The market for electric vehicles (EVs) continues to grow. To offset the immense weight of their batteries, a heightened focus has been put on lightweighting, which has brought many new materials and components into the global automotive supply chain. These components and assemblies, including polymer films used for Li-ion battery cell separators and the packs they ultimately form, require development and production quality testing to verify their mechanical properties.
One of the methodologies, static mechanical testing, is commonly performed by universal testing machines operated within manufacturing and research labs. Put simply, universal testing machines are designed to hold a test specimen and subject it to tensile (pull) or compression (push) forces by moving the machine’s crosshead up or down. Transducers, such as a load cell, capture valuable data that allows labs to analyze the physical properties of the test specimen for quality checks or exploratory work.
Within the automotive supply chain, raw material providers have long been under pressure to reduce the weight of their products while maintaining acceptable strength-to-formability properties. The substantial mass of liquid-cooled Li-ion battery packs reinforces the need to light-weight EVs to achieve an acceptable driving range for consumers.
Additional safety considerations are at play when hundreds of pounds of batteries are fastened to a vehicle. Accidents have the potential to damage the battery pack, triggering a hazardous thermal runaway. To mitigate these dangerous events, OEMs must ensure the structural integrity surrounding battery modules while minimizing weight. This has created a demand for stronger metals while maintaining formability. Plastic strain ratio (r-value) and the strain hardening exponent (n-value) are critical mechanical properties that define the formability of these products.
During a static tensile test, these formability properties can be determined if the test system is equipped with the appropriate transducers. For compliance and data integrity, labs adhere to the test procedures outlined in ASTM E517 (r-value) and E646 (n-value) or other standard bodies’ equivalent test standards. To determine n-value, axial strain needs to be measured after yield and determined at or between strain values. To determine r-value, the material’s transverse strain must also be measured. To accurately measure axial and transverse strain, a device referred to as an extensometer is used. Traditionally, two extensometers would be required, one for axial strain and the other for transverse. However, new technology has been developed by test system manufacturers that allow for simultaneous, dual-axis strain measurement.
Separator film testing
The membrane between a battery’s anode and cathode is referred to as the separator. These films must be capable of withstanding harsh conditions all the way from manufacturing processes to extreme climates. A critical part of Li-ion or other liquid electrolyte batteries, separators are films typically made of polymers. They must be mechanically strong enough to withstand the winding operation during the battery’s assembly. The critical mechanical properties that suppliers must control are tensile strength, elongation and puncture resistance.
ASTM D882 outlines the tensile testing of thin plastic film, which is suitable for polymer separators. Film sheeting is prepared into 1-inch wide strips and pulled in tension to failure with a static universal test frame. Tensile strength and elongation results can be evaluated to ensure necessary specifications are met. Regarding puncture tests, ASTM F1306 outlines the puncture testing of flexible barrier films to characterize the material response when a probe 3.2 mm (0.126 in.) in diameter is driven into a clamped specimen.
Testing nail penetration
A nail penetration test of an assembled, charged battery cell, module or pack is another form of puncture testing that battery and automotive manufacturers regularly perform to assess EV battery safety. High energy thermal runaway is a danger of an internal short circuit of Li-ion batteries. Puncture events are a known cause of internal shorts and therefore must be evaluated for consumer safety. As nail penetration tests are not well defined in international standards, identifying the equipment and procedure necessary can be a challenge.
SAE International has provided guidance to the industry with its J2464 standard, Electric and Hybrid Electric Vehicle Rechargeable Energy Storage System (RESS) Safety and Abuse Testing1. SAE J2464 has a section that defines a battery penetration test and fundamental parameters such as rod (nail) diameter, rate of penetration, and depth of penetration. Another well-established standard is GB/T 31467.3 (Chinese national standard), Lithium-ion traction battery pack and system for electric vehicles—Part 3: Safety requirements and test methods2.
Various laboratory equipment providers have developed single-purpose systems to perform nail penetration and crush testing of batteries. Fully enclosed, fire-resistant and ventilated for safe operation, the systems are configured with temperature and voltage sensors. Similar systems have been developed by universal test machine manufacturers, but given the extreme hazards, the integration of a third-party test chamber has filled this competency gap.
Component testing and cost
EVs introduce new components into the automotive industry that require mechanical testing for both development and quality-control environments. Battery packs consist of thousands of interconnected battery cells, resulting in thousands of potential failure locations. Each assembly’s quality is a critical concern for OEMs. Testing components can be challenging to fixture into a universal test machine, and depending on the number of components, it can be a frustratingly inefficient process.
Static universal testers offer automated and semi-automated testing solutions to improve throughput by removing the operator and the need to repeatedly reposition and fix specimens to jigs. Freed up from constant specimen preparation, operators can focus on more value-added activities. Vehicle OEMs are challenged to generate sustainable profitability with EVs, given their still-significant cost (mainly from the battery) deltas versus combustion-engine vehicles. Testing is one area that offers meaningful cost-reduction opportunities. Quality labs and R&D centers are now implementing more efficient processes toward that goal, with the help of automated universal test machines.
Dan Caesar is an Applications Engineer with Instron’s Static Electromechanical Applications Team.
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