Mechanical Packaging and Material Selection 133 pouch-type cell will require a set of frames made from either plastic or metal that the cells are mounted to and in which the cells are generally protected and the appropriate amount of pressure is applied onto the cells. For the large prismatic can cells, each cell may not need its only frame/structural piece. In this instance, the interconnect board (ICB) may be enough to provide the appropriate amount of mechanical structure. In some instances, the use of banding is included in order to assemble the complete module design.
However, there are a couple of long-term issues that need to be evaluated when it comes to the use of banding your modules. If you are using a plastic-based banding, the question comes down to what is the level of elasticity over the life of the material? If the banding material stretches enough over time, the cells may no longer get the amount of stack pressure that they need. If steel banding is used, it may have the opposite effect in that it will not stretch over time, but the cells will grow dimensionally over their life. Therefore, the amount of pressure that the metal banding provides will increase over time.
Another thought that should be considered in a module design is the need for serviceability. Some battery manufacturers have designed their modules in such a manner that the cell interconnections are made with mechanical components, bolts, and nuts. In this case, the cells can be replaced and the module can be serviced throughout the life of the module. However, there is always a risk that the mechanical connections will loosen over time thereby increas- ing the resistance and creating the potential for a failure mode in the battery. Other battery manufacturers believe more strongly in the use of welding for the cell-to-cell interconnec- tions.
The benefit here is a somewhat lower material cost (no costs for fasteners) and some- what higher reliability (will not loosen over time). However, there is also the risk that if you build a module with multiple cells and there is a failure of one cell, the entire module must be replaced instead of a single cell. For most battery manufacturers, the module is the basic building block for all of their system designs, so having a solid and reliable modular solution can be a very big benefit to driving commonality across multiple systems. Use of Metals in Battery Design We will begin by looking at the different types of materials that are used in battery enclosures.
Module and pack enclosures may be designed from plastics, steel, aluminum, fiber glass, or composite materials. And in almost all cases, a combination of these materials will be used in an ESS design. First, we will review the use of a stamped steel enclosure. Steel offers many benefits includ- ing high strength and relatively low cost. However, the use of a steel enclosure must also generally include some weldments or otherwise mounting attachments and may need to add mounting structures and to provide for strength. This adds to the processing time of the