Busbars

Q1:

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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

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  • 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.

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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.

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Module Size and Configuration

Q3:

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A3:

A few tradeoffs to consider here:

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• The layout of the modules within the pack may also be easier when working with smaller blocks.

Cooling Methods: Phase Change

Q4:

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In case the link ever gets taken down, here’s the PDF:

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Effect of Temperature

Q5:

We seem to be focusing a lot on thermal runaway for the next pack during the module design sprint. Our last one had cells very close together is there a reason I missed why we are changing this?

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