Battery Development New Testing Standard in Battery Research

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ZwickRoell GmbH & Co. KG

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Volume changes in battery cells influence performance, safety and ageing. Innovative testing systems make these effects measurable for the first time and provide valuable data for battery development.

Lithium-ion cells change their volume during operation: in the short term, this manifests itself as cyclical "breathing" during charging and discharging, and in the long term as permanent expansion during the ageing process. The causes are gas formation due to side reactions, structural changes in the electrodes, expansion of polymer separators and mechanical interactions between cell components.

This is not just a laboratory phenomenon for electromobility, as even small changes in volume generate mechanical stresses in modules and packs. As cells are densely packed and often pre-compressed, uncontrolled swelling can lead to increased internal pressure—with direct effects on electrical performance, safety and ageing.

It is also critical that current battery management systems do not directly record volume changes, but primarily measure electrical and thermal variables. The lack of mechanical measurement data increases the uncertainty of SOC and SOH estimates, especially with aged cells or high C rates.

Requirements for Modern Test Methods

In order for measurement results to be reliable and transferable, test benches must fulfill several requirements at the same time. They should be able to measure electrical, thermal and mechanical variables simultaneously and not only offer passive boundary conditions, but also actively control pressure and temperature.

High long-term stability is necessary in order to realistically reproduce ageing processes. In addition, test stands must take into account the different reactions of different cell types—such as pouch, prismatic and cylindrical cells. Equally important is the transferability of results from individual cells to modules and packs. Finally, precise calibration and traceability are essential to ensure the comparability of the data. Only if all these requirements are met can swelling effects be recorded not only qualitatively but also quantitatively and incorporated into the development of new cell generations in a targeted manner.

Classic Test Methods at A Glance

Climate chambers—tried and tested, but limited

Climate chambers are an established standard for controlling thermal boundary conditions. They reliably control the ambient air and allow defined temperature cycles. They are economical and efficient for screening tasks. However, temperature control remains indirect and pressure paths are usually passive. Mechanical effects can only be recorded to a limited extent.

Single cell tests with pressure plates

With this method, the cell is specifically compressed—ideal for pouch or prismatic cells. Electrical and mechanical parameters can be directly correlated. However, pressure homogeneity and temperature control are often inadequate. In addition, there is no way to actively control both parameters during cell operation.

Innovative Multifunctional Testing System: A New Standard?

For a long time, synchronous control of temperature and pressure during cell operation was hardly technically feasible. Climate chambers control the temperature indirectly via the air; active pressure control is practically impossible. Researchers at Zwick Roell and MBTS have therefore developed an innovative testing system that sets new standards. It enables precise regulation of the surface temperature and mechanical pressure on pouch and prismatic cells—during the charging and discharging process. This allows mechanical, thermal and electrical parameters to be recorded with high precision and the effects to be isolated in a targeted manner.

At the heart of the system are patented pressure plates with integrated thermal conditioning. They combine adjustable mechanical compression with direct surface temperature control. The entire system is fully automated and allows fast, reproducible test protocols with minimal user influence. The difference to conventional methods is fundamental: instead of indirect air temperature control and passive mechanics, this technology allows direct thermal conditioning, active pressure control and precise cell characterization. This results in high-quality models that can be used directly in research and development.

Comparison of Processes And Areas of Application

The choice of test method depends heavily on the objective of the test. Single-cell tests are particularly suitable for material screening and mechanical characterization. Climatic chambers, on the other hand, provide the necessary thermal conditions for long-term testing. Multifunctional systems, which can be used to specifically analyze the interactions between temperature, pressure and electrical load, provide even deeper insights. A combination of these methods is recommended for the development of modules and packs: controlled systems provide precise data models for battery management systems (BMS), while long-term tests provide robust information on ageing processes.

Practical Relevance And Regulatory Perspective

In industrial applications, swelling data is becoming increasingly important—for example in the validation of cell housings, the design of cooling concepts or the development of second-life strategies. Regulatory requirements, such as UN ECE R100 or ISO 12405, are also increasingly demanding mechanical tests as a supplement to electrical tests. The relevance for digital development processes is also increasing: Swelling data flows into simulation models that enable virtual prototypes and shorten development cycles.

Consequences for Battery Research And Development

Battery swelling is a complex phenomenon that combines electrical, thermal and mechanical effects. Specific practical measures can be derived from high-quality measurement data. For example, the cell pack design can be optimized through targeted pre-compression—but it is important to find a balance: Too much pressure can cause mechanical damage. The choice of material also plays a key role. Cell chemistries and separators with a low potential for gas formation and volume expansion contribute to stability.

In the area of thermal management, it is important to focus on the actual surface temperatures of the cells rather than just the ambient air. Finally, battery management systems (BMS) also benefit from models that take swelling effects into—especially when estimating the state of charge (SOC) and state of health (SOH), for example in marginal areas or during fast charging processes.

Integrated Testing Strategies As the Key to the Future

Battery swelling has a significant impact on the performance, service life and safety of electric vehicle batteries. Conventional climate chambers provide important temperature data, but are limited in their ability to control pressure and temperature. Single cell tests bring mechanical aspects into the laboratory, but struggle with reproducibility and homogeneity. Only the combination of both approaches—supplemented by modern testing systems such as those from Zwick Roell and MBTS—enables a realistic and reliable evaluation.

Future test methods must record electrical, thermal and mechanical data simultaneously and make it transferable at module and pack level. This requires stable data processes, consistent metadata and clear versioning in order to make measurement results comparable and efficiently usable across locations. This makes battery swelling a key factor in data-driven development—not only for performance, but also for the sustainability and reusability of modern battery systems.

Daniel Glanz is Content Manager at Zwick Roell

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