Manufacturing Lithium-Ion Battery Packs

When compared with competing battery chemistries, lithium-ion batteries have higher voltage, higher power density, and lower weight. New plug-in hybrid electric vehicle (PHEV) programs are possible due to the advantages of the lithium-ion batteries. A PHEV differs from a conventional hybrid electric vehicle (HEV) in that it can also be charged by plugging into the electric grid. One example of a PHEV, the Chevy Volt, can travel up to 40 miles on a single charge; more than most American's daily commute. After 40 miles, a gasoline engine (generator) kicks in to recharge the battery pack. This is also known as a series hybrid, because the gasoline engine does not directly power the vehicle. While a pure electric vehicle (EV) does not have a gasoline engine for recharging, lithium-ion batteries have afforded one EV, the Tesla Roadster^TM, the ability of traveling up to 244 miles on a single charge.

Assembling lithium-ion batteries, however, presents new manufacturing challenges. One major difference is that lithium-ion cell technology requires dissimilar tab materials. Assembling these batteries requires joining of the thin aluminum, copper, or nickel tabs, often in multiple layers and to thicker (and often dissimilar) interconnects. Additionally, PHEV battery packs often require welding hundreds of batteries in series with little or no space for redundancy. It is also important to mention that the battery packs are welded electrically live, albeit in usually smaller sections or modules, but still containing a significant amount of stored energy. Special precautions must be taken to prevent a short between the exposed cell tabs or contact with workers.

When compared with other potential welding processes for these applications, ultrasonic welding is able to consistently produce reliable joints. The process also produces minimal heat, requires no flux, can be used with coated materials, produces low-resistance joints, and is relatively low cost. The ultrasonic welding process does have some disadvantages; including unfamiliarity of the process, tool wear, and a lack of accepted standards and procedures. The process is seemingly straight-forward to automate, much-like a standard resistance spot weld head. A large series of designed experiments have been conducted by EWI to evaluate the significance of many of the process variables, such as the weld power, amplitude, energy, tooling designs, fixturing, and equipment alignment. Under the guidance of EWI and leading battery manufacturers, OSU welding engineering students are also conducting related experiments.

For more information, please contact Matt Bloss at mbloss@ewi.org or call 614.688.5246.