Eddy Current Method for Inspecting Foil-to-Tab Weld Quality in Li-Ion Batteries

Defective components in Li-ion batteries can lead to loss of capacity, reduced discharge rate, or thermal runaway. For electric vehicles (EVs), quality control is especially relevant. In this investigation, foil-to-tab ultrasonic welds connecting EV battery cells were examined using eddy current testing (ECT). Two types of foil-to-tab welds were investigated; aluminum foils welded to aluminum tabs, and copper foils welded to nickel-coated copper tabs, with up to 24 foils welded to a tab. An ultrasonic welder was used under standard nominal conditions, at underwelded conditions as low as 1/3 the energy, and for overwelded conditions as high as four times the energy input. The underwelds were characterized by small divot size and lack of fusion between foils, and overwelds by excessive foil penetration due to larger divots. ECT demonstrated an ability to differentiate between the various weld conditions, attributed to divot size increasing with welding energy. ECT was also shown to be sensitive to the number of either copper or aluminum foils. COMSOL modeling showing eddy current response varying with a number of foils was in good agreement with measured results. Foil-to-tab welds contaminated by plastic sheets inserted in between foils, simulating a potential error in the manufacturing process, could also be detected. These results could be used to design an automated eddy current instrument where defects, including folded or missing foils, contaminated welds, and nonnominal weld types, could be detected.