Welcome back everyone!
I’m Jordan Giesige and this is The Limiting Factor.
This is part sixteen of the Lithium Mine to Battery Line Series to break down and understand
what was unveiled at Tesla Battery Day.
This is the second video on the Structural Battery Pack and will cover the leaks and
hints about how the pack could be manufactured from the cell up.
Before we begin, a special thanks to my Patreon supporters and YouTube members.
This is the support that gives me the freedom to avoid chasing the algorithm and sponsors,
and I hope will eventually allow me to do this full time.
As always, the links for support are in the description.
Back to the structural battery pack.
First, it’s worth noting that even within Munro and Associates, there was vigorous disagreement
as to exactly how Tesla would design the structural battery pack.
Other websites, like Inside EV’s, had a different view than Sandy Munro.
In other words, at this point, all we have is conjecture that often conflicts, and that’s
from engineers who are in the best position to make educated guesses.
I don’t have an engineering background, but I do think there’s still a lot missing
from the conversation.
My approach is different than others because I’ve tried to incorporate all the leaks
and because I’m taking into account cell design.
This approach will inevitably leave me reaching beyond my expertise and what I’m suggesting
may be way off the mark.
However, it will provide food for thought and illustrate challenges I haven’t seen
discussed elsewhere.
As I’ve said in previous videos, the design of the battery cell dictates the design of
everything that’s built around the battery cell.
In the case of the structural battery pack, the way the battery cell is wired tells us
how it’ll be connected to the battery pack.
This is a piece of information we don’t have, but we can find clues in leaks and official
releases by Tesla.
Let’s review the evidence on hand.
Before Battery Day, we were treated to this leaked example of the roadrunner cell.
Note the copper flanges and skirt at the bottom.
We don’t have a clear view of the top, but we can see that it has a bevelled edge and
that it doesn’t appear to be crimped in any way.
At Battery Day, we saw this CGI image.
Note that the top is similar to what we might see in a typical battery cell.
It’s crimped and tapered.
The diameter of the crimped top is actually less than diameter of the cell body.
There are either no features on the bottom, or we can’t see them.
Finally, in the recruitment video, we again see something different.
This image would be the most reliable because it’s not CGI and it’s not a leak.
Note that at the bottom of the cell, there again appears to be a skirt.
It appears to be a skirt rather than a crimp because it’s wider than the diameter of
the cell body.
In other words, what we’re looking at here appears to be in line with the initial leak
that we saw before battery day.
There’s a skirt at the bottom and a bevelled top.
If that’s the case, then the image that Tesla showed us at battery day is a red herring.
It was something people could easily recognise as a battery cell because of its conventional
design.
It could have been 4D chess and way to throw competitors off Tesla’s trail, but usually
I avoid jumping to those types of conclusions.
These seem like minor details, but they matter.
For example, Panasonic and LG Chem, who are also designing 4680 battery cells, would need
to know the exact dimensions of the 4680 and how it’s wired in order supply battery cells
that work with Tesla’s structural battery pack.
So what are we to make of this cell design and what would the benefits be?
Honestly, I have no clue because we still can’t see inside the cell and how the electrical
connections are made.
I do, of course have some speculation.
At the top of the cell there’s a coin sized disc that appears to be electrically insulated
from the rest of the cell.
I’ll refer to this as the coin.
Inside EV’s suggested that the coin could be used to position the battery cells.
I’m not convinced of this for two reasons.
First, there are much better ways to position battery cells - ways that don’t add extra
manufacturing steps and waste metal.
We’ll come back to this later in the video.
Second, the coin provides a good sized target for a welder and an electrical connection.
Why would there need to be an insulator between the metal coin and the metal body of the cell?
The battery will need a positive and negative electrical connection.
In other words, you can’t connect both the aluminum current collector at the top and
the copper current collector at the bottom to the same piece of metal.
They must be electrically isolated from each other.
In a conventional battery cell, this is achieved with a highly engineered and complex crimped
cap design that contains the insulator.
The crimped metal and insulator create a positive terminal in the battery cap, and the body
of the cell becomes the negative.
The battery cap also includes safety measures such as the current interrupt device.
I’m not sure how Tesla deals with safety in their battery, and I’m not seeing any
evidence of a safety device in the images we have, but that doesn’t surprise me.
In prior videos I suggested these safety devices could be redundant with some clever wiring,
engineering, and electronics to safely isolate cells.
I haven’t investigated further but I also haven’t come across any evidence that my
suggestion is technically implausible.
Let’s get back to the coin connection.
If the coin is the positive electrical connection, it appears to be a much simpler design than
a crimped cap and therefor cheaper and easier to manufacture.
However, this leaves us with an engineering challenge.
The diameter of the coin is smaller than the diameter of the top.
In a conventional cell, the entire top is electrically insulated from the body and there’s
a single tab from the electrode to the positive terminal.
With a tabless electrode on top, it would create a large amount of exposed aluminum
and there would be a risk that the aluminum current collector would touch both the body
and the coin.
In other words, how would the aluminum current collector be attached through the top of the
battery cell to the coin while maintaining electrical isolation between the tabless electrode
and the body?
I haven’t solved this challenge, but I’m guessing it has something to due to the disc
at the top of the battery cell.
Many people have said this disc is made of metal.
I can’t tell what it’s made of due to the angle of the camera and I think it could
be metal or plastic.
If the disc is metallic, it would potentially create an electrical connection between the
aluminum current collector, the coin, and the body.
This would mean both the copper and aluminum are touching the cell can, meaning no distinct
positive and negative terminal, meaning it wouldn’t work.
The only way I can see around this would be an insulative gasket above the disc.
If the disc is an electrically insulative material, such as plastic, it might keep the
aluminum current collector from touching the body.
This would allow a separate electrical connection to be made from the coin, through a hole in
the top of the cell and a hole or holes in the plastic disc, to the aluminum current
collector.
If you have any ideas for a better way to do this, let me know in the comments below.
Regardless, without better insights or information from any source I’ve seen, we’ll continue
on the assumption that the body is the negative electrical contact, and the coin is the positive
electrical contact.
This is because the cathode of a lithium ion battery is painted on aluminum foil and the
anode is painted on copper foil.
This makes the aluminum positive and the copper negative.
It would make sense for the aluminum to connect to the adjacent coin at the top of the cell.
Meanwhile, the copper flange at the bottom of the cell would clearly touch the skirt,
making the body negative.
The assumption here is that the entire shell of the cell is conductive and electrically
unified from skirt to body.
If there’s an electrically insulative gasket between the skirt and body, I can’t see
one.
I’m looking forward to Sandy Munro tearing into one of these structural battery packs
to find out how the cell itself is wired.
The result could be mundane, but the internal wiring of the cell could also be one of dozens
of improvements that were hidden in the Battery Presentation.
As Elon said at Battery Day, “There’s a whole bunch of things that we’re keeping
a little secret sauce here that we’re not telling…”.
I think a lot of that secret sauce is in how the battery cell design, structural pack,
electronics, and cooling system nest elegantly with each other in the physical space of the
battery pack.
Let’s bring another leak into the picture see if we can work out anything about the
pack design and how that might dovetail with the cell design.
This leak is from Electrek and shows what appears to be a hard plastic or epoxy honeycomb
structure.
I’ll refer to it as epoxy because Elon said at Battery Day that there would be a flame
retardant epoxy filler.
This image could be a hoax, but Electrek has been good about vetting sources in the past
and the image looks legit.
Given how clean this honeycomb structure is, I’m assuming that the cells haven’t been
inserted yet.
If the cells had been epoxied in and removed, the honeycomb structure would have been destroyed.
If that’s correct and the honeycomb structure’s partially prefabricated, what could the pack
assembly process look like?
First, we start with the 4680 cell from the b-roll instead of the CGI image from battery
day.
Rather than using the coin to position the cells, the cell would be flipped over and
inserted head first into the flame retardant epoxy honeycomb.
The skirt would catch on the honeycomb.
The entire assembly would then be flipped over.
Note that some of the coin side and some of the skirt side would be exposed.
The exposed skirts would be placed on the bucketed tray shown at battery day.
The bucketed tray would include a caste in place or stamped cooling plate at the base.
At the top, a rigid printed circuit board or rigid gaskets would be welded to the cell
tops.
The lid would be sealed tightly over the circuit board or gaskets, holding it them place.
The entire assembly would be bolted together and injected with epoxy.
The epoxy would fill the slight gap between the cells, honeycomb, and face sheets to bond
the entire pack together in seamless and rock solid assembly.
In fact, mechanical engineer and friend of the channel Nick Butler has done CAD analysis
of a conventional battery pack vs a structural battery pack and found that it may be up to
5 times stiffer.
Thanks for the analysis Nick!
Although the assembly process described in this video is a Frankenstein of all the leaks
and engineering assessment out there, I’m relatively happy with what I’ve proposed.
Using a preformed mould could accelerate manufacturing by holding the cells in place while they’re
bonded to the upper and lower face sheets.
After topping up with epoxy to fill the voids, it would then harden into a fully unified
honeycomb structure, with zero joints or weak points.
This leaves us with the overall design of the battery pack.
A structural floor pan with cooling channels, the honeycomb matrix with cells, a wiring
matrix that’s flat and robust to being part of the structure, and then the top plate.
This is a good place to stop and point out a similarity between Tesla’s illustration
and the honeycomb structure that was leaked.
Notice on the right side of the honeycomb structure that there are triangle-shaped metallic
retainers and that the point of the triangle is up.
As I said earlier, imagine that we’ve inserted cells skirt up and flipped the whole thing
over.
Now the point of the triangle is facing down, with the broad side up.
Compare this to what we see in Tesla’s image.
The broad side of the triangle is up and if you look very closely, you can see the positive
coin contact on the top of the cells.
One of the counterarguments to what I’ve suggested here is that the triangles could
just be coincidence and Tesla’s image is a rough illustration.
For example, there are some discrepancies in Tesla’s illustration, such as the unequal
spacing on the left and the right.
It definitely may be the case that I’ve read too much into a connection between the
leak and Tesla’s illustration, but it’s also a strange coincidence and structurally,
it makes sense to use an inverted triangle here.
As a side note, the Electrek article stated that the black tubing that we see in this
image would be coolant loops.
I don’t buy that analysis for two reasons and I think it’s actually wiring rather
than coolant loops.
First, as I said before, my view is that the cells would be inserted skirt up and the entire
assembly would be flipped over, putting the wiring at the top.
This would place the wiring in closer proximity to the rest of the wiring in the vehicle.
Second, given that the coolant channels appear to run lengthwise down the vehicle, I would
think it would make more sense to put the connections for the coolant channels at the
front and rear of the pack rather than down the sides.
Wires, on the other hand would need to be wired in series and parallel, which would
require wiring connection points along the sides.
After I’d fully scripted and recorded this video, Sandy Munro released an image of what
he believes is the structural battery pack.
It looks authentic, but it doesn’t align with what I’ve described today based on
the other leaks.
I can only assume that this image is before the epoxy is added because we can still see
the cells.
I showed the epoxy as clear in my images as an artistic choice, but I’d be very surprised
if it’s actually clear.
Next, the gap between the side of tray and the pack is quite wide here, and it seems
like a waste of space and a structural weakness.
The way I dealt with this in my model is for the entire tray, not just the spaces between
the cells, to be injected with epoxy.
Because of that, I modelled a tight fight between the edge of the pack and the side
of the tray to minimise the amount epoxy used to fill, seal, and bind the pack together.
Next, the points of the triangles face up here, which doesn’t match what’s in Tesla’s
presentation.
I don’t think this pack is upside down because there appears to be two wiring chimneys at
the lower left which would connect to the rest of the vehicle wiring.
Finally, the cells are coated in what appears to be the same green shrink wrap that we saw
with the cells in this image.
I don’t see the purpose of shrink wrap if the cells are going to be surrounded by epoxy.
In other words, I’m not sure what to make of this leak and I find it difficult to square
with the other leaks.
But, I only had my first look at this leak a few days ago whereas the other leaks have
been stewing in my brain for several months.
It could be that the earlier leaks were of earlier prototypes.
Then again, it could be the reverse, and maybe the pack see here is a prototype that was
scrapped.
Either way, I’ve hopefully provided enough open ended thinking in this video to trigger
some brainstorming in the community.
All this conflicting information only makes me more excited to see the final product.
The last video contained a thorough summary of the benefits of the structural battery
pack so we’ll close it out here for the day.
I’m looking forward to the discussion and feedback in the comments.
In the next video, we’ll be discussing the gigacastings.
There’s been a lot of discussion on gigacastings, but I’ll attempt to wrap up all the key
points in one video along with a few bits and pieces that you might not know.
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