4680 Structural Battery Pack explained - How does it work?

[cvalue13]

This leads me to another question though, and that is if the pack is attached to the rear bulkhead,

by chance, just today, there’s a flyover of an upside down BIB

Also coincidentally, this upside down shot of the rear raises a question I was gonna ask earlier but didn’t want to muddle the convo - but here it is again:

what’s the deal with that bracket?

and relatedly, it looks like there’s a space between the top of the pack and the floorboard about the depth of that bracket?

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[JBee]

Bummeroonie...just refreshed the page and lost a load of text...

I found a video that shows some more CT underbody:



These are grabbed from that; so briefly and to the point:

You can see the pack front corner lines up with the rear of the front wheel arch.

You can also see the line ending in front of the rear arches.

I have done some math on our spreadsheet to work out how the dimensions interact.

If we take the same 6 cell wide per module and 4 module setup like in the MY but for a 15kWh pack it looks like this:

Pack Configuration calc			mm	inch
# Modules across vehicle width			4
Rows of Cells across module	#		6
Cell Gap	mm		1.0	0.04
Cell Offset	mm		4.0	0.2
Cooling	mm		10	0.4
Module Cell row width	mm	​	288	11.3
Module Separator	mm		30	1.2
Module Setback to edge of pack	mm		150	5.9
Pack width	mm		1542	60.7
CT Cabin Skirt	mm		70	2.8
Total Vehicle	mm		1682	66.2
Cells in column	#		61	2.4
Module Length	mm		2806	110.5
Total Cells Module	#		366
Total Cells Pack (comp. F52)	#	​	1464

Going by this calculation and keeping the same pack width as the MY we would need a 110" long pack, which won't fit.

That means if there is only 4 modules, then each must be wider than 62" so it doesn't loose so much area around the perimeter setbacks.

Probably something more like this going 7 cell wide per module (which is inconvenient for the cooling):

Pack Configuration calc (work in progress)		mm	inch
# Modules across vehicle width			4
Rows of Cells across module	#		7
Cell Gap	mm		1.0	0.04
Cell Offset	mm		4.0	0.2
Cooling	mm		10	0.4
Module Cell row width	mm	322​	336	13.2
Module Separator	mm		30	1.2
Module Setback to edge of pack	mm		150	5.9
Pack width	mm		1734	68.3
CT Cabin Skirt	mm		70	2.8
Total Vehicle	mm		1874	73.8
Cells in column	#		52	2.0
Module Length	mm		2392	94.2
Total Cells Module	#		364
Total Cells Pack (comp. F52)	#	1472​	1456

Or possibly even this 8 cell wide, that also lines up with the electrical configuration per pack as well to achieve the pack kWh and module voltage required for a split 800/400V pack. This reduces the setbacks though.

Pack Configuration calc (work in progress)		mm	inch
# Modules across vehicle width			4
Rows of Cells across module	#		8
Cell Gap	mm		1.0	0.04
Cell Offset	mm		4.0	0.2
Cooling	mm		10	0.4
Module Cell row width	mm	368​	384	15.1
Module Separator	mm		30	1.2
Module Setback to edge of pack	mm		110	4.3
Pack width	mm		1846	72.7
CT Cabin Skirt	mm		55	2.2
Total Vehicle	mm		1956	77.0
Cells in column	#		46	1.8
Module Length	mm		2116	83.3
Total Cells Module	#		368
Total Cells Pack (comp. F52)	#	1472​	1472

Overall, I think the pack "must" be wider, but I haven't really dialed in the pack constraints, in particular the electrical configuration of the cells to meet both the capacity and the voltage, as these determine the cell count per module as well.

 

[WHIZZARD OF OZ]

I thought I'd make a dedicated thread for this discussion to share thoughts on how the structural pack design works, in response to discussions on various other threads.

Model Y 4680 Structural pack as reference

These are some pictures I grabbed from Monroes teardown the 4680 to get an idea of the of the side buffer setbacks and dimensions on the MY. I think on the CT we should be able to use the same factors, to at least get to the ball park of the max space available for 4680 under the CT. These side buffers are for side impact protection of the battery packs, and they also offer a progressively rigid side impact protection.

Looks like about 175mm (7") between the cell and the edge of the pack. Which is fairly substantial. It looks like the pack screw holes are inside of the red cabin frame, and it slots up onto the grey cabin frame and cast. The red frame is about another 40mm, so about 215mm (8.5") from the outside wall to the first cell.

CT Pack cell Area

Using those setbacks I end up with a pink box that looks like the below, and inside that a "naked" 120kWh pack on a single layer that is this big:

It's pretty clear that a single cell high 120kWh pack does not reach the size capacity limit on the CT floor (which is the pink box around the cells), given that we now know that the battery can also extend under the vault cover in the rear (which is actually a bit further back than the pink box), where the "hump" known as the penthouse also is housed. We also know from the x-ray pictures, that the original battery "could" even extend under the front and in between the front wheel arches as shown by the pink box to the left.

Just for comparison this is what a single layer 185kWh looks like still staying within the same setback bounds as a MY design:

Now this is "just" naked cell density spacing without any structural or cooling components going through the pack. This could take up an extra 20-30% of the surface area depending on configuration. Especially if there is snake cooling between the cells that also has lines that have to be routed to the front cooling system

Edit: But if that cell cooling is no longer between cells anymore, rather just from the cathode side on the top of the pack. it would mean that it wouldn't add to the cell foot print area (like on 18650/2170). This is proposed here

Because the cells are stacked and attached, by the both top and bottom of the cell, by the structural layers, it would also form more of a structural webbing between the top/bottom layers. It's important to point out that both the cell wall of the cell (which is also a bit thicker than a 2170) and the pink foam that binds the cells together transfer load. The foam also helps maintain the cell wall geometry and stops it from buckling, it's also likely a fire retardant, to reduce fire propagation, and also adds some thermal capability as well.

The MY used 4 packs, which means three internal ribs separating the packs:

Each separating rib being about 40mm wide (1.5")

So just using the above, it's likely that they could get around 165-175kWh in one single layer, and still stay within the constraints of the pink box. BTW 165kWh at 400Wh/mile is well... around 420miles.

Now if we add 20% cell improvement over time we get 500mile range or a 200kWh pack.

But we do so within the same confines of the 165kWh, which fits within the 185kWh footprint shown above, and all without needing a dual layer pack at all.

This is more important than it might first seem, in that a structural pack is unlikely to ever be dual layer, because both the top and bottom steel "skin" layer of the pack form the structural chord and the battery cells themselves the "core" webbing. Now because the webbing is attached to the two steel layers, they can transfer load from front to rear. This is because like a truss, one layer skin goes into compression, while the opposite skin goes under tension, while the core keeps each skin geometrically separated. The core is compressing the cells, every time the top and bottom skins are subjected to forces.

Note this diagram will change depending on where the forces are applied, but the principal of the sandwich structure is the same.

With TWO layers of batteries however, there would have to be a third layer in between them, which in turn means that the lower layer battery will carry the top layer battery, but unless it is pre-tensioned, the top layer won't add much structure, and bottom will carry both as the two panels of cells "slide" over each other.

That's why you don't see many "truss" designs that use two trusses stacked on top of eachother.

This is because with core compression the vertical forces need to make it through both the vertical and diagonal webbing from the very top skin to the very bottom skin (chord). If they don't the top horizontal skin chord can't go into compression and the bottom skin chord can't go into tension (as per above truss loading), which is how load is transferred from one side of the truss to the other to the anchor points (red triangles).

There's also a bunch of cooling, cell terminations, and heat that needs to be considered when double stacking, and in particular assembly times etc. There's also height limits in the rear seat to consider, and the fact that the "cabin frame" would need would need to structurally connect to both the single and double layer pack, using the same cabin frame design. The difference in the skin heights means loads will be significantly different between a double and single layer pack.

Making two cabin frames, and possibly even two casts sounds expensive, just to have a longer range pack, which given the cell area above seems unnecessary anyway. From a battery perspective I seriously doubt we will need, or ever see a double layer "structural" pack. And the CT needs a structural pack for it's cabin design, otherwise there's no point making the pack structural in the first place, otherwise the pack will not have the desired effect of reducing the vehicle mass by displacing other load bearing structures in the vehicle.

You could do some "backpack" style under seat battery storage, but I don't think it is necessary given the space in the 1st layer, and adding cooling and connections to the backpacks is also non-trivial for the extra capacity they provide. Let alone they detract from interior space.

For example in the MY the open space under the front seats is quite generous and allows one to put feet underneath. In the CT the rear seats fold up (like the F150) to make a flat floor loading space as we have seen from the pictures. This is because that penthouse is behind the rear bulkhead under the bed, and not under the rear seat like in the rest of the SEXY range.

Hope this helps visualise what is going on.

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[JBee]

by chance, just today, there’s a flyover of an upside down BIB

Also coincidentally, this upside down shot of the rear raises a question I was gonna ask earlier but didn’t want to muddle the convo - but here it is again:

what’s the deal with that bracket?

and relatedly, it looks like there’s a space between the top of the pack and the floorboard about the depth of that bracket?

Well that's very fortunate!

I'm not quite sure, but it does look structural, and looks like it will be attached to the pack to the bottom.

It might be the frame that attaches to and holds up the rear seats and seat belt mounting points? Looks like there are a few pins that go through the top of it that might be the rear bottom seat cushion axis upon which it rotates upwards?

1) I think the red dotted line is the hard structural limits of the pack.
2) The pink dotted line the limits of the rear penthouse.
3) The yellow line the height of the structural pack throughout.
4) And the blue lines the point the rear bulkhead touches the pack. It doesn't look like there is much there to bolt it to the pack, so might only be glued, if that.
5) the orange seat mounting points on that bracket

I also like finally the seeing the webbing of the casts that follow all the load paths!
So pretty!

I also don't see much in the way of attachment points to connect them exo-skins either!!

 

[WHIZZARD OF OZ]

Well that's very fortunate!

I'm not quite sure, but it does look structural, and looks like it will be attached to the pack to the bottom.

It might be the frame that attaches to and holds up the rear seats and seat belt mounting points? Looks like there are a few pins that go through the top of it that might be the rear bottom seat cushion axis upon which it rotates upwards?

1) I think the red dotted line is the hard structural limits of the pack.
2) The pink dotted line the limits of the rear penthouse.
3) The yellow line the height of the structural pack throughout.
4) And the blue lines the point the rear bulkhead touches the pack. It doesn't look like there is much there to bolt it to the pack, so might only be glued, if that.
5) the orange seat mounting points on that bracket

I also like finally the seeing the webbing of the casts that follow all the load paths!
So pretty!

I also don't see much in the way of attachment points to connect them exo-skins either!!

Agreed.....
'Pretty In Pink' or any colour on the 'WEB'
Wait till Munro rip this Mothership apart!
Your conclusions are gonna be a 'Match Made In CyberHeaven'(!) 99.99999% ✓

 

[cvalue13]

1) I think the red dotted line is the hard structural limits of the pack.
2) The pink dotted line the limits of the rear penthouse.
3) The yellow line the height of the structural pack throughout.

agreed except for maybe the red line at back. instead, I think the rearmost edge of the pack maybe aligns with the ~rearmost pink line of the penthouse. though maybe I’m misreading your red line’s location because in 3D space, rather than reaching back too far, it’s just lofted.

Use the upside down features relative to this:

Note how in the rear that bulbous bit relates to the door rear pillar and the wheel arch

now overlayed on the body, and the known pack limit at the rear fender flare’s front edge meeting the SS

appears to me the pack itself ends just forward of your rearmost red dotted line, instead ~where your rearmost pink line is

in any event, just when I thought my screen measuring days were over, seems what we really need to know the width of the physical pack is a measurement of the width of your red dotted lines between cab rails

none too easy to get a perspective-adjusted/controlled photograph for measuring that. Only way to get a grip on it would be to have an directly overhead photo of the built truck, and know what points on the top of the truck are directly overhead of those lower cab rails

 

[JBee]

agreed except for maybe the red line at back. instead, I think the rearmost edge of the pack maybe aligns with the ~rearmost pink line of the penthouse. though maybe I’m misreading your red line’s location because in 3D space, rather than reaching back too far, it’s just lofted.

Use the upside down features relative to this:

Note how in the rear that bulbous bit relates to the door rear pillar and the wheel arch

now overlayed on the body, and the known pack limit at the rear fender flare’s front edge meeting the SS

appears to me the pack itself ends just forward of your rearmost red dotted line, instead ~where your rearmost pink line is

in any event, just when I thought my screen measuring days were over, seems what we really need to know the width of the physical pack is a measurement of the width of your red dotted lines between cab rails

none too easy to get a perspective-adjusted/controlled photograph for measuring that. Only way to get a grip on it would be to have an directly overhead photo of the built truck, and know what points on the top of the truck are directly overhead of those lower cab rails

I see the rear red line on the same plane as the pink lines, but further back. The rear red line is lower than the red lines along the side.

Those oval aluminum cast pads in the corners of those red line are the underside of the connection points shown by the yellow circles here:

I don't think those oval areas or the rear red line is further back than you have shown on the side view. So your side image for the battery extents matches from what I can tell.

The width could also be determined from a front or rear cross section view, where we can see the bottom. Or somebody handy with a measuring tape that stops sees a CT on the side of the road? I think there is some expo in Austin today that has the CT there.

 

[PilotPete]

none too easy to get a perspective-adjusted/controlled photograph for measuring that. Only way to get a grip on it would be to have an directly overhead photo of the built truck, and know what points on the top of the truck are directly overhead of those lower cab rails

Oh Mr @greggertruck!!! We have a work assignment for you!!! Pretty please?

 

[greggertruck]

Oh Mr @greggertruck!!! We have a work assignment for you!!! Pretty please?

That’s a tough one
Quickest easiest grab I got. It’s actually my Lock Screen rn

 

[RVAC]

@JBee using @cvalue13's estimated pack measurements of 92x62 in, what capacity would that yield?

 

[cvalue13]

Oh Mr @greggertruck!!! We have a work assignment for you!!! Pretty please?

That’s a tough one
Quickest easiest grab I got. It’s actually my Lock Screen rn

this is the challenge of it - from overhead it’s difficult to impossible to know where, “underneath,” are the lower cab rails

but we can work around this with a bit of creative data sourcing

 

[greggertruck]

this is the challenge of it - from overhead it’s difficult to impossible to know where, “underneath,” are the lower cab rails

but we can work around this with a bit of creative data sourcing

What do you mean by cab rails? The points of connection for the battery pack to the actual chassis?

 

[cvalue13]

What do you mean by cab rails? The points of connection for the battery pack to the actual chassis?

Here’s the upside-down casting, and the rails

The problem with measuring these from an overhead of the truck, is that the truck is bulbous in the middle beltline, so from above you can’t see underbelly width of the truck

 

[JBee]

That’s a tough one
Quickest easiest grab I got. It’s actually my Lock Screen rn

That's actually a pretty useful photo.

Now if I had a front, side and rear orthagonal shot like that, I think I could stretch it into a 3D shape in CAD, which we could take measurements from. In fact even just a good full frontal or rear end shot would be useful.

If we had like a orbit video or lots of pictures it would add fidelity, but those orthagonal shots make life easy. I was meaning to do that with all the photos we got from the original prototype in the Peterson, but there's a few days worth of work to do it, so it would be better to put the effort into the production model.

I suppose we have the crash test top and side ones as well. Just need a front and rear one, seeing an underneath one is unlikely. Although the car transport ones give you a glimpse.

 

[JBee]

@JBee using @cvalue13's estimated pack measurements of 92x62 in, what capacity would that yield?

There's 3 tables in a post of mine further up that give you the pack dimensions etc.

We're thinking it's around the 115-130kWh range, but it needs to run pretty efficient to get over 300miles from it.

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