As the core energy source for electric vehicles, the safety of the power battery under mechanical abuse is a paramount concern. Accidents involving thermal runaway following collision or impact highlight the critical need to understand a battery’s tolerance to deformation. This study presents a comprehensive, first-person experimental investigation into the failure thresholds of a specific commercial prismatic ternary lithium-ion battery when subjected to localized quasi-static extrusion from different directions and at various positions. Our goal is to quantify the critical displacement before failure for each distinct loading case, providing essential data for battery pack design, vehicle layout, safety protection, and simulation-based safety assessment.
The test subject was a 55 Ah Nickel-Cobalt-Manganese (NCM) / Graphite li ion battery with dimensions of 148 mm (length) × 27 mm (thickness) × 101 mm (height). All cells were tested at 100% State of Charge (SOC, 4.220 V). A custom fixture was employed to clamp the cell securely, mimicking its constrained condition within a module. A hemispherical indenter with a diameter of 25 mm was used to apply quasi-static compression at a constant speed of 0.1 mm/s. The test matrix was designed to cover three principal faces, subdivided into specific zones, as detailed in the methodology. Failure was defined by events such as a sudden voltage drop (＞0.1 V), rapid temperature rise, visible smoke, fire, explosion, or visible casing rupture with electrolyte leakage.
The internal architecture of the li ion battery is key to interpreting its mechanical response. Upon dissection, the cell was found to contain two jellyrolls (electrode assemblies) stacked along its height. The tabs connecting the internal electrodes to the external terminals are gathered near one end of the cell, creating an internal void or less densely packed region in the top section of the casing. This structural inhomogeneity significantly influences the stiffness and failure behavior when different locations are loaded.
To systematically analyze the mechanical integrity, we defined the large face (148 mm × 101 mm) as Face A, the thin side (27 mm × 101 mm) as Face B, and the bottom (148 mm × 27 mm) as Face C. Face A was divided into 9 zones (A1 to A9), Face B into 3 vertical zones (B1 to B3), and Face C into 3 horizontal zones (C1 to C3). The first phase of testing aimed to establish a rough failure displacement for each zone by compressing until a clear failure event occurred. The force-displacement data from this phase revealed distinct stiffness characteristics. The load-bearing behavior of a li ion battery under compression can be initially approximated by analyzing the stress-strain relationship, where the nominal stress $\sigma$ and strain $\epsilon$ are defined relative to the contact geometry and initial cell dimensions.
$$ \sigma = \frac{F}{A_c}, \quad \epsilon = \frac{\delta}{D} $$
where $F$ is the recorded compression force, $A_c$ is a characteristic contact area (evolving with indenter penetration), $\delta$ is the displacement, and $D$ is a relevant initial dimension of the cell (e.g., thickness for Face A compression). The stiffness $k$ for a given loading scenario is derived from the linear elastic region of the force-displacement curve:
$$ k = \frac{dF}{d\delta} $$
The energy absorbed by the li ion battery up to a displacement $\delta_f$ is given by the area under the force-displacement curve:
$$ W = \int_{0}^{\delta_f} F(\delta) \, d\delta $$
This energy absorption capacity is a critical metric for evaluating crash safety.
The comparative stiffness curves from Phase I are summarized in the analysis below. A key finding was that locations A1-A3, situated near the terminals, exhibited consistently lower stiffness compared to locations A4-A9. This is directly attributable to the internal void space in the upper part of the casing, allowing for easier inward buckling of the jellyrolls. Conversely, compression at the mid-height (A4-A9) loads the more rigid, fully packed central section of the jellyrolls, resulting in higher force for the same displacement. Furthermore, compressions on Face B and Face C showed markedly lower stiffness than Face A. When loaded on the sides, the stacked jellyrolls can buckle more freely, causing out-of-plane bulging on the large faces, which is a lower energy deformation mode compared to the direct in-plane compression of the jellyrolls on Face A.
Phase II testing was designed to precisely pinpoint the safety threshold—the maximum displacement before the onset of failure—for each zone. Using the failure displacements from Phase I as an upper bound, we conducted iterative tests with progressively lower target displacements until no failure (no voltage drop, temperature rise, or casing breach) was observed over a 600-second hold period. At least two non-failing tests were conducted to confirm the safety of a given displacement level. The detailed results for the large face (A) and the side/bottom faces (B & C) are presented in the following comprehensive tables. These tables consolidate the complete experimental campaign, listing for each zone the test sequence, the target displacement, the observed phenomenon, and the deduced safe threshold.
| Face | Zone | Cell ID | Target Displacement (mm) | Observed Phenomenon | Deduced Safe Threshold (mm) |
|---|---|---|---|---|---|
| A | 1 (Near +Terminal) | 1 | 9 | Casing rupture, leakage | 7 |
| 2 | 8 | Casing rupture, leakage | |||
| 3 | 7 | No anomaly | |||
| 4 | 7 | No anomaly | |||
| A | 2 (Near +Terminal) | 1 | 9 | Fire, explosion | 6 |
| 2 | 7 | Micro-crack on casing | |||
| 3 | 6 | No anomaly | |||
| 4 | 6 | No anomaly | |||
| A | 3 (Near +Terminal) | 1 | 8 | Casing rupture, leakage | 6 |
| 2 | 7 | Micro-crack on casing | |||
| 3 | 6 | No anomaly | |||
| 4 | 6 | No anomaly | |||
| A | 4 (Center-Left) | 1 | 9 | Rupture followed by fire | 8 |
| 2 | 8 | No anomaly | |||
| 3 | 8 | No anomaly | |||
| A | 5 (Center) | 1 | 10 | No anomaly | 10 |
| 2 | 10 | No anomaly | |||
| 3 | 11 | Rupture followed by fire | |||
| A | 6 (Center-Right) | 1 | 9 | Rupture followed by fire | 8 |
| 2 | 8 | No anomaly | |||
| 3 | 8 | No anomaly | |||
| A | 7 (Near -Terminal) | 1 | 11 | Fire, explosion | 8 |
| 2 | 9 | Rupture followed by fire | |||
| 3 | 8 | No anomaly | |||
| 4 | 8 | No anomaly | |||
| A | 8 (Near -Terminal) | 1 | 10 | Rupture, leakage | 8 |
| 2 | 9 | Rupture followed by fire | |||
| 3 | 8 | No anomaly | |||
| 4 | 8 | No anomaly | |||
| A | 9 (Near -Terminal) | 1 | 10 | Rupture, leakage | 8 |
| 2 | 9 | Rupture, leakage | |||
| 3 | 8 | No anomaly | |||
| 4 | 8 | No anomaly |
| Face | Zone | Cell ID | Target Displacement (mm) | Observed Phenomenon | Deduced Safe Threshold (mm) |
|---|---|---|---|---|---|
| B | 1 (Top) | 1 | 9 | Rupture, leakage | 7 |
| 2 | 8 | Rupture, leakage | |||
| 3 | 7 | No anomaly | |||
| 4 | 7 | No anomaly | |||
| B | 2 (Middle) | 1 | 10 | Rupture, leakage | 7 |
| 2 | 8 | Rupture, leakage | |||
| 3 | 7 | No anomaly | |||
| 4 | 7 | No anomaly | |||
| B | 3 (Bottom) | 1 | 10 | Rupture, leakage | 7 |
| 2 | 8 | Micro-crack on casing | |||
| 3 | 7 | No anomaly | |||
| 4 | 7 | No anomaly | |||
| C | 1 (Left) | 1 | 24 | Rupture, leakage | 23 |
| 2 | 23 | No anomaly | |||
| 3 | 23 | No anomaly | |||
| C | 2 (Center) | 1 | 22 | Rupture, leakage | 19 |
| 2 | 20 | Micro-crack on casing | |||
| 3 | 19 | No anomaly | |||
| 4 | 19 | No anomaly | |||
| C | 3 (Right) | 1 | 24 | Micro-crack on casing | 23 |
| 2 | 23 | No anomaly | |||
| 3 | 23 | No anomaly |
The data reveals clear and significant trends regarding the safety of this li ion battery design. The most vulnerable areas are the zones near the terminals on the large face (A1-A3) and the entire side face (B). The safety thresholds for these locations are critically low, ranging from 6 mm to 7 mm. This weakness is a direct consequence of the internal structure; compression in these zones leads to severe localized bending and shear stresses on the electrode assembly near the tab collection area, precipitating internal short circuits at relatively small deformations. The central regions of the large face (A4-A9) are more robust, with safety thresholds between 8 mm and 10 mm, due to the more uniform and supportive loading of the jellyrolls’ packed region. Strikingly, the bottom face (C) exhibits a significantly higher tolerance, with thresholds between 19 mm and 23 mm. This is because extrusion on this face primarily induces global buckling and bending of the entire cell structure, dissipating energy over a larger volume before causing a critical internal short circuit in the active layers. The failure of a li ion battery under mechanical abuse is governed by the onset of internal short circuit (ISC). The condition for ISC can be related to the critical internal strain $\epsilon_{crit}$ within the separator or electrodes. When the local deformation $\epsilon_{local}$ exceeds this material-dependent threshold, electronic contact occurs, leading to rapid joule heating. The heat generation rate $\dot{Q}_{ISC}$ can be described as:
$$ \dot{Q}_{ISC} = I_{short}^2 \cdot R_{contact} $$
where $I_{short}$ is the short-circuit current and $R_{contact}$ is the contact resistance. If this heat cannot be dissipated, it can trigger exothermic side reactions, leading to thermal runaway. The critical external displacement $\delta_{crit}$ we measured is thus the macroscopic indicator that $\epsilon_{local} \geq \epsilon_{crit}$ at some point within the cell.
The implications of these findings are substantial for the engineering of safe battery systems. This quantitative dataset provides clear guidance for the strategic placement of this specific li ion battery within a pack. The low-threshold zones (A1-A3, B) must be afforded maximum protection, potentially through strategic placement of internal module stiffeners, external pack armor, or by orienting cells so that these weak directions are not exposed to likely impact paths in a vehicle crash. The high threshold of the bottom face is advantageous but must be considered in the context of “ground strike” or “curb impact” scenarios, where deformation allowances can be designed accordingly. Most importantly, the precise safety thresholds serve as crucial pass/fail criteria for finite element simulation (FEA) of battery packs under mechanical loading. By simulating an impact and monitoring the simulated intrusion displacement at critical cell locations, engineers can compare the value directly against our experimentally derived thresholds (e.g., 7 mm for a side impact on zone B2) to assess the safety of a design virtually, saving immense time and cost in physical prototyping and testing. The safety performance of a li ion battery is a system-level property. While we characterized the cell’s intrinsic tolerance, the final pack safety is determined by the integration of cell thresholds with module and pack structural design. The energy management during an impact involves the entire system’s stiffness matrix $[K]$, governing the relationship between applied forces $\{F\}$ and displacements $\{u\}$:
$$ \{F\} = [K] \{u\} $$
An optimal design ensures that the load path distributes forces away from the weak cell axes or that the surrounding structure crushes in a controlled manner to limit the displacement $\{u\}$ on any single cell below its critical threshold $\delta_{crit}$.
In conclusion, through a detailed experimental campaign, we have successfully mapped the localized extrusion safety thresholds of a commercial prismatic ternary li ion battery. The study conclusively demonstrates that failure tolerance is highly anisotropic and location-dependent, primarily dictated by the internal cell architecture. The terminal-adjacent areas on the large face and the entire side face present the highest risk, with failure occurring at minimal intrusions of 6-7 mm. In contrast, the bottom face can withstand much larger deformations. This work provides an essential empirical foundation for data-driven, simulation-validated design of safer lithium-ion battery packs for electric vehicles.