This is meant to be a page to give an in-depth overview of the electrical powertrain system for someone with minimal electrical knowledge.
Direct any questions to Hubert Chen on slack
Accumulator
Cell
At the lowest level of the accumulator is the cell, a component that stores energy in the form of chemical potential. We are not very concerned about the chemistry since we do not make our own cells, so that aspect can be abstracted.
Example cell used for overview: https://www.molicel.com/wp-content/uploads/INR21700P45B_1.2_Product-Data-Sheet-of-INR-21700-P45B-80109.pdf
Discharge curves
Cells increase in voltage with higher charge, and decrease in voltage with lower charge.
In the above figure, we can see how the voltage of a cell decreases as energy is discharged. We can see that the cell voltage will range between 4.2v and 2.5v. Exceeding this range can damage the cell permanently and can be dangerous.
Discharge rate relevant notes:
State of charge estimation (SOC)
By measuring the voltage of a cell, or an average of all cells, we can estimate the total amount of energy left in the pack. This is often used along with coulomb counting to estimate state of charge. coulomb counting is when you integrate the power dissipated by a cell to estimate how much energy has been used/is remaining
Current/power limiting
When the voltage in a cell decreases, the available power of the cell also decreases
Throughout an endurance run, as voltage decreases the available power will also decrease, which affects vehicle performance
Loading effects
As seen on the graph above, the voltage encounters a higher immediate drop as a higher load is placed on the cell
Heat generation
Just like any other electrical component, heat is generated by cells as current flows through them, and is the primary source of heat inside the accumulator.
To determine the amount of heat dissipated by a cell, we simply use the current and the DC impedance. In this example, we can use a current of 20A and the following equation: P=I^2R
P = 20A^2 * 0.015ohm = 6w
In this example, if a cell is discharging 20A of current, it will also generate 6w of heat.
Heat generation relative notes:
This is used to provide calculations on how much cooling is required for the battery pack in order to keep the maximum temperature within the temperature range
Typically, FSAE teams will not design their cooling system to maintain a steady state temperature during endurance. Instead, the cooling system will only be powerful enough to keep the temperature under 60C during an endurance run.
Given constant current, a cell will heat up linearly over time. The cooling system only needs to reduce the slope of the line such that it does not exceed 60C at the end of an endurance run
Different cell types
The two main cell types that are relevant are cylindrical cells and pouch cells.
Cylindrical cells
Cylindrical cells have their own structure in the form of rolled aluminum, and are structurally stable, although in accordance with FSAE rules we cannot consider them structural.
While charging and discharging a cell, the chemicals contained within the cell will increase and decrease in pressure, but the structure of the cylindrical cell prevents physical expansion and contraction.
Cylindrical cells tend to be more gravimetric and volumetrically energy dense than pouch cells, meaning that for the same mass and volume, they will carry more energy than pouch cells.
Cells will typically be insulated with a coating. This is because the majority of the cell is at 0v potential, or the negative terminal of the cell, while only the top bit is the positive terminal of the cell.
Naming. Cylindrical cells typically have the following naming convention: _ _ _ _ 0
digits 1 and 2 denote the diameter in mm
digits 3 and 4 denote the length in mm
digit 5 is a 0 to denote that the cell is cylindrical (0 is a circle)
naming only denotes the physical form factor, not the electrical performance characteristics
18650
21700
proposed form factor for use NER 24
https://nerdocs.atlassian.net/wiki/spaces/NER/pages/318668802/Accumulator+24#Cells
Pouch cells
Pouch cells are rectangular in form factor
Pouch cells internal structures are not as strong as cylindrical cells, which means that additional structure needs to be made to prevent physical expansion of the cell
Pouch cells tend to be more gravimetrically and volumetrically power dense, meaning that for the same mass and volume, pouch cells can provide more power than cylindrical cells.
Cell safety
Lithium ion cells catch on fire under different circumstances:
Overcharging
When you charge a battery to above its rated charging voltage, they may catch on fire
Structural compromise
When cells are crushed, or expand too much (pouch cells), or punctured, they may catch on fire
Overheating
When cells are heated to above their rated temperature range, they may catch on fire
https://www.reddit.com/r/FSAE/comments/14bwoww/battery_fire_fsae/
In 2023 FSAE competition a team left a spare accumulator inside a rental car on a hot day, and it got hot enough for it to catch on fire
How to prevent/mitigate dangers
Store cells properly
Store in well cooled area
Store away from crushing hazards
Sand
When lithium ion cells catch on fire they are very difficult to put out, because the combustion of the chemicals creates its own oxidizer and heat. Sand can be used to prevent or put out fires because they have a high thermal mass and can possibly absorb the heat of a lithium ion battery fire
Run away
lithium ion dust is very toxic, and smells very funny
Fusible Links
In order to connect cells together, fusible links are used.