Model 3 Battery size

[stopcrazypp]

To be fair, the battery pack needs to be kept cooler than an ICE, which means you need more efficient cooling, especially with high ambient temperatures.

The Model S battery pack is kept at under 55C, using active cooling. If the ambient temperature is 40C, you only have a 15C temperature difference. If you need to get rid of 17.5 kW, that means you need to have a radiator heating roughly one cubic meter of air from 40 to 55 C every second.

It makes for a big radiator, but it's certainly not impossible.

Fair point, but the cooling capacity would be a factor of the fan and the radiator. The current battery cooling system is able to handle 500+kW of discharge power (when the reaction is exothermic, so actual heat is higher than when charging). Of course the car is moving at that point. When stationary it largely depends on how much airflow the fans can get going. They can use more powerful fans to offset the need for as large a radiator area.

Not an AC expert, so I'm not going to dive into how to do the temperature differential and BTU calculations. You would have to factor in that at most it will be near peak power for under 10 minutes (judging from existing supercharger sessions), while a ICE car would have to be designed to idle for much longer (for example in bumper to bumper traffic).

 

[Yggdrasill]

Fair point, but the cooling capacity would be a factor of the fan and the radiator. The current battery cooling system is able to handle 500+kW of discharge power (when the reaction is exothermic, so actual heat is higher than when charging). Of course the car is moving at that point. When stationary it largely depends on how much airflow the fans can get going. They can use more powerful fans to offset the need for as large a radiator area.

Not an AC expert, so I'm not going to dive into how to do the temperature differential and BTU calculations. You would have to factor in that at most it will be near peak power for under 10 minutes (judging from existing supercharger sessions), while a ICE car would have to be designed to idle for much longer (for example in bumper to bumper traffic).

True, also, you can expect the battery pack to be under 55C when the charging starts, except when you make several supercharging stops in a row. If the pack is 500 kg and starts at 40C, it can soak up about 2 kWh before reaching 55C. That's ~7 minutes at 17.5 kW of heating.

 

[sandpiper]

I think there is no information in Tesla's statement. I think they were deliberately setting low expectations, and that's exactly what they should have been doing. Nobody's unhappy when the final product exceeds the expectations.

They don't want the noise from GM (or the anti-Tesla contingent) to be that Tesla can't build a 35K car with the Bolt's battery capacity. This would put Tesla on the defensive, having to explain "yes, but it's more efficient and goes further". With every explanation/justification you lose some people - who either don't understand or don't think too hard about it. If they go with a 60, there's no crack for Tesla's opponents to drive a wedge into.

I stand by my prediction: 60/85.

So.... I've decided to go full Trump on this.

I never predicted that the big battery was going to be 85 kwh. FAKE NEWS! This is awful, mean, vindictive nonsense spread by the biased Liberal media. Bunch of sore losers! And Rosie O'Donnell sucks. I know because she tapped my my phone. Repeal and replace Elon Musk!

Just kidding.

 

[arnis]

but I don't see how it would be a challenge to design a cooling system than can handle the heat that 200kW charging might generate.

The current battery cooling system is able to handle 500+kW of discharge power

Well, battery can not handle 500+kW discharge. It is not the nominal vehicle power rating. It is peak power. This is why Tesla's struggle at German autobahns above 200km/h. They actually overheat (most likely it's the rotor that overheats before the pack, pack takes a minute longer).
Bimmers operate at near 100C glycol and oil. And those higher loads are while in motion. Supercharging happens while stationary. Fans can only push as much as they can. This is why BMW has a variety on fans available, from 300W to 850W - depending on engine, towing hitch, transmission, climate package etc.
Also you forgot that most of heat is lost with exhaust gasses, not radiator. EV-s don't have poophole.

It makes for a big radiator, but it's certainly not impossible.

Like I mentioned everywhere, we can add as many radiators as we want (Bugatti Veyron has 10) but they only act as thermal buffer if they can't dissipate that heat. And for that, we need air. And for that, we need air intake cross-section larger than we already saw. I would remind that M3 doesn't have two additional air input areas near fog lights. ONLY the center hole. I think it is even possible to calculate
maximum air throughput.

The way how Model S/X keep the charging process happening fast at 40*C ambient (to keep pack below 55*C) is with AC compressor mostly. Glycol is cooled down much closer to 0*C and AC evaporator can go 70-80*C for more performance per cubic meter of air.

I've asked anybody to measure AC compressor power but no data from Tesla owners. Leaf has around 2500W maximum electrical input to compressor. We can assume that Tesla might go up to 3000W for obvious reasons (2500W is already gigantic). That means 9kW of heat extraction is possible. I think it is possible to stack glycol loop in front of condenser and slightly add some heat extraction capabilities.
BUT THAT is pretty much as much as it is possible.

PS: With 3kW AC compressor while supercharging 3kW(compressor)+1kW(fans)+0,2kW(pumps) is just wasted for cooling, that's 3-4% of the whole (which lowers charging efficiency too).
I'm absolutely sure that with the similar chemistry (efficiency) nothing above 150kW is going to happen no matter the voltage. I don't expect 75kWh Model3-s will even do anything above 120kW.

 

[Yggdrasill]

You don't *need* to use the AC to charge at 250 kW. It depends on the size of the radiator and the amount of air the fans can push through the radiator. Thus, the AC power doesn't define the maximum amount of cooling possible.

As I also explained, the battery is a fairly large thermal mass and takes something like ~7 minutes to go from 40C to 55C while charging at 250 kW, so even assuming the max cooling is only 9 kW, that means you could charge for around 15 minutes at 250 kW. That works out to 62.5 kWh supplied to the battery. That's pretty respectable for 15 minutes, and should allow you to reach the next supercharger.

 

[arnis]

Ok, let's try to explain those charging speed reductions on green and red lines.

If the pack is 500 kg and starts at 40C, it can soak up about 2 kWh before reaching 55C.

How did you get 2kWh. You know specific heat capacity for the cells?

 

[Yggdrasill]

Ok, let's try to explain those charging speed reductions on green and red lines.

I don't know. Something in the SW probably explains the behaviour, but what exactly isn't easy to say. Could be the cabling, connectors, cells, etc. I'm quite sure it's something that can be resolved through engineering, and I expect it's been resolved on the Model 3.

How did you get 2kWh. You know specific heat capacity for the cells?

I used the heat capacity of aluminium. A lot of the pack is aluminium, and you also have components like lithium, which has three times the heat capacity of aluminium.

I also assumed the battery pack is completely isolated from the environment, which is it is not. The battery pack will transfer some heat through conduction to the chassis, as well as through convection on the underside of the car.

Edit: Here's a study showing the heat capacity of 18650 LCO cells are close to aluminium: Thermal Properties of Lithium‐Ion Battery and Components

 

[hacer]

...
I'm absolutely sure that with the similar chemistry (efficiency) nothing above 150kW is going to happen no matter the voltage. I don't expect 75kWh Model3-s will even do anything above 120kW.

I expect this is true for the initial release and some years to come. Eventually fast charging stations will have active cooling plates that press up against the battery to extract much of the excess heat, so that's a solvable problem. Long life at high C with high energy density still needs work.

 

[WarpedOne]

Heating up air is not the only possible cooling.
What about a heat pump that is evaporating water at its hot end?
One would need to fill up with distilled water before starting a SC session though.

 

[arnis]

Ok. The explanation is thermal throttling.

“If the Facts Don't Fit the Theory, Change the Theory” Einstein.


As M3 will be what Elon lately mentioned, there will be no advancements in supercharging due to new Model 3.
If any, they will start with bigger platform first and add functionality to the smaller platform later (3 and Y).

PS. I did my own calculation and got 1.6kWh of heat needed to raise 15 degrees (500kg pack). Aluminum is actually near, yes.
Convectional heat transfer is extremely small. As a Leaf owners, I know. There is nothing until delta T is at least 12-15 degrees.

I'm still searching for data about cell efficiency at different rates. I KNOW that at higher rates efficiency goes down.
So charging at Level2 is more efficient than supercharging (if we exclude cooling system requirements and converter losses).

Edit. Found some data that modules in S/X pack weight 25kg. 16 of them in the bigger pack, total 400kg.
Those 100kg are for case which doesn't actually take heat well enough (air insulation).
Therefore actual heat capacity is 1.3kWh.
If I estimate heat buildup (while charging at 120kW speed) is 5%=6kW, 1.3/6*60=13 minutes to raise 15 degrees.
But I don't expect such high efficiency at those charging speeds and I don't expect 300% AC COP. (Leaf heatpump is 200-250%).

All this boils down to: charging at 120kW = absolutely yes. Charging at 250kW = absolutely no.
Even if we can extract heat at 9kW rate, the opposite side of the cell will be hotter than the heatsink it is touching (glycol loop).
This is especially true for 2170 cells, as diameter is 15% bigger. Calculating deltaT manually is extremely hard.
The most precise method is to measure it.

Holy cow. It gets complicated. My mind is getting hot. Taking a break.

 

[Nuclear Fusion]

Wheelbase can fit 85kWh.
This is a marketing, not functional, decision.

 

[ItsNotAboutTheMoney]

Why does the Model 3 potentially get 300 miles with the 75 battery, but the S only 250? Is it simply the weight difference? Or better battery, or both?

-smak-

I think people are thinking 300 based on the combination of reduced weight and improved aerodynamics. The S is a _heavy_ car. A lot of the overall rating gain will come from weight reduction. So real world we should expect improvement for highway trips (rolling resistance is proportional to weight and increases linearly with speed), but nothing really dramatic.

And, of course, it's not just about the range: the Model 3 will be more efficient than the S, and will be easier on tires and road.

 

[arnis]

Golden rule:
Weight doesn't matter on highway. Drag does.
Drag doesn't matter in city. Weight does.
Rolling resistance is just a cherry on a cake.

Of course it is not black and white though semi-trucks weighting 36 metric tons use 30l/100km fuel at
steady cruise while Model S sized BMW 5-series weighting 1.5 metric tons uses 5l/100km, same speed.
Mass is 24x bigger, fuel consumption only 6x. Mostly due to 9m2 frontal area and 0.9Cd.
Whoever want to calculate: BMW figures: 0.28 Cd; 2.35m2 area.

All this applies to EV-s. If we make Model S as light as Model 3 while keeping body the same, we get
few extra miles of range on a looong trip between superchargers.
If we fully load Model 3 so it weighs as much as Model S it will still have the same range (few miles lost)
The only requirement is to have the tires filled appropriately to the load.

Model 375D weights slightly less than Model S75D but it has much less drag.

*this video is half the mass of the example.

 

[vinnie97]

So.... I've decided to go full Trump on this.

I never predicted that the big battery was going to be 85 kwh. FAKE NEWS! This is awful, mean, vindictive nonsense spread by the biased Liberal media. Bunch of sore losers! And Rosie O'Donnell sucks. I know because she tapped my my phone. Repeal and replace Elon Musk!

Just kidding.

Not that amusing...Rosie O'Donnell called (twitted) for martial law to prevent a legally elected President from taking office. She is what you call an unhinged leftie, and CNN is biased as hell (all it takes is a little research to see the statism and DNC talking points they peddle and have peddled over the years; they are not all that different from most other infotainment outlets, but their blatancy has been turned up to 11 lately). Oh yea, "incidental" surveillance conveniently resulted in the leaks that the biased media spread in their quest to undermine the new admin. The intelligence agencies don't even have control over their own rogue operatives. Accountability doesn't exist.

*rant over*

As for the battery sizes, I had hoped Elon's admission that <100kWh would mean at least 80, but we take what we can get.

 

[stopcrazypp]

Well, battery can not handle 500+kW discharge. It is not the nominal vehicle power rating. It is peak power. This is why Tesla's struggle at German autobahns above 200km/h. They actually overheat (most likely it's the rotor that overheats before the pack, pack takes a minute longer).

Well I didn't say it's continuous, but that is the peak power that it can handle. Supercharging is not continuous at peak power either, it ramps down. Also, as you point out the motor is the one that overheats quickly. I don't believe there is a source that shows how long it takes for battery to overheat. And on that note, one thing I neglected to factor in is that has the battery cells get hotter, their internal resistance actually goes down, which makes things a bit better (lower than the 17.5 kW I estimate).

Bimmers operate at near 100C glycol and oil. And those higher loads are while in motion. Supercharging happens while stationary. Fans can only push as much as they can. This is why BMW has a variety on fans available, from 300W to 850W - depending on engine, towing hitch, transmission, climate package etc.

The power I am quoting is idle engine speeds, when the car might be stationary. As I noted the 335i idles around 20hp (the dyno chart is at 800 rpm, I believe idle is around 700 rpm), which would put out around 35hp of heat into the cooling system (another 35hp goes through the exhaust). Of course, this is not a direct idle heat measurement, but I couldn't find sources for that, so I only have this rough estimate.

Also you forgot that most of heat is lost with exhaust gasses, not radiator. EV-s don't have poophole.

Actually I didn't forget, I factored that in here already, I did not include the other 35% that is lost through exhaust gasses, only the part through the cooling system:

In the second part I talk about the pack generating 17.5kW of heat using 200kW charging. In a typical ICE, 35% of total energy is heat through the cooling system and 20% is useful work, which means 17.5kW is equivalent to when an ICE is outputting 10kW (13.4hp).
http://www.saldanaracingproducts.com/Cooling System Principles.pdf

 

[mrkymarc]

The way how Model S/X keep the charging process happening fast at 40*C ambient (to keep pack below 55*C) is with AC compressor mostly. Glycol is cooled down much closer to 0*C and AC evaporator can go 70-80*C for more performance per cubic meter of air.

Sure, but the car knows the ambient temp and the pack temp. No reason it couldn't crank up the power to 250 if the ambient temp is lower than 40C and then adjust as needed.

 

[arnis]

Supercharging is not continuous at peak power either, it ramps down.

We can call a DC-charging process continuous (and stable) as it continues for minutes at the same power level. Stable (or full throttle) acceleration never takes more time than half a minute. And usually fluctuates. So it is not continuous but averages to something different (compared to peak). As we estimated, thermal capacity is around 1.5kWh for 15 degrees. It takes few minutes, therefore motor overheats faster (we have youtube videos about 0-250 accelerations on autobahns, some also continue to show that another 250 is not doable). We also estimated 15 degrees with 13 minutes (120kW). Acceleration is 4x more intense, therefore 2-2,5 minutes (more internal resistance due to higher load).

As I noted the 335i idles around 20hp (the dyno chart is at 800 rpm, I believe idle is around 700 rpm), which would put out around 35hp of heat into the cooling system (another 35hp goes through the exhaust).

Dyno shows maximum power output at specific RPM. Diesel engines eat 0.5l/h, gasoline 1l/h (bigger ones 1,5l/h). Power output is way smaller, something like 2kW for diesel. (3hp), more on gasoline. Most passengers cars are unable to race "continuously". They do overheat as air throughput is too small at lower speeds (racing includes curvy track with a lot of low speed periods) and full throttle accelerations.

No reason it couldn't crank up the power to 250 if the ambient temp is lower than 40C and then adjust as needed.

Actually, AFAIK, Model S is not capable doing 120kW (nominal maximum speed) for reasonably long at 40C ambient.
Like we can see on the graph, new chemistry is capable to absorb more juice. More than Tesla originally intended. Unfortunately SC had to slow down due that higher average charging speed resulted heat buildup. Getting maximum amount of range within 30-40 minutes is MORE important than getting slightly more during first 10 minutes and then much less later (so after 30 minutes result is worse). 250kW is not doable with today's 18650/2170 packs (55-100kW). And pushing 250kW for first 20-50 seconds doesn't count. Also pushing 200kW for 2-3 minutes is pretty much next to useless. And even pushing 150kW (instead of 120) for 10-20 minutes isn't worth if charging after that 150kW 10-20 period must slow down due to heat buildup (red graph line). Well, I could actually accept that 150 version as some people might only want to charge 30-40kWh. Though changing plug/voltage/wires due to that is overkill that will cost money.

 

[sandpiper]

Wheelbase can fit 85kWh.
This is a marketing, not functional, decision.

That's what I wonder. Alas, we won't know until somebody pulls one apart.

 

[Topher]

Wheelbase can fit 85kWh.
This is a marketing, not functional, decision.

Then why lie about it? Elon could have just said 75kWh will be the max for now...

Thank you kindly.

 

[arnis]

When Elon said: "100kWh won't fit" people were like: OK it will be 90kWh then.
Then Elon said: "It will fit 75kWh" people were like: OK it actually fits 85kWh then.
Soon Elon says: "It will have 75kWh" and people like: OK 80kWh is coming.

What next?
Elon says: "Model 3 will have 55kWh and 75kWh versions, period". People like: "So Model 3 will have 3 battery sizes?"