Battery Q&A

Owned by Micah Black
Last updated: Sept 14, 2023 by Katharine Laughton
12 min read

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?

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.

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?

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.

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:

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.

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?

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

A4:

Phase change material! Yes - this stuff is awesome! All Cell Technologies is where I first heard of it when looking into cooling for MSXIV: https://www.allcelltech.com/

Here’s another company that we looked into as well - Outlast Technologies:

I would absolutely love to do this if we can figure out a way. Increased safety is the biggest reason to push for this as it limits thermal runaway propagation. NASA also have a ton of presentations on this - just search ‘NASA 18650 Thermal Runaway Propagation’ on google and you should get a bunch of results.

We did look in to this a bit, and determined that it would be practically impossible to acquire some material to use (we did send an email to All Cell Technologies but they said they don’t sell the material directly, they will only sell it as built into a pack that you buy from them, which was super expensive). I did look into what it would take to make some, and determined it to be too difficult of a technical challenge. We also wouldn’t really be able to test the thermal runaway propagation technologies since the university wouldn’t really like us burning up cells on purpose. There may be a few battery labs on campus that we can reach out to about spaces to do this testing though. The main reason we did not pursue this route is that we needed something to work and be complete in a short time frame. We did not know if making a phase change material from paraffin wax stuffed with some kind of thermally conductive powder would even work, so we decided not to invest the resources into it. On this new cycle though, it would be great to reconsider!

Other resources for PCM:

https://ctherm.com/applications/phase-change-materials/

https://endless-sphere.com/forums/viewtopic.php?t=97512

https://endless-sphere.com/forums/viewtopic.php?f=14&t=84857&p=1241253&hilit=Outlast+phase#p1241253

https://www.allcelltech.com/pcc

 

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.