Q1:
A1:
2 main reasons for the busbars - material and shape. We want a low resistance in the busbars so that there is less heat produced in them (P = I^2 * R). With less heat production, we can support a higher maximum current draw for the same temperature target (the max temperature of the cells in the datasheet) - i.e. if P is constant (based on our cooling design), and we can reduce R in our design (based on material and shape) then our I increases, allowing us to give more power to the motors. Or, if our max motor current (I) is constant, then is we reduce R then we can reduce P (the power lost in the busbars) and thus reduce the module temperature.
Resistance can be calculated from the equation:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html
A resistivity table with Copper, Nickel, and Aluminum can be found here: https://www.electronics-notes.com/articles/basic_concepts/resistance/electrical-resistivity-table-materials.php
Resistivity is a material property, so we had to choose the material carefully to get a low resistivity.
The busbar material that we used is the EMS Sigma-Clad 60. More details about Sigma-Clad here: Module Busbars - EMS Sigma Clad 60 (read the designer’s guide especially). Sigma-Clad has lower resistivity than nickel, but is easier to work with than copper because it can be spot-welded with simple tools and it has better corrosion protection than copper.
Length and Cross Sectional Area depend on the design of the battery module
We were careful to ensure that the current flows in a straight path from one of the module to the other and does not wind throughout the module (ensure that cell connections to module bolted connections have short length and wide cross-sectional area).
As highlighted in the Design Guides for Sigma Clad 60 on the page linked above, a material with a relatively high thermal conductivity, it reduces the temperature difference between the hot and the cold parts of the busbar by spreading the heat better than nickel.
Q2:
A2:
The risk of shorting the cells when terminal touched was not really part of this decision. In the figure shown in the question, there is adequate separation between all the cell terminals, with an insulating material in between, which is sufficient.
Cell Can Shorts
It is also good to know that the can (the outside wall) of the 18650 is also part of the negative terminal. It is normally covered in colored heat-shrink PVC tubing, but in the case where the pack is crushed or a sharp metal object is dropped into the pack (screwdriver, allen key, bolt, wrench, etc.) there is a risk. If the object falls between 2 cells and punctures the PVC heat shrink tubing around the outside exposing the metal can walls, and if the object touched both cans at the same time it will cause a short circuit is the cells are connected in series. For series cells that are directly beside each other in the MSXIV modules, we added a piece of ‘fish paper' insulation (or vulcanized fiber insulation, often used in telecom and battery applications). Some images below. That covers cell-to-cell shorting on the negative side.
TODO - add a picture of this here as manufactured.
Shoulder Shorts
Another type of cell shorting to be aware of is ‘shoulder shorts’. Because the cans of the 18650s extend all the way from the negative end to the positive end of the cell and are crimped around the positive end, a single conductive object placed across the positive surface of a cell and touching the edges will cause a short. Read the section on shoulder shorts in the doc below. In the image below, the metal tab is spot-welded to the positive terminal of the cell, and when bent downwards, broke the insulation and touched the negative
We wanted it to be impossible for a wrench dropped on the pack to cause a short, so we made sure to make covers for all the exposed conductors (wires, busbars, or metal components touching those).
In order to protect against shoulder shorts, some pack builders will use fish paper insulation rings as shown below on the positive terminals of their cells.
This works pretty well, but is only required because the plastic holders that are available off-the-shelf and often used do not cover the whole ring around the positive terminal that is susceptible to shoulder shorts - they only cover the corners, so the extra fish paper insulation is required.
We designed our 3D printed cell holders to fully cover the area susceptible to shoulder shorts, so the ring insulators were not required.
Thermal Runaway Propagation
Now, the real reason that we avoided this configuration:
When a cell goes into thermal runaway, it pressurizes the inside and then explodes. Some cells are designed such that when the pressure builds up inside, it will be released from a specific location. They do this by weakening the can in the location that they want to cell to ‘vent' and release all the hot gasses built up inside. Often, the positive end of the cell is the location that the majority of the Thermal Runaway energy is release through. The presentation slide from NASA shown below confirms this, that 76.1% of the energy contained in the cell will be shot out through the positive terminal.
When the energy shoots out the positive terminal, it will heat up anything in its path. If there is another cell in its path it will heat that cell up and cause it to go into thermal runaway, which causes a chain reaction if many cells are connected in this way (thermal runaway propagation). If however, we remove all the cells from the path of this thermal runaway energy, we should be able to reduce the chance of thermal runaway propagation. This was the motivation to not have any cells stacked vertically on top of each other.