Welcome back everyone!
I’m Jordan Giesige and this is The Limiting Factor.
Today we’re going to be talking about Tesla’s alleged girthy cell, lovingly referred to
as the BFC, chubby cell, the biscuit tin, and the burrito.
Before we begin, a special thanks to Bradford Ferguson of Halter Ferguson Financial and
the Patreon supporters listed in the credits.
It’s the support on Patreon that keeps me grinding away on niche topics.
Let’s start by taking a look at the cell.
BRcooper released this analysis and image.
The assumption was that the 54 on the side could mean that the battery cell is 54mm wide
and the 98mm dimension was built from there.
However, both Rob Maurer from Tesla daily and Now You Know did their own measurements
and each arrived at around 45X75mm.
A 4575 cell is what we’ll work with for the rest of this video.
That’s very close to the 4070 cell I predicted back in March.
What else can we tell from these images?
One of the cells appears to be the cell with the outer shell removed.
There are copper flanges on the bottom, a plastic sleeve, and what appears to be a plastic
disc on the top.
I’ll provide my speculation here.
The plastic sleeve is probably the outer separator, and the copper flanges probably mate to the
skirt of the outer shell.
The plastic disc is mystery to me in terms of cell functionality but may be there to
protect the fragile tabless electrode edges until it’s mated into the battery pack.
The tabless electrode is an internal design improvement and we wouldn’t expect to see
clear evidence of that here, only hints.
What’s contained in the QR code?
My view is that QR codes are used in manufacturing to track things like where products are made,
their version, and unique characteristic to that batch.
Usually they don’t reveal anything trade secret.
However, I’ll indulge the curiosity.
Most of the code clearly has to do with versions.
70MPa could be the calendaring pressure that was applied to the cathode material during
manufacturing.
Calendaring is a step in the manufacturing process where the cathode is rolled to smooth
and compress it to reach the correct porosity.
This suggestion was made to me on twitter by someone who claims to work in the industry.
H2 could be anything.
My view is that there’s a 1% chance it stands for hydrogen.
For example, this could be the code of the calendaring press that the battery was made
on and the site number.
We do know that Tesla and Maxwell are at least on their second iteration of this line.
9.2 is probably grams of something, but I wouldn’t know what.
I’ll finish my speculation there.
Overall, I didn’t view this as significant information.
Let’s start with first principles.
Why is a larger cell better?
In the simplest terms, constructing a battery pack with larger cells is easier because fewer
cells are required.
This is why Tesla has made their cells larger in the past.
The Model S and X platform use an 18 by 65 mm cell and the Model 3 and Y use 21 by 70
mm cell.
This slight change in cell dimensions allowed for 33% fewer cell cans, jelly rolls, electrolyte
fills, and welds.
This reduced manufacturing time and cost.
A 4575 cell would have 5 times the volume of a 2170 cell.
This would mean 80% fewer cans, rolls, fills and welds.
Put another way, it would reduce the number of cells in a Tesla Model 3 or Y from roughly
4000 to 800.
Why didn’t Tesla implement a larger cell than the 2170 years ago when they had the
chance?
It comes down to thermal management.
As cells charge and discharge, they generate heat.
Automotive battery packs have thermal management systems such as liquid filled cooling ribbons
to remove that heat from the pack.
If the pack generates heat more quickly than the thermal management system can cool the
pack down, the heat will degrade the batteries and shorten battery life.
At extreme temperatures, the battery will explode.
Larger cells are harder to cool because they have a lower surface area to volume ratio.
Smaller cells are easier to cool because there are more surfaces to dissipate heat.
However, there are two more engineering goals that Tesla engineers would have in addition
to larger cells.
1) Faster charge and discharge rates.
Faster charge and discharge rates mean that cells would be generating more heat
2) A thicker layer of cathode active material.
Cathode active material is applied to a foil current collector.
The cathode material holds the energy and the foil transports the energy.
A thicker cathode therefore means more material to store energy, and higher energy density.
However, electrons and ions have to work their way through more material to enter and exit
the cathode, which generates more heat.
This brings us to the triangle I created to illustrate this challenge.
Within an any given battery cell, any move towards any one of these corners results in
a trade-off decision.
For example, if you want to engineer a larger battery to reduce manufacturing costs this
is your trade-off decision: The cathode must be thinner which would result in lower energy
density, or the discharge and charge rate would need to be capped.
The only way to beat the triangle is to shrink it.
There are three ways to do this: The first way is Maxwell’s Dry Battery Electrode
technology (or DBE) which has significantly faster charge and discharge rates.
When a battery cell is pushed hard, energy is being pulled out of the cell more quickly
than the internal chemistry can reach equilibrium.
It creates a kind of lag.
That’s what this graph from Maxwell is showing us.
Wet cell chemistry can’t keep up with high charge and discharge rates and develops a
large lag.
Dry battery electrode chemistry from Maxwell performs much better due to higher electronic
and ionic conductivity.
Stated the opposite way, the Maxwell Battery Cell has significantly lower internal resistance.
Resistance is like friction in a cell.
Lower resistance means less friction and less friction means less heat.
Maxwell’s lower resistance is what led me to make the prediction that Tesla would create
a larger cell.
However, I don’t understand anything about thermodynamic modelling and my guess at a
4070 cell size was essentially a best guess based on rumours.
In my view, the lower resistance of Maxwell DBE is the most important factor in increasing
the size of the cell.
There is no heat to remove if there is none being generated in the first place and it
would significantly shrink the triangle.
The second way to shrink the cell is through an improved cooling system.
This one is obvious.
If Tesla added more cooling ribbons to their pack it would improve the cooling.
However, cooling a battery cell through the sides means that the core of the cell is insulated
but the dozens of layers of the jelly roll and plastic separators, which aren’t thermally
conductive.
Counterintuitively, a cell can be cooled more efficiently through plate cooling, where the
heat generated by the battery cell is wicked out the aluminum and copper foils.
These foils are thousands of times or conductive than the plastic separator and are like superhighways
for wicking heat out of the cell.
Thanks to Sagan on Sustainability for bringing tab and plate cooling to my attention.
This brings us to the third way to shrink the triangle.
The Tesla Tabless Electrode Patent.
On screen is a conventional tabbed design on top and a Tesla Tabless electrode on bottom.
The tabless electrode shrinks the triangle in two ways:
1) The entire edge of the electrode is available to conduct heat into and out of the cell.
With the conventional design, the heat only has a small access point to exit and enter
the cell.
2) The same bottle neck applies to electrons.
With the tab, electrons travel the full length of the foil to enter and exit the cell.
They bump into other atoms on the way, generating heat.
With the tabless design they can take the shortest path, meaning less friction and heat.
Individually, in my estimation, the effect of each of these bottlenecks is less than
that of Maxwell’s Dry Battery Electrode.
This is because the electrodes in batteries are overengineered.
Battery manufacturers make battery electrode foils as thin as possible but have hit a limit
to how thin those foils can be because the foils start developing pinholes and tears.
This results in electrode foils that are about 10x thicker than they need to be.
However, the effect of removing that bottleneck should still have a significant positive effect.
Again, without thermodynamic modelling and more details on the cells, which is trade
secret, I can’t estimate further.
In my view, the tabless electrode appears to primarily be a manufacturing improvement.
With a tabless electrode, the tab no longer needs to be welded to the current collector.
This is difficult and error prone because the foils are as thin as the aluminum foil
you use in your kitchen.
By removing the tab, the jellyroll can be inserted directly into the cell to make contact
with the end caps, with no need for welding.
Welding is about 5% of the cost and space of a battery manufacturing line.
Why has no one else done a tabless electrode before?
In my opinion, the reason why a tabless electrode hasn’t been done before is for three reasons.
• First, these cylindrical cells were originally designed to be standalone components with
all the failsafe mechanisms built into the cell.
This is opposed to an automotive battery pack, where the failsafe mechanisms can be external
and there are thousands of cells in a pack.
• Second, this wouldn’t have been an easy engineering problem to solve.
Creating a solid electrical connection 100% of the time without a weld was likely difficult.
• Finally, the price of a vehicle containing thousands of cells is more cost sensitive
than something like a laptop or cell phone.
In other words, there was no technical need, no economic forcing function, and it was probably
a challenging problem to solve.
Finally, given that most companies are run by accountants rather than engineers, it makes
sense that Tesla was the first to tackle this problem.
What’s the likelihood that this is the alleged cell that Tesla will unveil next week?
It’s not 100%, but very likely.
Let’s look at the evidence: 1) Fred at Electrek confirmed through more
than one source that this cell is the real deal
2) Tesmanian reported on a rumoured 40mm cell earlier this year.
3) Sandy Munro said a few months ago that there was a rumour of a 50mm plus diameter
cell 4) Everything we discussed above indicates
that a larger cell is highly desirable, and that Tesla has the stack of technology to
make it happen.
5) This is something I need to research further, but large cells already exist and from what
I can see here, they appear to be used for LFP, which is a chemistry with high power
density and high heat tolerance.
Elon hasn’t denied the rumour, but I don’t think that tells us anything.
Sometimes he addresses leaks, and sometimes he doesn’t.
Let’s cover a few remaining speculation points.
The first is that Tesla could fit cells between cells.
Thanks to Andrew Wang for sending me this research paper.
Putting cells between cells would result in only minimal gains to energy density in comparison
to the effort required.
It would defeat the purpose of moving to a larger cell in the first place, which is to
reduce the number of parts.
Now You Know made comparisons between the 2170 and 18650 cells in terms of energy density
and power density.
I didn’t rely on the publicly available information about these cells because the
2170 and 18650 cells were different generations, they came from two different production lines
in two different countries, and may have had different performance goals for different
vehicles.
Theoretically, going from 18650 to 2170 should have increased energy density by around 2%.
So why does the publicly available information show lower energy density?
These cells may have been engineered for power density instead of energy density by making
the cathode thinner, for example, which may contribute to the higher power densities exhibited
by the 2170 cells.
With that said, Now You Know does make an important point about size and power density.
Larger cells do tend to exhibit lower resistance, which should translate into higher power density.
However, there are two counter points to this.
There are diminishing returns beyond the 2170 cell size and larger cells have more difficulty
dissipating heat due to the larger volume, which brings us back to the triangle problem.
A larger cell does have the potential for higher power density, but for two cells with
comparable engineering, the larger cell will hit its heat budget first and start developing
the lag we talked about earlier.
In summary, Tesla’s goal is to accelerate the world’s transition to sustainable energy.
To do this, they need to accelerate manufacturing.
A larger battery cell accelerates the manufacturing process and reduces costs.
A larger battery cell competes with charge rate and energy density for the heat budget
of the battery pack.
The only way around this is to remove heat more quickly or reduce the heat being generated
in the first place, because it eliminates the problem before it’s created.
Maxwell Technology Dry Battery Electrode technology is that solution.
It reduces the amount of heat generated.
A new thermal management system like plate cooling could further allow a larger cell.
Finally, the tabless electrode is the icing on the cake, both reducing the amount of heat
generated by a small amount and allowing plate cooling to work more effectively.
That’s it for today and I’m looking forward to seeing you on Battery Day with the Livestream!
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A special thanks to Stefan Cserepy, Stuart Steele, Steven Beyer, MJ Aminian, MumblingE,
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Freddie Lewis David Quan who’s also known as a random shaved head Asian dude who has
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