Laser welding holds firm for electric vehicle battery joins

Challenge

Laser welding process development and parameters optimisation for attaching busbars to cells for the assembly of 22 prototype electric vehicle battery modules.

Background

Helical Technology works predominantly with the automotive sector such as automotive manufacturers, motorsport teams, and as a component supplier for vehicle, powertrain, and, increasingly, emissions testing. It has test facilities for vehicles, engines and components, including low and ultra-low-emission vehicle technologies such as hybrid and electric.

An electric vehicle uses a large number of batteries, which need joining to attain the desired capacity without compromising on safety. The joints need to be reliable, have acceptable strength and electrical resistance, and also be free from defects. This Innovate UK-funded project focuses on the welding of tab-to-busbar materials in battery pack modules.

Helical Technology, a company that has been established for more than 50 years with production facilities in the UK, India and China, chose to work with the AMRC because it has the capabilities and previous experience of welding such cell terminals to the busbars using an 8kW laser welder at its AMRC North West facility in Lancashire.

Innovation

Electric vehicle (EV) battery assembly involves significant challenges that arise from inconsistencies in the cells and their connections. The battery materials are chosen mainly based on their electrical properties. The tab and busbar materials are mostly different in battery pack modules and hence there are differences in physical, chemical and mechanical properties.

The welding of dissimilar materials presents a number of challenges mainly, due to variations in melting point, chemistry, but also their thermophysical and thermomechanical properties. The tab and bus bar materials used in this project were aluminium, copper and nickel-plated copper, owing to their lightweight, good electrical properties and corrosion resistance.

The fusion welding of these materials using a laser results in the formation of intermetallic - a hard and brittle phase with reduced electrical properties which causes premature failure of the joints. The laser welding of these materials also produces porosities, blowholes and cracking. To overcome this, laser welding process development and parameters optimisation is needed to produce joints with acceptable strength and low electrical resistance.

A methodology was developed that used a knife, applied by a cobot, to secure the cell tabs on the busbars. This ensured a uniform contact between the metals that needed to be welded.

The laser welding was done as a series of ‘spots’ to build a seam to prevent any blowholes from propagating through the thin material. Welding parameters were also developed and optimised for both the types of joints: aluminium to copper and copper to copper.

Result

The minimum and maximum electrical resistance for the aluminium-to-busbar joints were 0.03mΩ and 0.08mΩ, and for copper-to-busbar they were 0.03mΩ and 0.04mΩ respectively. The minimum and maximum shear load for aluminium-to-busbar joints were 850N and 1016N and for copper-to-busbar were 735N and 986N respectively.

The weld pool formation, cooling rate, weld chemistry and heat input alter the solidification of the weld. By using a series of spot welds, rather than a linear line, it produced joints with good quality as the technique helps in creating and solidifying welds without any cracks. The produced joints had low electrical resistance and high strength.

Impact