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Thus, with a 36s36p configuration, our total pack would require 1296 cells. At 48.5g each, we would have a total battery mass of 62.9kg. If we wanted to aim for a 60kg pack, we could build a 36s34p pack, resulting in 1224 cells at 59.4kg. By our target of 1C, this would still safely support 108.8A.

Note: To clarify the nomenclature we're using here, 36s denotes that there are 36 individual units (modules) connected in Series, while 36p denotes that there are 36 individual units (18650 cells) in parallel.

Module Design

We define each set of parallel cells as a module. Our goal is to design the modules such that they will be relatively easy to manufacture, modular, and easy to replace. We have two major concerns - airflow concerns—airflow and electrical connections.

Our current design takes advantage of 8s6p 18650 brackets available from China. To maximize airflow without compromising total volume required, we have decided on the cell arrangement shown below, where the x's mark the location of cells within a particular module. This allows for an air channel for each row of cells, while still packing the cells reasonably closely. Each module would then be approximately 7cm x 12cm x 16cm. With airflow from the side, this should adequately cool the pack. Modules would be arranged with the two air channels are running parallel to the box bed.

To connect the cells electrically, we plan 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, 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.

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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 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.

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.

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