In battery research and development, understanding the mechanical behaviour of cells during cycling is as critical as electrochemical performance. Battery testing protocols that fail to account for mechanical parameters can yield unreliable or irreproducible results. Our advanced testing fixtures address this challenge by allowing precise control and monitoring of two critical factors: pressure and thickness. This post examines how these parameters impact battery performance and how our equipment enables more comprehensive testing. Additionally, you will learn which testing configuration would be best for your use case.
Test Setup
- Cell type: Commercial pouch cell (LiPo)
- Environment: Ambient
- Cycle profile: Standard charge/discharge cycles at C/5
- Instrumentation:
- Pouch Cell Holder
- Force Sensor Module
- Thickness Sensor Module
- Battery cycler
Understanding Cell Pressure Dynamics in fixed thickness mode
When a battery cell charges and discharges, it experiences significant mechanical changes. These changes manifest as:
- Electrode expansion (contraction) during (de)intercalation
- Gas evolution during cycling
- Mechanical stress redistribution
When the cell is tested in Pouch Cell Holder in fixed thickness configuration, i.e. without springs, these effects can be monitored using our Force Sensor Module. Our measurement data illustrates these phenomena on Figure 1, where a typical force pattern that occurs during cycling is clearly seen.
Figure 1 In-situ measurement of force changes during galvanostatic cycling of pouch cell in fixed thickness fixture configuration. Initial applied force was 500 N.
Notice how the force oscillates in direct correlation with voltage. As the cell charges (voltage increases), the cell expands, creating increased pressure against the fixed boundaries. This pressure reaches its maximum at full charge and decreases during discharge. It is so-called ‘breathing’ of the battery. Additionally, average pressure increases over time due to cell swelling, caused by gases accumulation.
Key Benefits of Fixed Thickness Testing:
- Early Detection of Gas Generation: Identify potential safety issues or quality control problems before they become critical
- Material Evaluation: Quantify expansion forces for new chemistry to inform battery design choices
- Pack Design Optimization: Simulate actual battery pack where cells are often under fixed dimension constraints
- Production Quality Control: Establish pressure profiles of "good" cells to quickly identify manufacturing deviations
Intermittent Monitoring Capability
A unique feature of our system is the ability to detach sensors during testing and reattach them later without losing measurement precision. This is demonstrated in Figure 2:
Figure 2 In-situ measurement of force changes during galvanostatic cycling of pouch cell. Sensor was detached from test jig for a couple of cycles and reattached afterwards.
The sensor was detached for approximately 20 hours during the middle of the test, yet upon reattachment, it continued to provide consistent data. This capability is particularly valuable for:
- Labs running multiple parallel tests with limited sensor equipment, reducing capital expenditure
- Long-term aging studies where continuous monitoring isn't necessary
- Cases where the sensor may be needed for other test stations temporarily
Fixed Pressure Mode
For applications requiring stable pressure conditions, our system supports a fixed pressure mode, i.e. with compression springs mounted in the Pouch Cell Holder. Figure 3 demonstrates this capability:
Figure 3 In-situ measurement of force changes during galvanostatic cycling of pouch cell in fixed pressure fixture configuration. Initial force was 250 N and green springs were used.
The yellow line shows the force maintained at approximately 200 N throughout the cycling process, despite the natural tendency of the cell to expand and contract. This is achieved through our spring system with selectable spring rates. While the applied force is fixed during the test, the volume changes of studied cell are unrestricted, which means that they can be monitored with our Thickness Sensor Module.
Key Benefits of Fixed Pressure Testing:
- Enhanced Comparability: Creates standardized test conditions for comparing different cell chemistries or designs
- Improved Cycle Life: Maintains optimal pressure for cell performance, potentially extending cycle life
- Performance Optimization: Helps determine ideal pressure settings for specific cell designs
Precision Thickness Monitoring
Our Thickness Sensor Module offers ultra-precise measurement with 0.1 μm resolution, capable of detecting subtle changes in cell volume, even for single layer pouch cells (SLP). Figure 4 shows thickness changes during cycling:
Figure 4 In-situ measurement of battery thickness changes during galvanostatic cycling of pouch cell in fixed pressure fixture configuration.
The data reveals a thickness variation of approximately 40 μm between charged and discharged states, providing valuable insights into:
- Cell ‘breathing’ behaviour
- Potential gas evolution
- Mechanical deformation patterns
- Gradual thickness increases that often correlate with cell aging
Intermittent Thickness Monitoring
Similar to our Force Sensor, the Thickness Sensor supports detachment and reattachment during testing, as shown in Figure 5:
Figure 5 In-situ measurement of battery thickness changes during galvanostatic cycling of pouch cell in fixed thickness fixture configuration. Sensor was detached from test jig for a couple of cycles and reattached afterwards.
This feature, similarly to Force Sensor Module, enables cost efficient use of equipment across multiple test stations without compromising data integrity.
When to Use Fixed Thickness vs. Fixed Pressure Mode?
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Choose Fixed Thickness Mode When:
- You need to understand how pressure builds up in constrained environments
- Simulating battery operation in rigid pack designs with fixed dimensions
- Developing cells specifically for applications with dimensional constraints
- Studying the relationship between state of charge and mechanical stress
- Collecting data for digital models used to predict failures in real-world applications
Choose Fixed Pressure Mode When:
- Replicating battery performance in flexible or pressure-regulated pack designs
- Establishing standardized testing protocols across different cell designs
- Comparing performance of different chemistries under consistent mechanical stress
- Optimizing performance in applications where pressure can be controlled
- Investigating volume changes independent of pressure variations
Summary
By integrating precise mechanical parameter control and monitoring into battery testing protocols, researchers gain access to a wealth of additional information that complements traditional electrical measurements. Our system's ability to operate in both fixed thickness and fixed pressure modes, coupled with intermittent monitoring capabilities, provides unparalleled flexibility for battery R&D environments.
These mechanical measurement capabilities open new avenues for battery research:
- Materials Development: Estimate expansion characteristics of new electrode materials
- Safety Analysis: Early detection of abnormal swelling indicating potential failure modes
- Aging Studies: Track gradual changes in mechanical properties over thousands of cycles
- Model Validation: Provide experimental data to validate electrochemical-mechanical models
- Pack Design Optimisation: Generate precise data on mechanical behaviour to inform battery pack engineering
- Manufacturing Quality Control: Ensure cells from production batches exhibit consistent mechanical behaviour
For researchers seeking to understand the complete picture of battery behaviour, accounting for mechanical parameters is no longer optional—it's essential.
Contact us to learn more about how our battery testing solutions can enhance your research capabilities.