Leak detection tests vital for EVs, autonomous vehicles
To ensure the quality of critically important hybrid-electric, electric and autonomous vehicle components such as sensors and battery packs, automakers and their suppliers are relying more on critically important “ingress” and “egress” leak detection tests.
Ingress testing today is being used to help protect a variety of critical systems from dangerous contaminants. Ingress leaks occur when water, dust or other contaminants enter components through seams, holes or porosities. Leaks can cause fires, degrade autonomous vision systems and even create air-bag explosions.
Automakers today are doing an incredible job of designing vehicles to go further with fewer repairs. However, the complexity of new generations of both electric and autonomous cars is putting greater pressure on engineers to ensure that components perform perfectly over the lifetime of the vehicle.
To help ensure component safety and durability requirements, many automotive components now are being certified using ingress protection ratings established by the International Electrotechnical Commission (IEC), which serves as an international standards organization. The Switzerland-based IEC has developed a rating system with ingress protection codes that include the letters “IP” and are followed by two numerical digits such as IP67. The first number (from 0 to 6) represents a product’s intrusion resistance to solid objects such as fingers, tools, wires, insects and dust.
The second digit in the IEC code (from 0 to 9) represents protection from water ingress over time. Numbers below 7 represent an unsubmerged component’s resistance to water such as splashes of water or high-pressure jets of water experienced in car washes. Numbers 7 and above represent leak resistance for components submerged under water to a certain depth for a certain period of time.
Previous Testing Methods
For decades manufacturing industries relied on either pressurizing a part, immersing it in water and then watching for bubbles (water bath testing), or pressurizing the part and observing pressure decay over a specified time. Water bath testing often requires overly long periods to complete and is operator dependent. Pressure decay testing is subject to the size of the part and nearby sources of heat or cold that might affect the validity of the test. Pressure also may damage sensitive or flexible parts.
Many industries have replaced these methods with tracer-gas leak detection procedures that are repeatable, quantifiable and meet cycle time requirements. Different pressure settings are required for various types of tests such as testing for thermal management systems or battery pack enclosures. Manufacturers often struggle to decide what leak-rate specifications should be used to achieve IP certification. INFICON recently completed a series of tests to help solve the problem, creating artificial leaks made from glass capillaries with a defined diameter and length to represent the size of a distinct leak path.
In its testing, INFICON investigated capillaries with 10 μm up to 100 μm in diameter and 10.5 mm length. Test parts were filled with water and pressurized to 100 mbar overpressure (~ 1.1 bar absolute pressure), equivalent to the pressure of being submerged in water at a one-meter depth. The outlet of the artificial leak was then observed for 30 minutes and the amount of water dripping out was measured. Results are summarized in the table below.
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For instance, it was calculated that the minimum leak path that would keep water from passing through would have a 12 μm diameter when made from glass. Equating experimental results with theoretical calculations proved the theory. Therefore, it can be assumed that theoretical results for other materials also would hold true in practical applications.
For plastic materials and steel, INFICON has calculated the leak path threshold to be approximately 30 μm in diameter. Similarly for aluminum, testing has shown that leak diameters down to 5 μm would leak water.
As the sale of battery electric vehicles, hybrids and plug-In hybrids ramps up, attention will need to focus on both ingress and egress leak detection for battery cells, battery packs and their cooling systems. For autonomous vehicles, ensuring leak tightness of advanced radar and LIDAR sensors becomes critically important as well.
When creating a battery cell, whether cylindrical, prismatic or pouch, there are several stages where tracer-gas leak testing can be used to ensure leak tightness. Cells with hard containers—cylindrical and prismatic—should be tested in a helium vacuum chamber before filling with electrolytes. The process involves pumping air out of the container, refilling with 100 percent helium and resealing. The chamber is evacuated, then connected to a helium leak detector. Once the test is completed the chamber is purged and helium reclaimed for reuse.
Once a lithium-ion battery pack is filled and sealed it also should be examined for leak tightness. INFICON has developed a method to reliably and quantitatively detect leakage from battery cells through the detection of escaping liquid electrolyte (typically dimethyl carbonate).
The “thermal runaway” of a single battery cell caused by the short circuit of internal electrodes can lead to a fire or explosion within a container with temperatures reaching 1,100 degrees centigrade or more. It is not surprising that many experts recommend that auto manufacturers carefully inspect incoming battery cells for quality, especially products from overseas battery-cell manufacturers.
Individual components that are part of a battery pack can and should be tested for leakage during production. These components include cooling plates, cooling tubes and any ECU cooler. All tests can be performed in a production environment. Batteries also should be tested for water ingress once assembled into packs.
The tests, IP67 and IP69K, reflect different materials (aluminum, PVC plastic, glass, steel or PE/PC/ABS plastics) and wall thickness. Permissible leak rates are in the 10-3—10-5 mbar-l/s range.
Advanced sensors that are integrated into the vehicle will be critical to the future of autonomous driving. Up until recently very little emphasis has been placed on leak tightness for vehicle sensors. The need for testing increasingly smaller sensor packages in adverse climates and over extended time periods, however, will become important.
Aluminum battery components, as well as aluminum wheels, also require leak-detection testing. Aluminum wheels, for example, are usually made through a casting process that can result in porosity leaks. Porosity leaks of such an extremely small size can’t be detected with traditional water bath tests. To test for leakage, INFICON recommends covering the wheel with a device that creates a sealed chamber around the exterior of the wheel rim, while at the same time sealing off a space on the inside of the rim.
The outer space around the rim is then charged with a helium-air mixture while a vacuum is created in the space inside the rim. An INFICON LDS3000 Helium Leak Detector can then be connected to the vacuum. Helium atoms will pass into the vacuum through micro porosities in the aluminum and the leak detector can determine if a leakage threshold has been exceeded.
INFICON is one of the world’s leading developers of leak-detection equipment for the auto industry. The company offers free of charge three white papers related to automotive leak detection including guides for robotic leak testing, battery leak detection and automotive industry leak detection in general.
Thomas Parker is North American automotive market sales manager for INFICON Inc.