Exposure to water vapor is a death sentence for the lithium-ion (Li-ion) batteries used in hybrid-electric and electric vehicles. If moisture enters the battery cell and reacts with the electrolyte, it creates acid. And that acid can create an exothermal reaction, leading to a thermal runaway – an internal fire.
Catastrophic cell damage is a worst-case scenario, but any cell leak that permits acid intrusion is problematic. “If an acid leak is widespread in the pack, that’s both a warranty issue and an unhappy customer,” said Thomas Parker, automotive market sales manager at Inficon, maker of instruments for gas analysis.
While using a tester gas and associated equipment is common today for battery pack leak detection, finding a leak at the cell-level is a vastly different task. Three main types of battery cell – cylindrical, prismatic and pouch – are currently used in EVs, with pouch cells gaining popularity due to their lighter weight. The vacuum-pressurized cell pouches typically undergo visual inspection by workers to determine if any have ballooned outward during storage and shipment.
“It’s important to know that a leak that would cause a swollen pouch in a week’s time doesn’t reflect the size of a leak that occurs with humidity ingression over months or years,” Parker told SAE’s Automotive Engineering. “We want to know about a leak that is a million times smaller than what a visual test would indicate.”
Inficon claims its newly released ELT3000 technology can detect Li-ion cell leaks 1,000 times smaller than what is possible with conventional testing methods using tester gases. The leak detection system employs a vacuum chamber, a gas control unit and a gas detection unit. Inficon engineers wrote the software that operates the gas control unit and the gas detection unit.
The system’s use of a quadrupole mass spectrometer and the way in which the vacuum chamber connects to the gas detection unit highlight the testing method’s uniqueness. The company has a registered patent for this solution, noted Dr. Daniel Wetzig, Inficon’s research manager for leak-detection tools.
During the ELT300 leak-detection process on electrolyte-filled pouch cells, the testing equipment protects the soft cell pouch. “The system provides gentle encapsulation of the pouch cell with a flexible membrane so that the sample being tested isn’t damaged,” Parker said. The flexible membrane’s surface includes a structure that serves as a separate boundary layer. “We want the pouch cell to be cozy and safe. We also don’t want the flexible membrane to seal a hole, essentially covering up a leak during testing,” he explained.
According to ELT3000’s developers, the system provides a highly accurate leak test on Li-ion battery cells, starkly contrasting traditional test procedures. “The leak test’s main advantage is the battery cells are already filled with electrolyte solvent, so there is no need for test gases,” Wetzig noted. The ELT3000 can handle different types of electrolyte solvent, including dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and polypropylene (PP). Inficon began rolling out its battery leak detection production equipment for pouch, prismatic and cylindrical cells in 2020.
Current conventional testing of empty hard-case prismatic or cylindrical cells is done by filling the cells with helium tester gas to detect leaks while the cells are in a vacuum chamber. Electrolytes are inserted into the cells after “dry” testing. A less common testing method involves placing the electrolyte-filled cells into a vacuum chamber and exposing the cells to helium under pressure, referred to as “helium bombing”. Leaks are detected when the helium test gas escapes into the vacuum chamber.
Inficon’s ELT3000 leak detection system was developed by the company’s researchers in Cologne, Germany using sensor technology from the firm’s North American headquarters in Syracuse, New York. Product development unfolded over 30 months. “The ELT3000 leak test gives a clear answer about the cell tightness and therefore about the lifetime of the battery,” according to Wetzig.Continue reading »