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Constraint/Requirements | Reasoning + Notes |
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Max Energy Capacity <= [FILL IN AFTER REG DROP5.25kWh] | Our maximum energy capacity is limited by ASC2026 regulations. We want our pack configuration to allow our energy capacity to be as close to this number as possible (if not exactly on this number). [1] |
Max Voltage < 165V | Constraint based on motor controller’s max cont. bus voltage [2]. Higher voltage is always better because we want to minimize current to reduce power loss from high current (P=I^2R). The max voltage is more-so a constraint on our series count. It’ll stay at 36 (37 is a prime number, 38 has 1, 2, 19, 38 as factors, so neither are very good for us when splitting the pack into modules) |
Minimize Weight | The battery is a big contributor to the car’s weight. The lighter it is, the more efficient our car will run. Additionally, a lighter pack is easier to remove, making serviceability and overall quality of life better. Minimizing weight relates to the cell’s gravimetric energy density and size (larger sizes are often more energy dense than smaller size, though this is not at all always the case). |
Minimize Volume | The battery takes up a considerable amount of space. As we aim for a slimmer car with MS16, minimizing volume is something to keep in mind. Minimizing volume relates to the cell’s volumetric energy density and size (larger sizes often have higher mAh capacity and so will allow us to use less cells for the same amount of energy → less volume taken up). |
Minimize Heat Generated | One area of improvement (or deletion) for the MS16 battery system is the removal of forced air cooling using fans. Minimizing heat generated from the pack may allow us to either move to passive cooling (cooling with no fans) or have no cooling at all. This can allow us to simplify design and also have less parts. Minimizing heat generated relates to the cell’s DC internal resistance and mAh capacity (because lower cell capacity means higher parallel count without decreasing series count → current draw is split across more cells → P=I^2R, less heat generated). |
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We can use the cell to configure the pack as close as possible to the reg’s max energy capacity. Because of the reg change, MS16 battery pack will have less energy than MS15 battery pack (likely by around 1kWh) [8]. If we’re going to have less energy this car, we want to have as much energy as we can still get.
The cell will strike a balance between lower capacity (which will lead to more total cells and a higher parallel count → less load for each individual cell → better thermals) and a “reasonable” weight/volume.
Relative to aerodynamic drag, weight has a small effect on the efficiency of the car (rolling resistance accounts for 14% of spent energy while aerodynamic drag accounts for 77% based on Forest’s calculation [89].
Cell Selection Organization + Timeline
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[5] https://www.batterydesign.net/dcir-acir/
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If 1.5kW is max expected power from the array and 90V (based on 36 series count) is the lowest possible voltage the pack can be, charge current shouldn’t exceed 1.5kW/90V = 16.7A. 16.7A charge current isn’t very much when you consider that it will likely be split among 8 or more cells in parallel. Most Li-Ion cells should be able to handle this easily.  |
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[9] https://docs.google.com/presentation/d/1C4GlFdyFLaSJIhRW-QnGZWU_KHUQxUT-ykzU_RGqzS4/edit?usp=sharing