Welcome back everyone!
I’m Jordan Giesige and this is The Limiting Factor.
This is part fifteen of the Lithium Mine to Battery Line Series to break down and understand
what was unveiled at Tesla Battery Day.
Today we’re going to talk about what I consider is the most surprising technology unveiled
at battery day, the structural battery pack.
This video will cover some of the key points of the battery pack and how it works and the
next video 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.
Before we look at how the structural battery pack works, we have to understand how battery
packs currently work.
In order to do that, we need a basic understanding of how the battery pack integrates with the
vehicle structure.
Bear in mind, I’m not a vehicle structural engineer and it’s not my area of expertise.
This is my understanding based off work done by Sandy Munro and some of my own research
on vehicle structure.
In a conventional ICE vehicle, there are rockers that run along the sides of the vehicle from
the front wheel wells to the rear wheel wells.
Along with the roof, the rockers connect the front and rear sections of the vehicle and
provide rigidity along the length of the vehicle.
However, without some additional structure, the vehicle frame would be flimsy and twist
easily.
Resistance to the twisting is called torsional rigidity.
The way ICE vehicles deal with torsional rigidity is by inserting a structural floor pan.
The stronger the structural floor pan, the greater the torsional rigidity, and the greater
the torsional rigidity the more responsive the vehicle is to driver input.
A vehicle with low torsional rigidity feels more like driving a boat and a vehicle with
high torsional rigidity feels more like a go-cart.
There are many other factors involved in vehicle structure and suspension, and how they affect
the driving experience, but torsional rigidity is one of the major foundations.
Let’s move from a conventional ice vehicle to the image Tesla showed at Battery Day.
Currently, Tesla’s battery packs carry the cells as cargo.
There are a series of cross members and a surround, which are both steel and marked
in red.
The cross members run longitudinally from the front to the rear of the pack and the
surround wraps around the pack.
Basically, it’s a steel box that holds battery cells and the cells themselves are under no
stress.
That steel box replaces the structural floor pan to provide torsional rigidity.
Furthermore, if we look at the cross section, we can see that there’s redundancy.
Not only do Tesla’s current vehicles have a rocker, they have a steel surround at the
perimeter of the battery pack and the longitudinal cross members.
The rocker, the cross members, and the steel surround are doing the same thing.
They’re connecting the front of the vehicle to the rear of the vehicle and provide rigidity
along the length of the vehicle.
This makes the rocker redundant and it’s why the rocker is marked as red in the top
image.
Let’s take a look at the structural Battery Pack.
In the bottom image, there’s no steel surround.
This means the rocker is no longer redundant and serves a purpose as the primary link between
the front and rear of the vehicle as well as provides rigidity along the length of the
vehicle.
This is why the rocker’s no longer marked in red.
In the conventional battery pack, the steel box that formed the pack carried the cells
and provided torsional rigidity.
How is torsional rigidity being provided now, and how are the cells carried?
To understand this, we need to understand the pack design of the structural battery
pack.
In the simplest terms the structural battery pack is where the battery cells are bonded
to each other and bonded to an upper and lower face sheet in what’s called a honeycomb
structure.
This provides enormous strength and torsional rigidity along every axis.
For example, as we can see in the Model 3 battery pack, there are a few cross members.
Between those cross members there’s no structure.
This means no matter how strong you make the cross members there is always some degree
of play in the areas that have no structure.
With a honeycomb and face sheets, you have a contiguous structure along the length and
width that evenly distributes stress across the entire length and width of the pack.
Furthermore, the two face sheets on the top and bottom combined with the cells bonded
in between are able to provide shear transfer between the top and bottom of the pack.
The simplest way I can think to explain shear force is that if the top face sheet is trying
to move in one direction and the bottom face sheet is trying to move another, the fact
that they’re bonded together with hundreds of cells prevents them from sliding.
In other words, shear transfer prevents twisting by locking the top and the bottom face sheets
together.
If there are engineers out there, let me know if there’s a better way to explain this
or if I’ve totally butchered it.
In other words, the cell itself becomes the load carrying structure along every axis,
with the face sheets distributing the forces amongst the cells.
Battery cells have extremely thin walls, so how do they provide so much structure.
Thin walled metal cylinders that are filled with liquids or solids can carry absolutely
enormous loads.
For example, a full, unopened soda can has a compressive strength of 653 kilograms.
If you want to know where I got that number, I’ve linked a video in the description.
This means one unopened can of soda under each wheel of a Tesla Model 3 could support
the weight of the vehicle.
The structural battery pack could have up to 960 cells and they’ll be bonded to the
face sheets which will distribute the load across all the cells as the vehicle flexes
during turns.
Where did I get 960 cells?
First, Sandy Munro’s calculation.
Second, this leak from Electrek.
If you count the holes in the honeycomb structure and assume a termination point slightly off
screen in the upper right, it appears to be exactly 40 cells by 24 cells, or exactly 960.
As Sandy notes on his whiteboard, 40% of the cells can be removed to achieve the range
of a Model Y.
I’d go one step further and say 50% of the cells could be removed because the structural
pack adds 14% to the range.
If 50% of the cells were removed, the empty spaces could be filled with epoxy or lightweight
dummy cells.
However, given the rigidity of the structural battery pack, these might not be necessary.
Either way, the centre of mass of the battery cells would be clustered more tightly around
the core of the vehicle and there would be no steel structural surround at the perimeter.
As Elon said at battery day, having the weight tightly held near the core of the vehicle
improves the polar moment of inertia.
Polar moment of inertia is determined by how tightly the weight of a vehicle is centred
around its core.
The closer to the core, the easier it is to spin that object.
In the case of a car, the spin we’re talking about is car turning around a corner.
The best way to illustrate polar moment of inertia is to imagine holding an object with
a fair amount of mass, like milk jugs, at arm’s length and twisting.
Then imagine holding the objects near your chest and twisting.
You should be able to imagine that you can twist much easier and more quickly holding
the objects near your chest.
The same thing happens when the battery cells are placed closer to the core of the vehicle.
The car becomes more nimble and feels effortless when stalking around tight corners.
What about the general structure of the battery pack?
My view, like Sandy Munro’s, is that the wiring will be at the top and the cooling
will be that the bottom.
Starting with the wiring, if all the positive and negative contacts are on top of the pack,
a series of conducting and insulating gasket layers could be used to expose conductive
pathways in the right sequence to get the desired voltage.
This could also be done with some type of circuit board.
Either way, my guess is that the wiring would be flat, compressible, and require minimal
tedious handling of fine and fragile wires.
This is because I expect the wiring in the structural battery pack to be more rugged,
better integrated into the structure of the pack, and easier to manufacture.
As a final note, I would also think that with the wires on top, they’re closer to the
battery management system and wiring of the rest of the vehicle, meaning less wiring.
As for the liquid cooling at the base, if the caste or stamped in place coolant lines
are punctured, the vehicle could continue to operate relatively normally.
More importantly, if this leak by Electrek is correct, the copper current collector is
clearly at the bottom of the cell.
Copper has superb thermal conductivity - significantly more than aluminum - so it makes sense for
the copper to be at the bottom against the cooling plate.
Thanks to the tabless electrode, the 4680 has a nice chunk of copper at the base that
creates a thermally conductive pathway through the core of the battery cell that acts as
a heat sync.
For those wondering, convection doesn’t play a role in cooling battery cells because
they’re solid objects.
Solid objects aren’t flowable, so they don’t allow for the convection currents that liquids
and gasses do.
This means it doesn’t matter if the cooling plate is on the top or on the bottom.
As a final note, and this is just a guess, water at the very bottom of the vehicle might
lower the centre of gravity which is good for handling.
Inside EVs suggested that the electrical connections were at the bottom of the pack and the cells
could be cooled from the top.
Inside EV’s had some great coverage of Battery Day, but I’d disagree here.
It doesn’t make sense to have liquid suspended above all your electronics, for obvious reasons.
Second, if the wiring is below the pack and the vehicle hits an object that slices through
the plate on the bottom of the vehicle, it would sever vital electrical connections.
One thing that I haven’t seen covered elsewhere and I’m hoping is a feature of the structural
battery pack is modularity and interchangeability.
The video here playing behind Drew and Elon shows the front and rear gigacastings being
mated to the structural battery pack.
My assumption would be that this full underbody assembly would bolted and/or welded to the
upper structure of the vehicle.
This is reinforced by the bolts we see in this image.
Although the placement of the bolts has changed, the rest of the structure is relatively similar.
If Tesla can design the structural pack so that it takes minimal retooling to switch
between a structural battery pack and a conventional battery pack, it would make the process of
upgrading from a conventional pack built around the 2170 to the structural pack a seamless
process.
I’m often asked about the recyclability of the structural battery pack.
The short answer is that I don’t know, but I’ll speculate.
Nothing indicates to me that a structural battery pack would be more difficult to tear
down than a typical Tesla battery pack.
Just because it has the word structural in it, doesn’t mean that it’s harder to disassemble,
it simply means that the internal engineering is done in a smarter way.
Instead of the cells being bonded to each other and encased in modules, they’re bonded
to each other and the face sheets.
In other words, both the old pack and the new pack both appear equally difficult to
recycle.
With the old pack, the cells need to be pried out with a hammer and chisel and the same
would be true for the new pack.
The stacking it up slide indicates that the structural battery pack saves 7% per kilowatt
hour at the pack level and provides a whopping 14% range.
The 14% range increase is achieved through a 10% weight decrease.
In order to achieve a 14% range increase, the weight decrease would have to be at the
vehicle level, not at the pack level.
A 10% weight decrease would be about 200 kilograms for a Model Y.
A Model Y battery pack probably weighs around 500 kilograms for a long range variant if
we assume a pack level energy density of 160wh/kg.
If we assume these rough estimates are relatively accurate, the effect of the new pack on the
vehicle weight would mean it would be the equivalent of a 300 kilogram battery pack.
The batteries themselves would weigh around 330 kilograms.
In other words, the structural battery pack makes 330 kilograms of battery cells feel
like 300 kilograms to the vehicle.
I think this is what Elon meant by negative mass.
Typically, a battery pack adds 50% additional weight beyond the weight of the battery cells.
By switching to the structural battery pack, it not only eliminates that 50%, it’s as
if the batteries weigh less at the pack level than at the cell level.
How can this be possible?
The batteries now serve a dual use in the vehicle.
Not only do they store energy, they provide structure.
This means the energy density at the pack level is effectively above the energy density
of the individual cells because dead weight has been eliminated in other parts of the
vehicle.
In summary, the structural battery pack turns everything we know about battery packs upside
down.
Instead of adding weight, the structural battery pack reduces the total weight of the vehicle.
It goes far beyond the cell-to-pack or advanced cell-to-pack that I was expecting at battery
day in terms of cost, complexity, and weight savings.
Beyond all that, it will make Tesla vehicles handle and ride even better.
I don’t hear much talk about this, but for me, this’ll be one of the biggest benefits
of the structural battery pack.
In the next video, I’ll take a stab at the design considerations for the 4680 Battery
Pack from the cell up, and how it might be assembled.
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