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

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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.

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 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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Module Size and Configuration

Q3:

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A3:

A few tradeoffs to consider here:

• A manageable assembly size. A single module with 200 or 300 cells is pretty heavy and would need 2 hands to hold it, harder to move around and keep extra modules. At ~50g/cell, 200 cells weigh 10kg and 200 cells weigh 15kg. Smaller modules are easier to handle.

• If a module fails, we need to replace the module. If we had only 4 modules in the pack, then we may only have the budget for a single spare module. Also, since there are more cells in a larger module, there is a greater chance that a cell within that module fails. Smaller modules means we can have more spare modules, and will statistically fail less often.

  • It is beneficial to make all modules exactly the same. Then we only have to keep 1 type of spare module, and any spare module can be swapped in for another. MSXII had 2 types of modules (and end module and a middle module) and we ended up running out of spare end modules, while still having a few spare middle modules unused.

• More modules mean that we have more inter-module connections. This increases the number of failure point. Each inter-module connection also has a small resistance associated with it, so more connections means higher resistance in the pack’s current path, and more power loss (P = I^2 * R).

• If you compare the weight of the cells in a module to the weight of the rest of the items (cell holders, support brackets, cables, bolts, etc.) then the weight of the extra stuff in a larger module will be smaller per cell (LM = Large Module, SM = Small Module):

(LM mass of extra items / LM # of cells) < (SM mass of extra items / SM # of cells)

The extra items can also be considered ‘overhead weight’ and would be roughly constant for all module designs. More modules = more extra weight.

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

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• P = I^2 * R).

• If you compare the weight of the cells in a module to the weight of the rest of the items (cell holders, support brackets, cables, bolts, etc.) then the weight of the extra stuff in a larger module will be smaller per cell (LM = Large Module, SM = Small Module):

(LM mass of extra items / LM # of cells) < (SM mass of extra items / SM # of cells)

The extra items can also be considered ‘overhead weight’ and would be roughly constant for all module designs. More modules = more extra weight.

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

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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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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