65 Battery leak testing | Product focus E-Mobility Engineering | March/April 2025 In ultrasonic testing, microphones listen for sound waves at frequencies higher than 20 kHz that indicate escaping gas. Non-invasive and quick, it is typically used to identify structural defects in cells and modules. It can be used in combination with compressed air that forces leaks, or passively to listen for unforced leaks. Ultrasonic testing does not require a vacuum chamber or tracer gases and identifies leaks in real time. However, it has similar sensitivity limitations to pressure decay testing, it can be susceptible to false positives from background noise, and the interpretation of signals requires expertise, as ultrasonic leak detectors do not read a quantitative leak rate. In storage, batteries are monitored for electrolyte leakage or gas emissions. One of the technologies used is gas detection using sensors that react to gases such as hydrogen, carbon dioxide, or volatile organic compounds (VOCs) emitted by leaking electrolyte or during thermal runaway. Although cost-effective, these systems are not selective, responding to all hydrocarbons and solvents. Another is thermal imaging using infrared cameras, which reveal the heat signatures of leaks and short circuits. Thirdly, optical emission spectroscopy analyses the light signatures of gases and is highly sensitive to those of interest, enabling early warning. In operation, gas detectors can be used inside battery cases, complementing the indirect detection capabilities of the battery management system (BMS) and isolation monitoring circuitry. Continuous processing of voltage, current and temperature measurements by the BMS in real time can indicate leakage or internal battery faults. Several testing methods are employed in manufacture, including tracer gas (helium and hydrogen), pressure decay, mass spectrometry and ultrasonic crack detection. With tracer gas testing, parts are filled with He or H2 and placed in a sealed chamber, and gas detectors show any leakage. Broadly, there are two approaches: the vacuum method and the sniffing method. In the first, the battery case is placed inside a vacuum chamber and pressurised with helium. A mass spectrometer-based leak detector inside the chamber but outside the battery case detects escaped helium. In the sniffing method, the battery case is filled with helium again, but this time a sniffer probe is moved along potential leakage points such as seams, welds and connectors to detect and locate leaks. The main advantage of tracer gas testing is its sensitivity, being able to detect leaks as small as 10⁻⁹ atm cc/s (0.000000001 cc of the gas escaping per second under a pressure of one atmosphere). Also, it does not damage the battery and it is highly reliable. The principal drawbacks are the expense of the required vacuum chambers and mass spectrometers, the complexity arising from the need for well-sealed environments to avoid false positives, and the handling risks, cost and scarcity of helium. In pressure decay testing, gas is injected into the coolant circuit and, separately, the battery casing, and the pressure monitored. After a set dwell time, the pressure is measured again, and if the pressure drops below a threshold a leak is confirmed. Increasingly seen as best practice in volume production, advanced pressure decay testing takes seconds or minutes, minimising effects of temperature variation. However, it is less sensitive than helium/mass spectrometry testing, only detecting leak rates down to 10⁻³ atm-cc/s, and temperature variations can cause significant errors in some systems. ATEQ’s F620 is a differential pressure decay tester with a range of leak measurement covering pressure change sensitivities of 20 Pa, 50 Pa, 500 Pa or 5000 Pa for small to large volume applications (Image courtesy of ATEQ) The main advantage of the tracer gas testing is its sensitivity, being able to detect leaks as small as 10-9 atm cc/s
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