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:

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.

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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.

Note then in the final module design, we had an acetal sheet placed on top of the cells, and some cells in the module had the positive end facing down, so there is more resistance to gasses trying to escape from the positive end since there are more things in the way. We also used a potting compound to cover the spot welds, and decided on a semi-flexible compound instead of a rigid compound to give less resistance to any TR ejections trying to escape the top of bottom. We did not test any cells going into thermal runaway, but my guess would be that there would be some cells that blow out the side and cause thermal runaway propagation due to the increase in resistance out of the positive end compared to in free air as the tests in the NASA research were done.

Another great presentation on TR propagation minimization:

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:

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?

A5:

In our last pack, I'm pretty sure that thermal runaway would have propagated from one cell to the next if one did happen to heat up too much.
For production packs in EVs, etc. the consensus is that the pack should be designed to deal with thermal runaway WHEN (not IF) a cell goes into thermal runaway, as there will be so many packs on the road, it is bound to happen.
For our pack, it is possible to monitor it and test the cells/modules regularly to evaluate degradation and temperatures, but there is always some risk.
I wanted to go with a thermal runaway tolerant design for MSXIV but none of the solutions that I found matched our timeline, budget, or space and efficiency constraints (placing  cells further apart, machining heatsinks for the cells, using PCM material, etc.). I focused more on keeping the pack compact, well-monitored (voltages and temperatures), well-cooled (airflow channels between cells rows, and more fans than I think we actually needed), efficient, and easy to manufacture.

Q6:

Are there any situations where might want to heat up the batteries? If we do, is that gonna be something involving strategy?

A6:

I'm assuming you've seen the track mode preconditioning for the lucid and tesla plaid cars, and that's why you're asking, correct? We don't care about power output that much to install a whole heating system - wastes too much energy to generate the heat. We might as well just be driving to heat up the batteries through the internal resistance.

The internal resistance of the batteries does go down a little bit with increasing temperature (up to a point) from what I've read, so this can give us a bit of an improvement. But allowing cells to heat up decreases the margin for cooling, and we may need a better cooling system to get up the hills (i.e. smaller temperature rise before we hit the max temp).

The best way to take advantage of the lower resistance at higher temperatures would be to strategically cool the batteries at specific points so that we get low resistance (slightly higher ambient temp) for most of the race, but still cool the batteries more before a large hill so that we don’t risk overheating as we climb.

The solar panels do get hot and are more efficient when they are cooler, so there could be a potential solution to transfer heat from the solar panels to the battery to get the best performance. Generally though, we are already close to max temp for the batteries - 30 degrees ambient in the middle of the desert in the US on the route typically, then 5 or 10 degree rise for normal operation puts us pretty close to the 45C limit on some of the cells.

I would see this as an optimization issue to try and tackle once we have a working vehicle. We can make some modifications if we have time, but our focus should be on creating a reliable system first.

When the battery’s resistance drop with increased heat, we can pull more current because the resistance is lower, but we'd blow our fuses well before that extra current would be helpful - we can already hit full power even without the lower resistance.