Overview of Lithium Battery Packs with Laser Welding Connections

With the electrification of the automotive power train, battery manufacturers are looking to get the most from well-established and proven manufacturing technologies to make increasingly dense and complex battery packs.

Within the packs, modules are electrically connected in parallel and series combinations to produce the overall power characteristics for the intended application. And within the modules, cells (typically 12 or 24) are electrically connected in parallel, certainly for EV applications.

Interconnects – which are cell-to-cell, cell-to-busbar and within the battery management system (BMS) electronics – are typically made in one of two ways: laser weld or ultrasonic wire bond.

Before comparing those techniques, let’s remind ourselves of the cell’s structure. A typical lithium-ion cell comprises four key elements: an anode, a cathode, an electrolyte and a separator. In a cylindrical cell, the anode and cathode are foils that are rolled-up together with a dielectric separator material (to prevent short circuit) in-between. This “Swiss-roll” is then placed into a suitable housing (or ‘can’) for electrolyte fill and sealing.

The Interconnects:

For laser welding, these typically use 99.99% copper busbars or “tabs” (sometimes nickel plated), ranging in size from 5 to 10mm wide and up to 0.5mm thick. Other widths and thicknesses are easily accommodated in laser welding, but these must be prefabricated (for the application) prior to welding. Laser welding of aluminium is also emerging as a potential alternative to copper laser welds.

For wire bonding, these typically use 99.99% aluminium wires or ribbons, in diameters ranging from 200 to 500µm (ribbons up to 2000 x 400µm).

Laser weld:

Laser welding is a typical weld process where two compatible materials are heated and diffuse into each other; the laser providing sufficient energy to melt the busbar to the battery terminal. For this process to be successful, the busbar and battery terminal must remain in close contact throughout, which does pose challenges to manufacturing setup and fixture design tolerances.

Also, beware thermal issues. Localised heat from the welding process penetrating the negative terminal can alter the cell chemistry and lead to catastrophic thermal runaway. The ‘floating’ positive terminal is less vulnerable because of the air gap.

Ultrasonic wirebonding:

Ultrasonic ‘wirebonding’ is well established (60+ years), dominating power electronics manufacture as a flexible and robust method of making electrical interconnects in hybrids, switches and regulators.

Ultrasonic bonding is effectively a ‘friction welding’ process, however, the majority of the energy transfer occurs within the bonding materials; with minimal localised heating to the wire or battery surface.

Therefore, aluminium wirebonding is known as an ambient temperature process, so there’s no risk of altering the cell chemistry and having thermal runaway issues during manufacture.

In ultrasonic wirebonding the controlling variables that determine the process are:
• ultrasonic energy;
• bonding force; and
• (bond) cycle time.

Some wirebonding systems now offer ‘response-based’ programming where the process engineer inputs process requirements (bond width, strength) and variables (wire diameter, ribbon cross section) and the machine offers parameter suggestions based on these criteria.

Conclusions

Laser welding is a good choice for joining battery tabs during battery pack assembly. The process is fast and efficient. It produces high-strength welds for all material combinations. The process is easily adapted to a broad range of joint designs. Laser welds of aluminum, copper, and nickel to themselves have higher shear and peel strength than comparable ultrasonic and resistance welds. Laser welding is particularly successful when used to join aluminum to aluminum and copper to copper. If integrators can develop off-the-shelf systems for cell and pack manufacturers, laser welding can compete with these established techniques for the electrified vehicle pack and module assembly market.