Extrapolation of Range and Battery Size(s)

[AWDtsla]

I didn't know about the 20% frontal area decrease compared to the Model S. Do you have a reference ?

20% reduction in size has been mentioned several times in interviews. Since the car is effectively a surface, being mostly empty space inside, then that means both 20% reduction in structural mass and 20% reduction in surface area, including the front.

 

[SageBrush]

20% reduction in size has been mentioned several times in interviews. Since the car is effectively a surface, being mostly empty space inside, then that means both 20% reduction in structural mass and 20% reduction in surface area, including the front.

hmmm .... not so sure about that. I'm more inclined to think that 'size' is a volume with a width, height and length -- sort of like a long box. I'll not be surprised to learn that compared to the MS, the M3 height is a bit less, and the length quite a bit less, while the width is not that much different. The frontal area is of course not affected by the change in length.

 

[Saghost]

hmmm .... not so sure about that. I'm more inclined to think that 'size' is a volume with a width, height and length -- sort of like a long box. I'll not be surprised to learn that compared to the MS, the M3 height is a bit less, and the length quite a bit less, while the width is not that much different. The frontal area is of course not affected by the change in length.

I'm expecting to see a decrease in all three directions, based on the pictures and video we have. The 20%/80% is more likely to be volume, though - which if it's balanced across the three would be 93% linear in each, or 86% cross sectional.

The two reference cars we've been told about are the BMW 3 series and Audi A4. The 3 sedan is 182 inches long, the A4 is 186. Compared to the S's 196, that's 93% and 95% respectively - which means if they really are reducing the *exterior* volume to 80% and modeling dimensions off of the above pair, the cross section has to fall to ~86% or a little lower.

 

[AWDtsla]

hmmm .... not so sure about that. I'm more inclined to think that 'size' is a volume with a width, height and length -- sort of like a long box. I'll not be surprised to learn that compared to the MS, the M3 height is a bit less, and the length quite a bit less, while the width is not that much different. The frontal area is of course not affected by the change in length.

It's definitely less wide, we have comments from the event.

 

[SageBrush]

I'm expecting to see a decrease in all three directions, based on the pictures and video we have. The 20%/80% is more likely to be volume, though - which if it's balanced across the three would be 93% linear in each, or 86% cross sectional.

The two reference cars we've been told about are the BMW 3 series and Audi A4. The 3 sedan is 182 inches long, the A4 is 186. Compared to the S's 196, that's 93% and 95% respectively - which means if they really are reducing the *exterior* volume to 80% and modeling dimensions off of the above pair, the cross section has to fall to ~86% or a little lower.

Sounds reasonable to me, so I'll avoid weighing down the thread with yet another graph and instead suggest eyeballing a virtual curve between the two plotted.

 

[Snowdog]

It also says "Hopefully .21". Also cars tend to be more consistent on width than length.

I expect it will still have 60KWh pack. Realize you use power just standing still on climate control, lighting, computers, sound, battery temperature maintenance etc...

So it isn't like you drop your drag by 20% and gain a full 20% in range.

 

[zer0cool]

I had thought maybe a 50kwh battery would achieve 215 miles. However given the battery size in the Chevy Bolt, I think it's harder to gain efficiency at the lower end, due to certain fixed energy usages and that the car's frontal surface area isn't really much less, etc. So I think it will be a 60 and a 80 battery.

 

[SageBrush]

It also says "Hopefully .21". Also cars tend to be more consistent on width than length.

I expect it will still have 60KWh pack. Realize you use power just standing still on climate control, lighting, computers, sound, battery temperature maintenance etc...

So it isn't like you drop your drag by 20% and gain a full 20% in range.

I'm playing the game of estimating energy consumption per mile and pack size based on EM's statement of 215 EPA miles.

 

[Snowdog]

I'm playing the game of estimating energy consumption per mile and pack size based on EM's statement of 215 EPA miles.

Yes, and I think you estimates are too optimistic.

Every percent of drag reduction isn't going directly convert into a similar reduction in pack size. You have all the residual, non motion drains. You also want a healthy reserve, probably even more healthy than before because you plan to ship a lot more of them, and a little more reserve is a little more protection against people complaining about capacity.

Even if you go with the most efficient EV on the market (Carbon Fiber i3) it would need about 58 KWh battery for 215 miles, and that doesn't count how much efficiency would go down given the increase in weight for the much larger pack.

So even 60 KWh might be too optimistic for 215 miles.

 

[Cebe]

i3 advertises 206.4Wh/mile (12.9kWh/100km) without the details on the speed, using 16.7kWh out of the 18.8kWh battery capacity to do 81 miles. Adjust that to 215 mile range, you get 44.3kWh. They should be able to do a 50kWh model and hit the 215 miles, but knowing what a disaster it would be dip under 200 miles, I'm guessing they're aiming for at least 55kWh if not 60 as others have suggested. Probably not worth the risk (to Tesla, dipping under 200 mile range) to save a few bucks on the base model...

 

[SageBrush]

Yes, and I think you estimates are too optimistic.

If it helps, here are the numbers used for my estimates...

1. Cd 0.21
2. Frontal area 90% of MS
3. 65 mph
4. 50% aero, 50% other losses at 65 mph

Then I (as my son loves to say) plugged the numbers into the drag equation for Wh/mile and then used

1. 215 mile range
2. 15% battery reserve

To calculate the battery pack size.

 

[Snowdog]

i3 advertises 206.4Wh/mile (12.9kWh/100km) without the details on the speed, using 16.7kWh out of the 18.8kWh battery capacity to do 81 miles. Adjust that to 215 mile range, you get 44.3kWh. They should be able to do a 50kWh model and hit the 215 miles, but knowing what a disaster it would be dip under 200 miles, I'm guessing they're aiming for at least 55kWh if not 60 as others have suggested. Probably not worth the risk (to Tesla, dipping under 200 mile range) to save a few bucks on the base model...

I thought we were talking about EPA range, not advertised numbers.

i3 has a 22 KWh pack and delivers 81 EPA miles of range. So it is pretty straight forward:

215miles/(81miles/22KWh)= 58.4 KWh

EPA test heavily impacted by vehicle weight and there is no way the Steel/Aluminum M3 is lighter than a Carbon Fibre i3.

 

[SageBrush]

i3 advertises 206.4Wh/mile (12.9kWh/100km) without the details on the speed, using 16.7kWh out of the 18.8kWh battery capacity to do 81 miles.

i3 has a much higher Cd, and I suspect a greater height as well.

Due to the low weight it gets great city energy economy, but much less impressive on the highway.

 

[Snowdog]

i3 has a much higher Cd, and I suspect a greater height as well.

Due to the low weight it gets great city energy economy, but much less impressive on the highway.

i3 is still about 10% better than a Model S on the EPA highway as well. EPA "highway" test has lots of stop and go. It is unrealistic to expect the Model 3 to come with less than 60 KWh battery and get an EPA range of 215 miles.

 

[omgwtfbyobbq]

Cd of 0.21 is a bit better than fantastic.
And all the more surprising given the air dam in front.

We can guess about 24 ft*ft frontal area, for a CdA of 5.04
Once we know car weight and tyre RR, we can get a good estimate of energy economy.

By the way, google says that a kWh of Li-x battery weighs about 6.7 Kg, so about 335 Kg for 50 kWh before packaging.

If my arithmetic is right, starting from

CdA - 5.04 ft*ft
Air density - 1.225 kg/meter_cubed

I get a power requirement of 7.3 kW to oppose the air at 65 mph.
If that is about half of total power, then ~ 15 kW to for flat road, steady state 65 mph travel, or 224 Wh/mile

Using that CdA, I'm getting ~11kW at 65mph assuming a ~4k lb car (w/ me in it) and 17" LRR tires (.007 Crr).

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

 

[SageBrush]

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

I apologize for being lazy and not checking the calculator you linked to. The general friction equation in wide use is:

tyres: mass*G*RR
air: 0.5 * rho * CdA * speed * speed

metric units
mass: kg
G: 9.8
rho: 1.225 kg/cubic meter
CdA: meter*meter
speed: meters/second

 

[Cebe]

I thought we were talking about EPA range, not advertised numbers.

i3 has a 22 KWh pack and delivers 81 EPA miles of range. So it is pretty straight forward:

215miles/(81miles/22KWh)= 58.4 KWh

EPA test heavily impacted by vehicle weight and there is no way the Steel/Aluminum M3 is lighter than a Carbon Fibre i3.

I was getting the 18.8kWh number from the BMW site: BMW i3 : Technical Data, but I understand they also mention the 22kWh.

 

[Snowdog]

I was getting the 18.8kWh number from the BMW site: BMW i3 : Technical Data, but I understand they also mention the 22kWh.

It's a 22 KWh pack, 18.8KWh is the amount they use from the pack. Just like Tesla won't use all of it's pack.

22kWh lithium-ion battery pack

It's straight forward calculation. Even the most efficent EV you can buy today would require almost 60KWh to have 215 miles of EPA range. And that would only be true if it gained no weight while upsizing the pack from 22 KWh to 60 KWh and we know that isn't true.

EPA test is heavily influenced by weight, both on highway and city numbers, so anyone fiddling around with numbers showing packs significantly smaller than 60KWh is just blowing against the wind.

 

[Cebe]

It's a 22 KWh pack, 18.8KWh is the amount they use from the pack.

22kWh lithium-ion battery pack

So, 18.8kWh at 129Wh/km gives about 146km, or about 91 miles, not 81. But, I suppose that's "this is what it can do" vs. EPA.

 

[ELRev]

Does anyone have/remember the figures on the original cost of battery upgrades on the S? I believe the original S started with a base 40kWh battery, with 60 as the upgrade. Is that right? I know they dropped the base model quickly cause no one was buying it, but does anyone know the history of the battery upgrade costs?

Right now, the 70D to 90D upgrade is $13000. Elon said earlier today on Twitter that the D upgrade on the 3 would cost less than it currently does on the S (5k). I'm trying to piece together as much info as I can to find the right direction for my upgrade options given my budget.