Electric vehicles need powerful, longlived batteries. For high power, often 20 or more lithium ion cells are combined in one battery. For this purpose, a rugged conducting connection resistant to vibrations must be established between metal parts. The lithium ion cells are welded in airtight film bags from which protrude lugs of aluminium or copper. Current bars, also made of aluminium or copper, establish a conducting connection between the different lugs. Laser welding is now mostly used to connect the bar with the lugs. The lugs must be welded to the bus bars in such a way that the airtight seal of lithium ion cells is not damaged by excessive heat. Moreover, the weld must be wide and deep enough to guarantee sufficient current carrying capacity. If the conductivity is too low, the contact resistance rises and the bus bars heat up. In the worst case, this may cause the battery to burn down. Scientists of the KIT Institute for Microprocess Engineering (IMVT) have optimized laser welding in such a way that the cross section of the weld can be set exactly. In addition, they achieve high deposition rates which allows the heat impact to be very short and closely confined in space. The desired width of the weld is obtained by the scientists by laterally deflecting the laser beam and, in this way, passing it over the surface like a wave. The laser power, the deposition rate, amplitude and frequency of lateral deflection and, finally, the position of the laser focus determine the shape of the weld. By setting the laser for the materials to be joined, the scientists thus can define exactly the width, depth and shape of the weld. Its high precision makes this process also eligible for manufacturing microcomponents. The KIT is looking for partners interested in applying the technology.