# Tesla Battery Packs: close to 400 Wh per kg!



## garsh (Apr 4, 2016)

During a discussion of electric airplanes, Elon dropped a little information that seems to have mostly gone under the radar.

At minute 198:20, he says *" ... you'd want Wh/kg at the pack level to be over 400. So we're pretty close to that."*

Most estimates seem to put Tesla's current batteries at around 260 Wh/kg
Now, perhaps he's referring to the upcoming structural battery packs. But still, it wasn't that long ago that Elon was thinking that we were 3-4 years away from reaching this level.

Tesla Air? Elon Musk Hints Tesla Could Mass Produce 400 Wh/Kg Batteries In 3-4 Years


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## TrevP (Oct 20, 2015)

He’s said in the past they need to be 500Wh/KG minimum to be viable for plane use


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## JWardell (May 9, 2016)

What I don't understand is why it's not more common to have hybrid systems in aircraft. They need the power to weight advantage (and rocket effect) of liquid fuel to fly, but a hybrid system that can augment thrust means they could have significantly smaller and cheaper gas engines. As always, engines must be sized for their peak demand, for maximum performance in an aircraft. But you never need that performance for long...while accelerating to <100mph in a car, while climbing over a mountain or short takeoff in an aircraft for 5-10 min. A relatively smaller battery and electric thrust system could augment an aircraft in these situations, while their ICE systems can be sized for cruising instead, saving a ton of weight, fuel, cost, and maintenance. The engines could then slowly recharge the pack again in cruise.


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## Feathermerchant (Sep 17, 2018)

As a pilot, I can say that airplane engines are internal combustion but past that they are very different than car engines. Due to physics you do not want your propeller tips to exceed the speed of sound. Makes more noise but not more thrust. A larger diameter propeller is more efficient than a smaller diameter. So for example our 1957 (not a typo) Beechcraft had an O-470G which is a horizontally opposed 6 cylinder 470 cu. in. (7.7L) engine. It had a redline of 2,600 RPM and an 84" propeller. Do the math and that's a tip speed near supersonic. It was certified to operate at 100% sea level power (240HP) continuously. Normal cruise power for that plane is 75% power. We carried 60 gallons of fuel and at 75% we could fly for about 5 hours no reserve. So airplane engines are designed to operate at high power settings continuously unlike car engines.

Altitude - At about 8,000 ft MSL an un-boosted ICE can only produce about 75% of sea level power. So if you install a smaller engine in that same plane, you will give up performance. Above 8,000 ft the horsepower output continues to drop. Many areas in the US you want to be able to fly as high as 10,000 ft to safely clear terrain. 

By comparison, My Tesla model 3 seems to run at about ~22kW or so on the highway but has the capability of about 350 kW in short bursts. At 22 kW on my 75 kWh battery I can drive about 3.4 hours with no reserve.

BTW Thanks to Wardell for the canbus info.


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## JasonF (Oct 26, 2018)

JWardell said:


> What I don't understand is why it's not more common to have hybrid systems in aircraft.


Airlines set an "optimal cruising speed" for pilots - a speed (relative to altitude) that allows the engines to reduce to a relatively low RPM and consume as little fuel as possible. Where a hybrid situation would work best is at this point. The plane could take off with a full battery, using the electric assist during initial takeoff, and then reclaiming that energy during the climb when the engines throttle back a bit but are running at high RPM. And then when they hit optimal cruising speed and slow the engines to the minimal fuel consumption point, the electric assist could really shine by cutting fuel consumption further, by very large amounts.

So why hasn't this happened yet? Mostly, I think, because in the commercial airline industry the engines are interchangeable. The airline doesn't want to have to first buy a more expensive engine that has electric assist, and then require the plane to be of a design that includes a battery array to power it. If they're going to have to buy lots of new aircraft or retrofit existing aircraft in order to get more efficiency, they want a _huge_ return on it. Something like that is more likely to happen first in short-hop prop and small jet aircraft where one company basically builds and delivers it as a single unit.


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## Feathermerchant (Sep 17, 2018)

One great thing about burning fuel is the longer you fly the lighter you are and the more performance you have. Not to mention that the landing gear can be built lighter. Airliners are restricted to a maximum landing weight that is much let then the maximum takeoff weight.


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## JasonF (Oct 26, 2018)

Feathermerchant said:


> One great thing about burning fuel is the longer you fly the lighter you are and the more performance you have. Not to mention that the landing gear can be built lighter. Airliners are restricted to a maximum landing weight that is much let then the maximum takeoff weight.


The most mind-boggling part about aircraft I learned was something that I bet a lot of people don't know: The aircraft weight has nothing to do with whether it can fly or not: It's the wing surface area and propulsion. You could fly a concrete Manhattan office building if you had enough engine power and wingspan. The whole obsession about aircraft weight is for fuel efficiency...just like cars.

A pretty close second is how much resistance air actually creates at high speeds. At some point you're running the engines at max speed just to overcome the resistance and maintain speed and avoid slowing down. And that the plane "flying apart" because of excess speed is a secondary worry - the biggest overspeed worry is not having enough power to maintain lift, and then you start falling. You could actually have a plane drop out of the sky with the engines at maximum.


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## Feathermerchant (Sep 17, 2018)

Well sort of. Don't tell a glider pilot he needs an engine. For commercial/airline aviation, every non-revenue pound costs you money. Most fly at or near gross weight on every flight because that's how they make $. They have enough fuel capacity for very long flights but by reducing fuel load so they can make the flight, they can carry more paying passengers and cargo.

As far as flying goes, generally speaking a wing generates lift proportional to speed squared and angle of attack. Making lift also means making drag. So to maintain speed thrust must equal drag. As an airplane climbs in to thinner air, speed or angle of attack must be increased to maintain the same lift because air gets thinner. But there is a limit to speed - engine power and also angle of attack because every airfoil quits flying (stalls) at a certain angle of attack. (an airplane can 'stall' at any speed and any attitude) So the higher you fly, the smaller the flight envelope becomes. As fuel burns off, weight decreases and the envelope becomes larger.

An electric airplane does not weigh less after a flight so his approach speed will be higher and his landing roll will be longer and require more braking.

Back to our 1957 Beechcraft, it's gross weight is 2900 lb. Gas weighs 6lb/gal so 60 gallons is 360 lb. If I land with 10 gal left that's 350 lb less than when I took off. That is very noticeable.


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## ULEWZ (Mar 14, 2020)

Planes do not fall out of the sky, ever (unless your wings come off LOL). It is all based on relative motion and lift. If you are at full thrust and still losing altitude, dump the nose.


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## JWardell (May 9, 2016)

JasonF said:


> So why hasn't this happened yet?


Well it HAS happened yet, heck I just did it last week. And there are several other startups out there working on hybrid aircraft. The whole point is to reduce cost and maintenance with a smaller/cheaper engine. We were chatting about jumping into hybrid turboprops too. Significant savings for certain situations.

Separately, this was recently posted, but they discuss a different angle than I am...


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## Feathermerchant (Sep 17, 2018)

I watched the video. Mostly it covered reduced emissions by using a centrally located gas turbine for power and driving electric fans on the wings. This would allow room for aftertreatment of the gas turbine exhaust to reduce emissions. If you only have one engine, you have no redundancy so you would want at least two. The weight and volume penalty would be pretty ridiculous. Not to mention noise because it would be located in the fuselage. You would also lose lots of cargo volume and with it, revenue. It just doesn't seem practical.

The cover picture (rendering) put me off quite a bit. How many jets have you seen with wing struts?


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## jsmay311 (Oct 2, 2017)

I came for the confounding thread title. Moderators can edit titles, right?


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## garsh (Apr 4, 2016)

jsmay311 said:


> I came for the confounding thread title. Moderators can edit titles, right?


If you tell me how it was confusing to you, I'll edit it.


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## iChris93 (Feb 3, 2017)

garsh said:


> If you tell me how it was confusing to you, I'll edit it.


Is it kW/kg or Wh/kg?


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## tomc603 (Oct 6, 2018)

I feel like if the title meant to discuss specific energy, and the value was supposed to be 400 kJ/kg, that's not at all impressive.

If it was meant to talk about specific energy and the value was supposed to be 400 Wh/kg, that would be impressive but it's unlikely we're near there right now and there are likely major safety hurdles before ever putting such an energy dense battery into a plane. There are huge development hurdles that need to be overcome before LIB cells are reliably near 400 Wh/kg, and most of the research papers being published in this area note that nothing close has been reached outside of specific lab conditions.

I'm not sure 400 kW/kg makes sense as a measurement, though.


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## garsh (Apr 4, 2016)

Oh goodness, I didn't even notice all the typos! Sorry people!
It really is meant to be Wh/kg.


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## garsh (Apr 4, 2016)

tomc603 said:


> If it was meant to talk about specific energy and the value was supposed to be 400 Wh/kg, that would be impressive but it's unlikely we're near there right now


That's why I was shocked when Elon said that "we're pretty close to that", and thought it was worthwhile to start a thread.

If Tesla is really that far along, then no other EVs are going to be anywhere close to competitive for quite some time.


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## tomc603 (Oct 6, 2018)

garsh said:


> If Tesla is really that far along, then no other EVs are going to be anywhere close to competitive for quite some time.


Totally agreed. I know when they acquired Maxwell, there were a ton of presentations online that have since been scrubbed. But basically, Maxwell was extremely confident that they had a process that was producing 300-350 Wh/kg in their lab on their test lines. The details were a bit sparse, but a lot of the research papers we were able to find at the time seemed to indicate that they could produce layers with greater thicknesses without some of the drawbacks normally attributed. But they hadn't done any manufacturing or testing at scale, so it's still to be proven out "in the real world".

Pretty much everybody is moving full force in the direction of optimizing energy density at the materials and cell level. Beyond just the conjecture that electric jets could be possible, which there are still serious questions about, the revolutionary change to the transportation and energy storage market would be world changing in a very literal sense.


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