Aluminium bus bars for marine battery systems

Electric drivetrains, either through fully electric or hybrid solutions, are one of the main solutions that can contribute to the emission reduction goal for maritime transportation. The battery system is a key element in an electric drivetrain and represents a substantial part of the total system weight and cost. Consequently, development of safe, robust and cost-effective battery systems is critical to secure competitiveness of these new and environmentally friendly solutions. Marine battery systems are typically based on a modular design, where each module consists of a set of smaller battery cells. Moreover, the battery systems have internal bus bars connecting all cells in one module, as well as external bus bars connecting modules into a complete system. While copper is currently the most commonly used material for bus bars, high conductivity and low density makes aluminum well suited as an alternative material. However, challenges related to strength and durability need to be solved in order to establish competitive solutions. The overall objective of the project is to establish type approved aluminum bus bar solutions that are tested and ready for pilot installation. This will be achieved by combining available knowledge and competence on aluminum technology, marine battery systems as well as battery powered vessels. In addition to the internal resources within the project partners, the project will establish extensive cooperation with NTNU and SINTEF through NTNU Aluminium Product Innovation Centre (NAPIC). This is a center that has been established to promote aluminum-based product innovation at NTNU and is a cross disciplinary collaboration between several NTNU departments and SINTEF. Aluminum based concepts for external busbars and a setup for testing of these have been developed. The test setup is a combination of exposure to corrosive environment and thermal cycling. The thermal cycling has been performed in several steps with increasing temperature amplitudes to identify differences in robustness for bolted connections. While the nickel coated samples have proven stable performance, cycling at higher temperatures have revealed varying performance for uncoated busbars. Since uncoated busbars are favorable due to lower production cost, more systematic testing of the bolted connections has been performed. The test results show that robust performance relies on proper surface preparation of the contact areas. Consequently, bolted connections for uncoated aluminum bus bars are only recommended for applications where the joining process is fully controlled. This is an important finding for marine battery systems where increased joint resistance could represent a safety risk. For welded internal aluminum busbars, the main challenge is the connection between aluminum and battery terminals for each individual battery cell. The project has focused particularly on cylindrical battery cells, where the terminals consist of nickel-plated steel. A suitable welding process must provide a robust connection with sufficient connection area, and at the same time limit heat input and temperature increase to a level where the battery cells are not damaged. As an initial process screening, laser welding, CMT from Fronius and ultrasonic welding have been tested and evaluated. The conclusion from the welding tests is that laser welding is the most promising process for welding aluminum to cylindrical battery cells. To document performance and establish robust process parameters, extensive testing and characterization of laser welds has been performed. The results of the project have enabled implementation of internal aluminum bus bars for two Corvus marine battery systems. Both systems are based on laser welding between the bus bars and the battery terminals.

The project results have been key enables for implementation of aluminum bus bars in new Corvus marine battery systems. Safety is always the main priority when developing marine battery systems, and well documented and stable performance is required for implementation of new solutions. Results from the extensive testing have shown that stable performance of bolted connections relies on proper preparation of the joint surface for non-coated aluminum bus bars. To avoid potential safety issues related to joint performance, Corvus has decided to base the initial implementation of aluminum bus bars on laser welding and utilized project results to establish robust laser welding solutions. The aluminum bus bar solutions are expected to be DNV approved and implemented for serial production during 2024. In addition to the described implementation in marine battery systems, the project results are used by Hydro to promote aluminum in conductors towards other industries. Especially towards electrification of the automotive industry, the potential for weight and cost reduction through implementation of aluminum as replacement for copper conductors represents a growing market. The project results on alloy selection as well as joint design and performance are valuable in supporting development and implementation of the new aluminum solutions.

The main objective of the project is to establish a solution for aluminium bus bars for marine applications as a direct replacement of the current copper-based solutions. With 60% of the electric conductivity and 30% of the density compared to copper, aluminium represents a potential 50% weight reduction of the bus bars. With the current LME prices, changing to aluminium also represents a significant reduction in cost. Marine battery systems have internal bus bars connecting all cells in one module, as well as external bus bars connecting modules into a complete system. For external bus bars, the main challenge related to use of aluminium is stable electrical conductivity of bolted connections. For internal bus bars, the main challenge restricting use of aluminium is related to joining of aluminium to the material used on the battery cell terminals, such as copper and nickel-plated steel. Due to a large number of separate cells used in a marine battery module, this joining process needs to allow short cycle time and be easily automated. Moreover, the heat input needs to be limited to avoid damage to the battery cells. Through utilization of available knowledge on metallurgy, corrosion, structural analyses and processing technology within the Norwegian aluminium industry, new and innovative aluminium bus bar solutions will be developed. These solutions are most likely found as a combination of alloy selection, surface treatment and design of the connections. Reducing both cost and weight, aluminium bus bars will increase the potential market for large marine battery systems, and thereby support development towards zero emission solutions. This is of special importance for high speed and long-range vessels, where cost and weight of the battery system is a major limitation.