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To connect the cells electrically, we plan We originally planned on spot-welding a grid of nickel strips to the cells. This is a standard procedure for building 18650-based packs, and is much safer , and more reliable , and easier than soldering or a purely mechanical solution. In order to carry the current, we plan on running 10 awg copper wire in the gaps between the blocks of cells which then run to blade connectors. To support our peak current draw of 100A, we should have 6 or more 10 awg wires connecting each module. Ideally, these should be evenly distributed for current sharing.

To reinforce the copper wire, we can punch holes in the nickel strip grid where the wire will be run and source thin copper discs to be placed where a cell would normally go. Then, we can solder the wire directly to the copper disc, providing some support from the 18650 brackets and reducing stress on the nickel strips. The purpose of punching holes in the nickel strip is to increase the copper to copper surface area. The goal is that during this, the nickel and copper would also be joined.  At the same time, it provides some structural strength by filling up the empty circles where an 18650 would normally go. This operation could be done before spot welding to reduce heat spread to our cells, and further reinforced with glue or some other adhesive.

Note that this design is still in development and is subject to change. We have yet to determine how we'll be mounting these modules to the box itself and how it's supposed to withstand 20g's of force. We also need to determine which connector best meets our design requirements. We would then solder 6 or more 10 AWG wires to the grid, terminating the wires in a blade connector. However, we have identified a number of concerns with this approach. The available connectors have a low lifespan of 25 mating cycles and require enough slack in the wire and clearance to actually connect and disconnect them. In addition, spot-welding all of our cells and soldering all the wire would be a labor-intensive task.

To reduce labor, we are considering a purely mechanical pack. This approach uses dimpled copper strips in place of the nickel grid, relying on compression through foam (and possibly magnets) to ensure an electrical connection. Even pressure will applied using a backing plate, likely made of plastic. This will either be attached to the foam with tape or bolted through the module to the other side. Our approach to series connections is the use of busbars through compression. Although ideally we'd be able to bolt busbars together, doing so safely would be difficult, requiring ample space for tools and both terminals to be on the same face. Instead, we can bring the copper strings from the battery terminals to the other side of the backing plate, adding a busbar that connects all cells in parallel. The insertion of the module into the box would then complete a series connection through copper busbars embedded in the mounts for each module. All exposed copper other than the faces of the busbars for the series connections must be insulated. Combined with an intelligent mounting solution, this should help reduce the risk of accidental shorts.

When designing a method for mounting modules to the battery box, we must consider the clearance required to access any fasteners used and electrical safety. For these reasons, we are considering a tool-less compression fit. Our modules would be held in place with a grid high enough to prevent rocking, possibly lined with foam. We can also embed copper busbars in this grid to form the series connections between each module, resulting in minimal exposed copper. For added strength, we can add magnets to keep the modules locked in place. These should be polarized to prevent modules from being placed in reverse.

Layout

An approximately 60kg pack will be difficult to lift. With additional electronics and the added weight of connectors, brackets, and the box itself, we expect the pack to be between 70-80kg total. To make this easier to manage, we are considering splitting the pack into two boxes. Note that we have approximately 40" x 47" x 9" of space to work with.

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