Why leak testing is vital to an EV future
The future belongs to electromobility. The number of EV / HEV (electric / hybrid electric) vehicles is growing continuously. The heart of an electric vehicle is its battery. In the manufacture of traction batteries, special attention is given to quality assurance because the batteries contain combustible electrolytes and must also retain capacity throughout their entire service life.
Leak testing plays a crucial role in the production of individual battery cells, the smallest component of traction batteries. Specifically, tracer gas procedures are acceptable for determining adherence to extremely low-limit leak rates. However, new methods of construction allow manufacturers to do more to provide leak-tightness for their cells. There are now new lid designs for improving the leak-tightness of battery cells. Using Glass-to-Aluminum-Seal technology (GTAS), battery electrodes are sealed with specialty glass instead of conventional plastic seals. GTAS permanently prevents the penetration of moisture into the battery cell. After all, no customer wants the battery range of their expensive electric luxury vehicle to reduce by half after just a few 2019s of service.
Permanently High Capacity
Traction batteries form up to one-third of the value of an EV. These drive batteries are mission-critical wear parts and even potential sources of danger in cases of malfunction when driving the vehicle, since the electrolytes in the battery cells are flammable. The target standard applied to the service life of a traction battery is that it should retain 80 percent of its capacity after 10,000 charge cycles – a lofty goal in the traffic-laden streets where EVs / HEVs are found. To achieve a long service life, it is essential that the electrolytes do not leak out of the battery cells and water / humidity do not penetrate into the cell: electrolytes could potentially react with water to become fluoric acid. Subsequently, the individual battery cell’s housing must be completely gas-impermeable. A leaky cell will slip out of its normal range of capacity after far fewer charge cycles than the targeted 10,000. Penetrating humidity can even completely destroy the battery cell over time. Modern tracer gas procedures are suited for testing to establish this indispensable gas impermeability.
Cells in Different Designs, Configurations
Battery cells are generally produced in three different configurations. Prismatic cells and commonly found round cells both have rigid housings. These are unlike the housings of pouch cells, which are flexible. Regardless of form, production requires testing for leak-limit rates in the range of 10-5 to 10-6 mbar·l/s.
Older testing procedures, such as water quenching, leak-detection sprays, and pressure drop measuring are not suited for detecting such small leak rates. A modern, comprehensive tracer gas method in a vacuum chamber with helium as tracer gas is recommended for quality assurance in production. Specifically in the production of prismatic cells, it often makes sense to test the tightness of a cell early, even before the cell is filled with electrolytes.
Helium Testing in the Vacuum Chamber
Helium testing in a vacuum chamber provides crucial advantages: it is an automated and highly precise test procedure with very short cycle times and does not require interventions. The rigid housings of prismatic and round cells are evacuated, backfilled with helium and sealed. The cells are placed in a vacuum chamber, the chamber is evacuated and a leak detector detects any helium escaping from a leaky cell. For pre-testing of pouch cells the pouches may be filled with low pressures of helium and the seals of the pouch may be scanned with the sniffer tip of a sniffer leak detector. This can be done manually by an operator or via robotic sniffing. After filling, only one more leak test of subareas of the cell is required.
Pole Contact Pins Are Potential Weak Points
Prismatic cells, like all cell designs, have a number of potential weak points that need careful consideration. The first is the seal between the cell lid (through which the anode and cathode contacts run), and the corpus of the cell, which is often a single piece of deep-drawn aluminum.
The aluminum lid, the fill opening, and the safety valve are laser-sealed in prismatic cells, which normally ensures a permanently tight connection. Other components of the lid are more problematic because polymer seals are used in the two pole contact pins. Since all organic materials, including polymer seals, are subject to natural aging processes, there is a high risk that the required tightness will not be maintained over time.
To combat this issue, different types of lithium batteries (such as high-capacity lithium-thionyl-chloride batteries) use instead glass-to-metal sealed lids for decades, which serve as a complement to their long service lives.
New Technology: Glass-to-Aluminum Seals
These problem points in the lid of prismatic cells can be remedied with a constructive approach that uses a new technology: glass-to-aluminum seals (GTAS). The principle of using glass as sealing material for metals is not new. Glass-to-metal seals (GTMS) have long been established on the mass market. They are utilized in large numbers in the automotive sector for lithium-thionyl-chloride and other lithium primary batteries, sensors and control modules. Experiences with this technology and the large amount of data on the reliability of glass-to-metal seals provided the basis for development of the new glass-to-aluminum seals for prismatic and cylindrical cells.
These GTAS seals are produced using specialty glass that is carefully formulated to adapt to the properties of aluminum. The thermal expansion coefficients of both materials have been precisely aligned with each other.
Strong Compression Sealing
GTAS technology provides a seal as impermeable as it is durable thanks to its specialty glass that has a precisely calculated thermal-expansion coefficient. The principle of compression sealing is applied in the manufacturing process. A prefabricated ring of specialty glass is placed around the two pole contact pins, which are made of aluminum or copper. Another larger aluminum ring encircles them. When the um expands faster than the glass. When the materials have cooled, the aluminum presses from outside against the glass ring through which the pole contact pin runs.
This pressure provides a very strong mechanical seal. In an application such as traction batteries, in which the battery cells must be especially strong for many 2019s, this can be crucial to a long service life. Additionally, GTAS technology not only enables longer service life of the cells, it also reduces the complexity of their structure. In lids of conventional prismatic cells, there are often up to eleven different components made of plastic, copper and aluminum. In GTAS lids, there are only a few components for each pole contact: the contact pin of aluminum or copper, the sealing ring of specialty glass, and around it the ring of aluminum.
This enables an intelligent, less-complex structure for reliable gas-tightness of prismatic cells. The use of GTAS technology on the pole contacts and careful laser welding of the lid on the corpus of the cell are two significant improvements that can help extend the service life of prismatic battery cells.
Electromobility Needs Quality
Battery cells, the smallest component of a traction battery, make up about 60 percent of their value. The importance of quality assurance is correspondingly high. Leak tests are required in downstream production steps if the battery cells are connected to battery modules which form battery packs. To avoid the risk of short-circuiting, at a minimum water must not penetrate from the outside into the housings of modules or packs. Testing these housings for impermeability does not require helium testing in the vacuum chamber: a sniffer leak test makes more sense in this scenario, using either helium or forming gas, and can be conducted automatically by a robotic system.
Moreover, a traction battery must never overheat, which is why the cooling circuit of the drive battery must be protected from coolant loss. This is usually checked after the battery has been completely installed via sniffer leak test in which the refrigerating agent itself (either R1234yf or CO2) serves as tracer gas.
With the continuously evolving modern triumph of electromobility, countless new sealing demands will be made on suppliers and manufacturers. It is sensible to tackle these challenges from multiple directions: with intelligent construction of components and consistent seal testing.
Daniel Wetzig is head of basic development for Inficon. Helmut Hartl is a research and development executive for Schott.