Battery tabs have been getting more conductive and thicker over the last several years, as customers keep seeking better performance and higher currents from their battery packs. These thicker battery tabs are primarily made of nickel, but nickel-plated copper tabs are also being introduced due to their higher conductivity. We have had success welding thicker tabs, but have found that the nickel-plated copper tabs can be very difficult to weld. In any case, adding slots and projections to the tab design is required as they focus the current and minimize current shunting.
Welding success also depends in part upon the nature of the battery itself. Those with thick caps can handle the high force and current needed to weld the thicker tabs. If the battery caps are too thin, you may deform or blow through them when welding very thick, conductive tabs. In summary, welding the thicker, more conductive tabs used in today’s more efficient, higher capacity battery packs can be challenging, but welding success can be achieved by designing the parts correctly and selecting equipment that is best suited to the application.
There are many joining requirements in battery manufacturing, including both internal and external connections and can or fill-plug sealing. Laser welding is often used to join the internal electrode materials that make up the internals of the cell. These are typically constructed from thin copper and aluminium foils. The remaining joints, including the connections inside the can and the external terminal tab connections, are suited to both resistance and laser welding. The decision to use one technology over another is determined both by the type of weld required and production requirements, such as high volume and material compatibility.
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.
These factors drive the range of techniques for constructing a battery pack, from resistive and ultrasonic welding to micro arc welders, highpower 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.
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.
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 and a big advance fort laser Welding.
When planning an automated or semi automated solution based on our Wobble cube, the primary factors to consider are loading/unloading, motion and tooling that ﬁt the planned production ﬂow and production rate.
Loading and unloading can range from manual to conveyer or pick-and-place, motion options center around whether the laser head or the part will be moved, with options including XYZ tables and gantry’s or robotic manipulators. For tooling, the laser is non contact, so tooling of the parts can be achieved either by using a ﬁxture that the batteries and tabs are loaded into, or using actuated tooling that is deployed prior to the welding process.
The most suitable technology and process for battery pack manufacture relates to a number of factors including the pack size, thickness and material of the tab itself, and the necessary production rate. Laser welding processes enable high quality volume production, and, of the two joining technologies today used, spot welding and laser welding, the selection is usually made based on the speciﬁc requirements in each situation, but laser welding is taking over very fast from the spot welding, especially with the excelent wobble laser welding technology.