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Busbars
Q1:
I see that many battery packs have a busbar that is strapped to spacers and more narrow/less surface area - I'm wondering what the pros/cons are of last season's busbars vs these ones?
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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.
Shorting Cells
Q2:
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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 I've heard that the original design was changed to now hold the battery cells upright because the positive terminals were touching the negative terminals and would have posed safety issues if there was problems.
Would there be any benefits to stacking the cells like shown in the picture on the right if there was adequate spacing between the cell terminals or if the top row was inverted so positive would be in contact with positive?
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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.
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Cooling Methods: Phase Change
Q4:
I read a little bit about battery cooling in this publication (see below), where they mentioned PCM Cooling and its benefits. I couldn't find a lot of information in past confluence documents about it but I'm wondering if we could include PCM materials such as aluminum fins/paraffin wax/pcm cooling vest pods in between the cells/modules?
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In case the link ever gets taken down, here’s the PDF:
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