Pack level specific energy density vs cell level

tidmutt

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Hi All,

Something I have been wondering for a while and I'm hoping someone here can help me understand relates to pack level energy density versus cell level.

I keep seeing this interesting numbers for the growth in cell level specific energy for the cells Tesla use. Numbers for cell level energy density in the Model S are close 270 Wh/kg. This has grown significantly and there is discussion about the 4680 cells being 300 or more.

However, when you look at pack level energy density it seems to have hardly moved since the original Model S. Some sources say the model 3 improved between 2019 and now somewhat, growing from 150 Wh/kg (at the pack level) to 166 Wh/kg. However, this isn't really much of an improvement over the Model S. I haven't found good numbers for pack level 4680 energy density but some sources say it won't be much different.

I believe I understand why pack level specific energy is lower than cell level. When you combine the batteries into a pack it takes structure and wiring and insulation and other things which means the overall energy stored in the mass of the pack is lower. Plus perhaps loses to heat when energy is conducted out of the cells and out of the pack etc. But why has the pack level energy density stayed relatively flat where as cell level as improved so much?

Thanks in advance!
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Crissa

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Any improvement is an improvement! Even from 150 to 166 is a 6% increase.

Pack level density with the smaller 18650 cells has a bit of a ceiling: The pack itself needs to waste alot of space on structure. Thermal management and wiring also takes up space, too.

That's one reason why the big cans of the 4680s are so important: They allow a different arrangement of the cells, which can be incorporated into the structure of the pack. 'Tabless' can also be cooled from the ends, which increases the efficiency of cooling.

The wiring upgrades probably made their way into the Plaid S (refresh S) pack. Using a sheet of laser-etched copper, the traces of the all the wires and sensors in the pack would be inserted and welded in a single step.

It's all about arranging the parts differently to increase pack density. A game of packing the suitcase.

-Crissa

Here's some resources that are easy to approach. I really recommend Jordan Giesige's series, it's filled with friendly animations about how modern lithium battery cells work, and the variations being tried.

 

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There are 3 places where batteries add weight.
  • Basic Pounds / kWh of the cell.
  • Pack level density (adds weight of the pack).
  • Additional structural required to support the batteries.
Tesla is attacking all three of these components with the 4680 cells.

I think they are also reducing the weight of the cooling system required by the battery by cooling it from the bottom which should require fewer channels and less fluid.
 
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tidmutt

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Any improvement is an improvement! Even from 150 to 166 is a 6% increase.

Pack level density with the smaller 18650 cells has a bit of a ceiling: The pack itself needs to waste alot of space on structure. Thermal management and wiring also takes up space, too.

That's one reason why the big cans of the 4680s are so important: They allow a different arrangement of the cells, which can be incorporated into the structure of the pack. 'Tabless' can also be cooled from the ends, which increases the efficiency of cooling.

The wiring upgrades probably made their way into the Plaid S (refresh S) pack. Using a sheet of laser-etched copper, the traces of the all the wires and sensors in the pack would be inserted and welded in a single step.

It's all about arranging the parts differently to increase pack density. A game of packing the suitcase.

-Crissa

Here's some resources that are easy to approach. I really recommend Jordan Giesige's series, it's filled with friendly animations about how modern lithium battery cells work, and the variations being tried.

Thanks!

I'm familiar with both those channels... The Limiting Factor is incredible, but one has two kids and an ex-wife and a partner and can't watch all 18 parts of the battery series!

I'm still not sure why pack specific energy has not increase at least somewhat inline with cell level?

Edit: Yes, I'm too lazy to watch them so I'm asking here. LOL
 
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tidmutt

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There are 3 places where batteries add weight.
  • Pounds / kWh.
  • Pack level density (adds weight of the pack).
  • Additional structural required to support the batteries.
Tesla is attacking all three of these components with the 4680 cells.

I think they are also reducing the weight of the cooling system required by the battery by cooling it from the bottom which should require fewer channels and less fluid.
So they claimed a 16% increase in energy density from the 4680... but that is at the cell level. I've seen claims the pack level won't improve that much.... that just doesn't make sense to me. How could they reduce weight while at least retaining if not increasing cell level density and not get a large jump in pack level energy density?

I feel like I'm missing some critical piece of the puzzle. Perhaps it's the can I'm missing my my 6 pack. :)
 


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So they claimed a 16% increase in energy density from the 4680... but that is at the cell level. I've seen claims the pack level won't improve that much.... that just doesn't make sense to me. How could they reduce weight while at least retaining if not increasing cell level density and not get a large jump in pack level energy density?

I feel like I'm missing some critical piece of the puzzle. Perhaps it's the can I'm missing my my 6 pack. :)
On the F150, you have a pack with the batteries slung inside the very heavy ladder frame of the truck.

The Rivian I believe is a unibody truck and has to reinforce the unibody frame to support the battery pack.

The 4680 pack supports the Cybertruck.

Even if the 4680 pack weighs as much as the Rivian or Ford battery packs, it is also eliminating the need for hundreds of pounds of frame/ unibody supports.

** Obviously pure theory here **

Until we get the beta trucks and actual shipping curb weight on the truck it's all speculation.
 
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tidmutt

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On the F150, you have a pack with the batteries slung inside the very heavy ladder frame of the truck.

The Rivian I believe is a unibody truck and has to reinforce the unibody frame to support the battery pack.

The 4680 pack supports the Cybertruck.

Even if the 4680 pack weighs as much as the Rivian or Ford battery packs, it is also eliminating the need for hundreds of pounds of frame/ unibody supports.

** Obviously pure theory here **

Until we get the beta trucks and actual shipping curb weight on the truck it's all speculation.
Right, so the truck is more energy dense but the battery pack is the same. I get that.

Still not sure why cell level energy density increases don't equate to pack level energy density increases. :)

I'm going to watch the Sandy video and see if he can get through my lead-lined skull.
 

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Thanks!

I'm familiar with both those channels... The Limiting Factor is incredible, but one has two kids and an ex-wife and a partner and can't watch all 18 parts of the battery series!

I'm still not sure why pack specific energy has not increase at least somewhat inline with cell level?
Maybe this is a foul, but I will ask a question to answer your question. The energy in the pack must be managed to be useful, so wouldn't this dictate the size of the battery pack? Years back, Tesla had to invent a new way to handle the power of the 100KW battery pack because until then the power of the battery pack destroyed the power management system. It looks to me that Tesla isn't making massive battery packs because they do not need them to be massive to dominate the market. This and the added weight works against the daily efficiency... my Ford truck is always 4800 pounds whereas my more fuel efficient Dodge pickup is at most 3000 pounds.
 

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I'm still not sure why pack specific energy has not increase at least somewhat inline with cell level?
Well, cell advances are ahead of production by a few years.

Also, some of those advances in power density are used up in gaining reliability, safety, or charging speed. Basically, the less of the density you use on a day to day basis, the more of the others you get (hence being told to charge only to 80%).

-Crissa
 

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<Snip>

I'm still not sure why pack specific energy has not increase at least somewhat inline with cell level?
Its “weight” you might be missing at the pack level.

My takeaway was the 4680 14% improvement derived from cell design enabled increase in RANGE and provided density increase at the cellular level.
Tesla Cybertruck Pack level specific energy density vs cell level D7668AF1-2235-4ED1-8705-F0354F7309F9

SO there’s the big innovation piece.
Tesla Cybertruck Pack level specific energy density vs cell level 175B90C8-68E6-439D-AFE6-BBE852DF2FC2

The next part is the trick!
Tesla Cybertruck Pack level specific energy density vs cell level 073492CB-71F9-421C-9B1C-8D500B224102

Tesla gains Range from 4680 cell design, ton of improvement in Anode with silly 69% cost savings 4680 enables by eliminating ~82% material, handling and processing. Now forget that and focus on Pack level.

See what they did there? Same energy density at 4680cell level(+ 14% Range) for the same amount of cell weight. Trick-y.

Then Structural Engineering enjoys designing the new pack(read lighter weight) but Tesla holds that tidbit as trade secret. Instead showing the overall impact on the vehicle(200kg savings). Weight in car lingo translated 1sec/mi quicker velocity per 100 lbs. weight saved. Each 4680 equipped Tesla gains 4.40 sec/mi velocity acceleration FOR THE SAME WEIGHT BATTERY.

The through line trick to understanding is that cell-to-structure has the same impact as Negative Mass. BUT the trick isn’t completed yet for the Mathematics.

SteveJobs famously said technology should not be seen but work like plumbing hidden so that water just comes out of a wall. Pack weight measured indexing negative weight impact yields the effective improvement in PACK DENSITY == 271 Whr/kg. A 112 Whr/kg improvement at the vehicle level.

Trick! Not magic. Technology(nod to SteveJobs)
 


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tidmutt

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Well, cell advances are ahead of production by a few years.

Also, some of those advances in power density are used up in gaining reliability, safety, or charging speed. Basically, the less of the density you use on a day to day basis, the more of the others you get (hence being told to charge only to 80%).

-Crissa
Ah.... okay, now we're cooking with gas. :)

So I get the cells being ahead of production... but then again, 2710s are roughly 267 Wh/kg at the cell level for reals out in the wild... but drop to 166 at the pack level... wow... So you would think the 4680s would get a pretty big energy density improvement even just theoretically at the pack level. Given the larger form factor means less metal casing for given volume, better packing efficiency, less wiring due to tabless electrodes. I'm ignoring the structural part because I think we should consider that vehicle weight savings and not part of the battery pack mass. Unless, as Crissa mentioned, they gave us faster charging, cooling etc. at the expense of energy density.

Good video, points for it being an Aussie but most of the discussion is at the cell level.



Maybe the fact that the pack is structural means the battery casings are beefier? An overall mass reduction for the vehicle, but an increase in mass for the pack maybe?

From what I can conclude the 4680s are more about battery life due to better thermals, light weight due to packing efficiency etc. and cost. So the CT and Model Y V2 etc. will be cheaper to produce for a given range and lighter due to overall mass reduction in the vehicle but the pack itself is not some big leap in energy density... Does that sound right?

The disconnect for me in all of this is Elon and others keep talking about increases in energy density are starting to make electric planes viable.... but if the pack level energy density is not really improving, then how is that true? Is Elon blowing smoke?
 
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tidmutt

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Its “weight” you might be missing at the pack level.

My takeaway was the 4680 14% improvement derived from cell design enabled increase in RANGE and provided density increase at the cellular level.
D7668AF1-2235-4ED1-8705-F0354F7309F9.png

SO there’s the big innovation piece.
175B90C8-68E6-439D-AFE6-BBE852DF2FC2.png

The next part is the trick!
073492CB-71F9-421C-9B1C-8D500B224102.png

Tesla gains Range from 4680 cell design, ton of improvement in Anode with silly 69% cost savings 4680 enables by eliminating ~82% material, handling and processing. Now forget that and focus on Pack level.

See what they did there? Same energy density at 4680cell level(+ 14% Range) for the same amount of cell weight. Trick-y.

Then Structural Engineering enjoys designing the new pack(read lighter weight) but Tesla holds that tidbit as trade secret. Instead showing the overall impact on the vehicle(200kg savings). Weight in car lingo translated 1sec/mi quicker velocity per 100 lbs. weight saved. Each 4680 equipped Tesla gains 4.40 sec/mi velocity acceleration FOR THE SAME WEIGHT BATTERY.

The through line trick to understanding is that cell-to-structure has the same impact as Negative Mass. BUT the trick isn’t completed yet for the Mathematics.

SteveJobs famously said technology should not be seen but work like plumbing hidden so that water just comes out of a wall. Pack weight measured indexing negative weight impact yields the effective improvement in PACK DENSITY == 271 Whr/kg. A 112 Whr/kg improvement at the vehicle level.

Trick! Not magic. Technology(nod to SteveJobs)
This is so excellent, thank you. That last slide is a good one. I saw that video but I didn't parse that slide sufficiently as it has the answers, at least some of them.

So would it be fair to say that we're kind of blurring the line here about pack level energy density. You could say the pack level energy density is essentially the same as a traditional pack, but because it's a structural pack it has the almost magical effect of negative mass as you said and hence the pack is "technically" a much higher energy density, 271 Wh/kg as mentioned in the slide?

If so, this is touching on what the other replies mentioned... you could kind of call it vehicle energy density. Not sure if that's a thing or not.
 

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Well, the 4680 is more efficient in some ways, but this is why you're not seeing large pack-level advances yet. The 4680 isn't in production packs.

-Crissa
 
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tidmutt

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Well, the 4680 is more efficient in some ways, but this is why you're not seeing large pack-level advances yet. The 4680 isn't in production packs.

-Crissa
yep I know, but the predicted pack level efficiency from various sources is essentially the same.
 
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tidmutt

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https://www.cybertruckownersclub.co...it’s-not-cell-to-pack-it’s-cell-to-body.1788/

So I guess this was somewhat addressed in the above thread. Should have searched. :p

The other part of my question remains... how does any of this make electric planes more of a reality? When people say planes require about 400-450 Wh/kg to be viable is that at the cell level or at the pack level? It seems to me the negative mass trick won't work with planes... if the cells for a plane need a pack then their energy density drops to 166 Wh/kg... or there abouts. That's a LONG way from 400-450.
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