Today’s rapid and unexpected growth in the production of vehicles with alternative drive systems is giving carmakers and their manufacturing partners a wide range of leak-detection challenges to ensure vehicle quality.
Traction battery systems manufactured for Battery Electric Vehicles (BEVs) and Plug-In Hybrid Electric Vehicles (PHEVs), for example, must be protected from water and humidity that can reduce battery life or cause fires. Fuel-cell Electric Vehicles (FCEVs) also have unique leak-testing requirements, especially for hydrogen tanks, fuel cells and the batteries that drive their electric motors.
INFICON has published a comprehensive 50-page guide to leak testing electric and fuel-cell vehicles for manufacturing and quality control engineers. The guide discusses leak-detection methods for a wide variety of applications, including battery cells, battery housings, electric motors, motor cooling circuits, fuel cells and hydrogen tanks. Electronic components, control modules and ADAS sensors are also covered.
EV Safety Concerns Grow with Sales
It is not only passenger cars that will go electric. A recent survey by McKinsey & Co. and the World Economic Forum sees a major shift from combustion engines to electric drive systems for commercial vehicles. As EV production increases, quality concerns will multiply as well.
Reliable leak-testing is critical throughout the production process. EV battery cells, battery packs, battery cooling circuits, electric motors and other systems modified for EV applications all require leak testing to assure both quality and safety. Leak testing makes certain that battery-cell electrolyte does not leak or come into contact with water during every stage of battery production. It is also important to assure the integrity of battery modules and battery pack housings. Why? Battery cell electrolyte is highly flammable and can cause vehicle fires.
Damage to battery cells while in transit to an OEM’s assembly plant also needs to be considered. The “thermal runaway” of a single battery cell can cause burning electrolyte to reach temperatures up to 1,100°C (2,012°F).
Leak Testing Methods for Battery Cells
Today, carmakers expect a lithium-ion battery to have a service life of up to 10 years or more. To achieve extended cell life, leak rates for prismatic and cylindrical battery cells must fall within a range of 10-6 to 10-8 mbar-l/s. Pouch cells need to be tested for large or so-called gross leaks, as well as for extremely small “capillary” leaks.
New leak-detection systems based on mass-spectrometer technology now are capable of catching leaks 1,000 times smaller than previously possible. An INFICON ELT3000 test device, for example, can identify leaks with a diameter of just a few microns. A flexible test chamber designed to prevent damage to pouch cells also has been developed by INFICON to support vacuum testing of pouch cells.
Leak Testing for Battery Pack Housings
Battery pack housings call for specific leak detection requirements since they protect battery modules and cells from water. Depending on where they are located, housings must meet IP67 or IP69K protection-class requirements. Housings for electrical components such as lithium-ion batteries, power-control units, electric motors and electronic modules often are designed in accordance with IP67. (Testing according to IP67 requires that a component be fully functional after immersion in water at a depth of 1 m for 30 minutes.)
The fastest and most accurate way to test components on the production line is to test for helium tracer gas in a vacuum chamber. Another option for testing both assembled and unassembled housings is an accumulation test, which requires longer cycle times.
If a manufacturer wants to test the integrity of gaskets or seals on an already assembled battery pack, vacuum testing is not an option. Pressure differences with this type of testing might damage gaskets or destroy already installed capacitors. As an alternative, trace-gas-based “sniffer” leak detection is recommended for battery packs and assembled housings.
FCEVs and Their Components
Leak testing for FCEVs that use hydrogen technology also is indispensable not only for their hydrogen tanks, but also for the fuel cells and battery packs that supply power to their electric motors. FCEVs and BEVs share a number of components with similar leak-detection requirements. Both are driven by electric motors powered by lithium-ion batteries, although FCEV batteries are much smaller and have less storage capacity.
FCEVs, however, generate their own electrical energy and their fuel-cell stacks; high- and low-temperature cooling circuits; and hydrogen tanks, lines and recirculation systems all must be leak tested.
Sensors and Electric Drive Motors
Whether it is a BEV or FCEV, a vehicle’s sensors, control modules and electric drive motors all require some form of leak testing. Water is the primary enemy of every vehicle’s electrical components. Water and humidity tightness, therefore, are critically important, especially for autonomous or Advanced Driver Autonomous Systems (ADAS).
Vehicle sensors often are tested with less-sensitive, strongly temperature-dependent pressure decay tests. ADAS manufacturers, however, follow a zero-defect strategy that is 1,000 times more reliable than a Six Sigma approach that tolerates 3.4 errors per million cases. Sensors used for radar and LiDAR technologies not only must be water tight, they also must be gas tight—completely sealed off from humidity.
Edited by Special Report Editor Bill Koenig from information supplied by INFICON.