The importance of electromobility is increasing steadily worldwide. With the increased production of Electric Vehicles (EV) and Plug-in Hybrid Electric Vehicles (PHEV), the number of batteries required is also growing rapidly. The traction battery is a critical part of wear as well as a potential source of danger. In the age of electromobility, consumers do not want to marvel at their burning luxury vehicles. They are also not interested in investing a lot of money in a replacement traction battery after only a few years, to get back to the vehicle’s usable range. Automobile manufacturers and suppliers must use suitable leak testing methods in their production processes, because the electrolyte of the battery cells must under no circumstances escape or come into contact with water and humidity. INFICON, one of the world's leading manufacturers of leak testing equipment (www.inficonautomotive.com), has put together the following checklist for requirements at different stages of the manufacturing process of traction batteries.
When electrolyte reacts with water, hydrofluoric acid is produced, which will damage the battery cell. At the end of the life of a cell, the concentration of water in the electrolyte should be as low as possible. That is why even the smallest units of a traction battery - the battery cells - must be air and water tight. Three types of battery cells exist: prismatic, round and pouch cells. After ten years, the concentration of dissolved water in the electrolyte should be less than 80 ppm. Assuming an average air humidity of 50 percent, this results in a maximum permissible leak size of 10-6 mbar∙l/s. This requirement for reliable gas-tightness can only be verified by modern test gas methods, for example by helium testing in a vacuum chamber.
Currently, most battery cells come from Asian manufacturing facilities and unfortunately, battery cells may be damaged during shipping. This can have fatal consequences during transport. For example, lithium-ion batteries and cells can no longer be transported as cargo in passenger aircraft because they are considered a fire hazard. Numerous shipping containers already have been completely destroyed as a result of the so-called “thermal runaway” of a single battery cell, caused by an internal short circuit. These single cells can potentially initiate an explosion of the entire shipping container, due to the burning cell electrolyte heating up to 1,100 °C. This is reason enough for suppliers to carefully check their cells for leaks.
Because of these types of experiences, many experts and scientists are of the opinion that an efficient incoming-goods test is essential for manufacturers and suppliers. In principle, processors could use empty battery cells from Asia and locally fill, close and format them with electrolyte. Then, of course, a leak test is still necessary. The susceptible areas of prismatic cells include, for example, the welds between the cover plate and the electrode contacts; in round cells the crimped connections between the cylindrical housing and the electrodes; and in the soft pouch cells, among other things, leaks at the seal of the bag.
Battery cells are initially assembled into battery modules, which then become battery packs. Some OEMs already are taking over these production steps themselves, while others are purchasing their complete battery packs from German tier 1 suppliers. Battery-pack housings must protect the contained modules and cells from water and conform to IP67 or IP69K protection ratings, which require protection from potentially exposing the enclosure to the blast of a high-pressure washer. The border leak rates depend on the housing material. For plastic or steel housings, leakage rates in the range of 10-3 to 5 x 10-3 mbar ∙ l / s are tested. With aluminum housings, it is 10-5 mbar ∙ l / s. For this purpose, sniffer leak detection with test gases, which is often carried out automatically with a robotic arm, is recommended.
The reliability of cooling over the lifecycle of a traction battery also affects its reliability and service life, as batteries heat up during driving and during charging. Therefore, both the battery cells and the electronic control unit (ECU) of the traction battery must be cooled. In principle, passive air cooling or active liquid cooling are possible. In the latter case, there are either water-glycol mixtures or refrigerants such as R1234yf. The specific tightness requirements depend on the cooling medium. For a water-glycol mixture, the limit leak rate is 10-3 mbar ∙ l / s. A refrigerant such as R1234yf should be tested against a leak rate of approximately 10-5 mbar ∙ l / s, which also requires the use of test-gas methods.
For more information on why you should check for leaks during battery manufacturing, download our new white paper: "Electric Cars: Requirements and Leak Testing Methods for Assuring Quality."