Note for People Just Starting
For an introduction to what kind of specifications are given for cells, read this: Battery Specification Guide (WIP). Additionally, take a look at the datasheet for the cell we used for MS15, LG’s INR21700 M50 cell: M50 Datasheet. Looking at what kind of specifications are given will give you an idea of what criteria can be used to choose a “good” cell for our pack.
Pack Constraints & Requirements
Constraints and requirements relating to the pack is important to keep in mind when doing cell selection. Consider what pack configuration would be appropriate for each cell, and what that implies for the pack.
Constraint/Requirements | Reasoning + Notes |
|---|---|
Max Energy Capacity <= [FILL IN AFTER REG DROP] | 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 prob stay at 36S or bump up to 38S. |
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). |
Cell Selection Criteria
There are many criteria to select our cell from, I believe the ones shown below are the most important.
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If you’ve found a cell that looks good, add it to this Excel sheet: MS16 Cell Selection Candidates.xlsx
Resources
[1] Insert ASC2026 regs here after it drops
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