Assembly - December 2008 - (Page 48) Leak Testing “It really depends on the specifics of the battery design and the process itself,” adds Sheaff. “For example, whether it is possible to perform a leak test on the dry unfilled container. If the battery cannot be leak tested without fluids present, any leaks could be masked and result in a test showing ‘good,’ but the battery then failing in service.” Size, temperature issues, weld techniques, leak specifications, test speed and cycle time, and the number of channels to match the tooling must be addressed when testing batteries. “Fixturing and sealing of the part on the assembly line is always an important part of any leak testing,” says Sheaff. Another big issue with battery testing is health and safety issues due to the types of materials and liquids being used, as well as the waste issue. “Batteries and fuel cells are often tested in multiples,” Sheaff points out. “Therefore, a high channel capacity system is often a more economical choice.” Leak testing usually involves a pressure difference between the inside and outside of a part. “This can occur both when the part is charged with trace gas (helium bombing) or during the leak check test itself,” says John McLaren, strategic marketing manager at Varian Inc. (Palo Alto, CA). However, this can stress a battery casing well beyond what it will see in its lifetime. “In addition, many batteries are designed with built-in pressure relief or vent ports that further complicate testing, because they are designed to leak on purpose,” adds McLaren. “They cannot tolerate the pressure difference involved in testing. When this is the case, we [usually] develop a partial vacuum test process that the battery can tolerate.” A Many battery manufacturers seal potentially harmful cells with laser welding technology. Photo courtesy Johnson Controls Inc. allowed electrolytes to leak from one cell to another,” recalls Jacques Hoffmann, president. “This is a relatively archaic application—alkaline car battery leak testing—and we haven’t had many requests for automotive battery testing for quite some time.” Hoffmann says his company has seen more applications for testing computer batteries in recent years. “This is not leak testing per se, but rather burst testing for many components of these batteries,” he explains. “Given the very high production rates for batteries, such as 100 units per minute, audit testing is generally used,” says Hoffmann. “But, given the high volumes, these are still automated test systems. Because the battery designs aim for lighter weights and thinner constructions, testing becomes more and more critical. “You aren’t generally dealing with the same high volumes when you are testing medical devices,” adds Hoffmann. “Much of our work is in testing battery caps, not the complete battery. If gas builds up in a battery and cannot escape, you have a problem. The relief valves eliminate pressure in the caps. Burst testing validates that these relief valves are functioning correctly.” Testing Challenges Batteries come in a variety of shapes and sizes, which have their own unique leak-testing challenges. “Modern batter- ies tend to be hermetically sealed, with very little or no headspace inside,” says David Selin, European sales manager at Uson LP (Houston). “This makes it difficult to measure a transfer of gas through the barrier (wall) to or from the battery, as there is nowhere to go. “Many medical devices either are not hermetically sealed, and therefore have a connection point for filling, or they are sealed but have a known headspace with which we can work,” adds Selin. “Leadacid batteries could be tested directly, as they had removable plugs for topping up with distilled water, and therefore provided an access point for test line connection. Some smaller batteries do have a useable headspace and can be tested using trace gas by the ‘bombing’ technique.” The type of testing method that’s used to check batteries for leaks typically depends on the type of battery and where it is used. For example, helium testing is used for cardiac pacemakers, or for any similarly internally planted medical device with batteries, where extremely tight tolerances are required. Cumulative helium leak detection is used to test very fine leaks in sealed packages. “Batteries come in many styles, shapes and sizes and can be easy or difficult to leak test,” notes Sheaff. “They all have their difficulties, depending on the specific configuration. Issues will include upstream processes and test stability, cycle time and throughput, size and volume, and test specifications. ASSEMBLY ONLINE To learn more about batteries, click www.assemblymag.com to read these articles: Nanotechnology Transforms Lithium-Ion Batteries. Thin-Film Batteries Hold Promise. Ultrasonic Welders Assemble LithiumIon Batteries. 48 ASSEMBLY / December 2008 www.assemblymag.com http://www.assemblymag.com http://www.assemblymag.com
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