not scale linearly, highlighting the critical effect of mechanical constraints on cell behavior and the need for module-level (not single-cell-extrapolated) data when sizing swelling support pads for EV batteries and pouch cell swelling pads.

These insights are valuable for designing better cell swelling management systems and effective EV battery cell swelling compensation strategies. Future work will focus on modeling mechanical behavior under different boundary conditions and elucidating electrode-level swelling mechanisms.

5. References

[1] Yongkun Li, Chuang Wei, Yumao Sheng, Feipeng Jiao, and Kai Wu. Swelling Force in Lithium-Ion Power Batteries，Ind. Eng.Chem. Res，2020, 59, 27, 12313–12318.

[2] Oh K Y, Epureanu B I ,  Siegel J B , et al. Phenomenological force and swelling models for rechargeable lithium-ion battery cells[J]. Journal of Power Sources, 2016, 310(Apr.1):118-129.

[3] Martin Wünscha,Kaufmana, Dirk Uwe Sauer. Investigation of the influence of different bracing of automotive pouch cells on cyclic liefetime and impedance spectra. Journal of Energy Storage 21 (2019) 149–155.

[4] Qiu Shitao, Chen Chaohai, Jiang Jibing. Effect of foam performance on module cell swelling force. Guangdong Chemical Industry, 2020, 47(22): 1-3d

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