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A battery pack in an EV consists of a large number of individual battery cells that are held together mechanically and connected electrically. Making those mechanical and electrical connections poses several challenges, including the joining of multiple thin, highly conductive materials of varying thicknesses and potential damage through thermal or mechanical shock. T hese factors drive the range of techniques for constructing a battery pack, from resistive and ultrasonic welding to micro arc welders, high- power lasers and even high magnetic fields. The choice also varies with the type of cell, whether it be cylindrical, pouch or prismatic. The different cell types have different mechanical requirements, but they all need to be protected against high temperatures during the construction process. Electrical challenges The key aim for the electrical connections is to produce a joint with a low electrical resistance to reduce the energy loss through resistance and thermal heating, and so maintain the efficiency of the pack. This also helps to keep the temperature of the pack as low as possible during operation. A high-temperature process such as resistive welding can expose the cell to enough heat to melt or disturb the safety vent, compromise seals or cause internal shorting in the cell. It can also create more fatigue in the cell, compromising the long-term reliability. Materials challenges A battery pack has to use different materials, and this creates a challenge for joining dissimilar materials. It can create brittle intermetallic layers with higher electrical resistance and a brittle nature compared with the parent materials. Highly reflective surfaces can be a challenge for processes such as laser welding, while surface coatings or oxide layers can be a challenge for resistive or ultrasonic bonding. The joint strength is of course vital, and a stronger bond takes longer to create with many techniques. However, the bonding has to be created with minimal vibration that can be transferred into the cells – a key challenge for ultrasonic systems. Ultrasonic bonding Nevertheless, ultrasonic metal welding is one of the most commonly used methods. It has been used for various electric cars, including the Nissan Leaf and GM’s Chevrolet-Volt, Spark and Bolt, says Abhishek Das, senior research fellow at the University of Warwick’s Warwick Manufacturing Group (WMG) in the UK. The method uses high-frequency ultrasonic vibration, typically between 20 and 60 kHz, to join substrate materials by creating solid- state bonds when clamped. The high- frequency vibrations create progressive shearing and plastic deformation between the metal surfaces, producing an atomic bond. However, the clamping needs access from both sides of the join, with a sonotrode – which creates the ultrasonic vibrations – on one side passing the ultrasonic energy through the join. Ultrasonic welding can be used for multiple thin foils, dissimilar materials or highly conductive materials such as aluminium or copper, especially for pouch cells. However, it may not be suitable for terminal-to-busbar joints of cylindrical or prismatic cells, as vibration under pressure can cause damage. Ultrasonic welding is most commonly used for bonding the wires, with bondable surfaces made from electrolytic nickel, aluminium and ENIG or ENEPIG (electroless nickel immersion gold and electroless nickel Nick Flaherty explains the pros and cons of the various welding techniques for connecting cells to form battery packs Bonding session 42 Spring 2020 | E-Mobility Engineering The key aim for connecting packs is to produce joints with low electrical resistance (Courtesy of PST Products)