Today we are discussing designing & creating lithium-ion battery packs for electric vehicles & industrial applications, focusing on the interconnections of the battery cells & the cell-to-busbar connections. Come along as we delve into the two primary techniques involved in this process, laser welding & ultrasonic welding/wire bonding.
Contact: Betsy Barry
Communication Manager
706.206.7271
betsy.barry@acculonenergy.com
The design and creation of lithium-ion battery packs for electric vehicles or any industrial or off-road vehicle involve connecting numerous li-ion cells to customize voltage, capacity, and power output to meet the application’s specific requirements while also providing safety, reliability, and thermal management.
The interconnections within a lithium-ion battery, including cell-to-cell links, cell-to-busbar connections, and components within the battery management system (BMS) electronics, are typically established using one of two primary methods: laser welding or ultrasonic wire bonding. How do you choose which method is best for your particular application and design specifications?
First, let’s look at the distinctions between laser welding and ultrasonic welding/wire bonding:
Laser Welding: Laser welding makes use of a tightly concentrated laser beam as its energy input. The laser beam efficiently melts and melds the materials at the welding spot, leading to the creation of a robust connection.
Ultrasonic Welding/Wire Bonding: This technique harnesses the power of high-frequency mechanical vibrations produced by an ultrasonic transducer. These vibrations are then directed into the materials that need to be joined, resulting in localized heating and the formation of a bond.
Now, let’s look at some differences between each technique.
The laser welding procedure follows a classic welding technique, in which two compatible materials are heated and blended, aided by the laser’s energy that melts the busbar onto the battery terminal. To ensure the success of this operation, it’s crucial to maintain continuous contact between the busbar and the battery terminal, which can present difficulties in manufacturing setup and fixture design tolerances. Additionally, it’s important to be cautious of potential thermal concerns. The concentrated heat generated during welding, when it reaches the negative terminal, has the potential to disrupt the cell chemistry and trigger a dangerous thermal runaway. Conversely, the ‘floating’ positive terminal is less susceptible to this issue due to the presence of an air gap.
Laser welding offers a distinct set of benefits. The tightly focused laser beam can create strong connections quickly, making it a suitable choice for high-volume production lines, such as those used in electric vehicle battery assembly. Laser welding produces minimal material spatter, reducing the risk of contamination within the battery pack. This is crucial, especially in applications where cleanliness and low impurity levels are critical, as is often the case in medical devices or aerospace. The precision and cleanliness of laser welding often eliminate the need for extensive post-welding cleanup. Additionally, laser welding is energy-efficient as it delivers focused energy directly to the weld area, reducing wasted energy and heat dissipation. This is an advantage in terms of environmental sustainability and cost-effectiveness.
Finally, laser welding is notably fast, primarily due to its reduced reliance on mechanical movements. The process entails the manipulation of only ultra-fast mirrors within the laser head to direct the laser beam, thereby significantly cutting down the time required for precise positioning in each weld.
When designing lithium-ion battery packs for electric vehicles & industrial applications, choosing between ultrasonic welding/wire bonding & laser welding depends on various factors such as the application, materials used, production volume, & desired performance characteristics.
Where laser welding might benefit from speed, wire bonding is superior when it comes to precision. Wire bonding provides precise quality control through a specifically tuned set of program-defined parameters. The bond head makes physical contact with each unique surface, accounting for micron-specific height changes and material compositions throughout the battery module. Each of these surfaces is assigned a unique combination of force, time, and energy necessary to achieve a proper ultrasonic bond. Careful control of variables allows for the intricate and case-specific tuning of each bonding process.
Additionally, statistical data of each bond is monitored and compiled into a database in order to ensure that consistent safety and quality standards are met. An onboard camera is used to geographically scan and map the battery module so that a wire matrix can be designed. Images of each bond surface are stored and used by the machine to form a consistently recognizable pattern to ensure the accuracy of the defined matrix. An integrated pull test provides a means of monitoring these standards, as well as ensuring each electrical connection has been made properly. These wired connections serve not only as the conduit but as a fuse between each connection, adding a precise measure of safety to a module that may be subjected to excessively rigorous conditions.
Ultrasonic welding generates minimal heat during the process, which can be advantageous in battery pack design. Excessive heat can potentially damage sensitive battery cells or electronic components, so this method is preferred when thermal management is a concern. Finally, ultrasonic welding can be used with a variety of materials, including aluminum, copper, and even dissimilar materials. This flexibility makes it suitable for a wide range of battery pack components, from busbars and connectors to thermal management elements.
The choice between ultrasonic welding/wire bonding and laser welding in lithium-ion battery pack design depends on various factors, including the specific application, materials used, production volume, and desired performance characteristics. Both methods have their advantages, and the selection is typically driven by a careful assessment of the project’s requirements and constraints. Ultimately, the goal is to create a safe, reliable, and efficient battery pack that meets the demands of modern electric vehicles and other energy storage applications.