E-Mobility Engineering 014 l InoBat Auto dossier l In Conversation: Brandon Fisher l Battery monitoring focus l Supercapacitor applications insight l Green-G ecarry digest l Lithium-sulphur batteries insight l Cell-to-pack batteries focus

in certain chemistries you can get some very large cylindrical cells, but not in the hundreds of amp-hours you would need for the average car,” he says. Physical size, energy capacity and structural strength are not the only important factors, however, as electrical connectivity also has to be taken into consideration, the lithium metal battery developer emphasises. “Prismatic cans are best suited for cell-to-pack and cell-to-chassis battery designs owing to the presence of both electrical terminals on the top of the cell, allowing for access to make the electrical connections to build up the voltage and battery as well as installing and connecting the battery management system [BMS],” he says. “Cylindrical cells typically have a terminal at each end of the cell and will be problematic to access the bottom terminal for cell- to-cell electrical connections.” Cell size is also important for assembly, the specialist EV builder says. “As a general rule of thumb, a larger cell format will reduce the number of connections needed, and will be better suited to high-volume applications where are you are looking at reducing manufacturing time.” Battery case design While the loss of internal module structure in cell-to-pack designs has to be taken into consideration, the pack case does not have to very different from one enclosing a conventional module-to-pack battery, the lithium metal battery specialist argues. “Tesla dramatically improved the Model 3 battery pack design over the Model S by decreasing the number of modules in the battery pack from 16 to four. At the same time, Tesla was able to communise the modules resulting in only two variants, a 23S and 25S module,” he says. (Communisation is the practice of using the same components across several vehicle models.) “To be successful in the EV battery pack business; communisation leads to greater efficiencies of scale.” The vehicle technology and engineering services specialist emphasises that it is not necessary to reinforce the enclosure in all cell- to-pack designs. “The important consideration in this design philosophy is to take a holistic view of the vehicle as a whole,” he says. “Larger format cells could play in role in providing some structure to the vehicle if you can design the systems to transfer load in a safe, controlled way. The trade-offs should be viewed at a vehicle level.” For all battery designs the pack housing structure, or case, makes a vital contribution to the overall mechanical integrity of the battery pack, the adhesives, sealants and thermal interface materials expert says. “The loss of internal structure provided by the module can increase requirements on the housing, although this effect is considered moderate. “Some battery manufacturers aim for a cell-to-pack design that also omits the use of extrusion profiles and cross- struts to maximise volume use and packing efficiency of the cells. In this case structural support through, for example, reinforced battery housings with strengthened bottom or top plates that act as a rigid outer skeleton can be required to improve overall stiffness and crash resistance.” The EV consultant and battery manufacturer emphasises that battery case design, whatever the kind of architecture used, the same fundamentals apply. “As a consultant, a lot of my time is spent explaining the principles of battery design to good mechanical engineers because, really; it’s a mechanical engineering job and Efficiency of packaging, structural performance and weight reduction are potential benefits from using cell- or module-to-chassis battery construction by reducing structural duplication. (Courtesy of Watt EV) 70 Summer 2022 | E-Mobility Engineering