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We originally planned on spot-welding a grid of nickel strips to the cells. This is standard procedure for building 18650-based packs, and is much safer and more reliable than soldering. 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. Spot-welding requires trained operators and a calibrated machine, which are major bottlenecks considering around 3000 welds would need to be done.
To reduce labor, we are considering a purely mechanical pack. This approach uses a dimpled copper strips grid 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.by running threaded rod through a number of modules in series with backing plates between each module. To achieve our series connection, we plan on extending the copper grid vertically and adding holes so that adjacent packs can be bolted together. We will likely use a copper block as a spacer, which can serve a dual purpose as our voltage sense tap. For safety, we plan on covering all exposed copper with insulated caps.
Inspiration
Original solderless pack design
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The AFEs that we are considering support up to 12 modules each, so with 36 modules, we will most likely put 24 modules in one box and 12 in the other. The pack with fewer cells will also contain the power distribution and BMS systems. If necessary, we can split the boxes differently, but this would require an additional AFE and processing of an incomplete AFE in the middle of our daisy-chain.
Both packs will also need an HV relay. The main box will also need to contain our auxiliary battery.
Follow-up Discussions
2017-03-01 Battery Pack Design Review
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