Tesla Model S P85D: Heavier When Charged? (Video)

JUL 10 2015 BY JAY COLE 48

Is an electric vehicle heavier when it is fully charged over one that is not?  We know petrol cars lose weight as fuel is burned off, but how about as onboard power is depleted?

Tom Scott, With Some "Help" From Robert Lleweln Answer The Age-Old Question Of "Is It Heavier"

Tom Scott, With Some “Help” From Robert Llewellyn Answer The Age-Old Question Of “Is It Heavier”

For those of us with a background in science/physics, or at least stayed awake during those classes in high school, we probably have a strong idea what the answer is.

However, famous YouTube personality Tom Scott (feel free to go have a looksee at his entertaining videos after watching this one) asks that exact question from inside a Tesla Model S P85D – driven by Fully Charged star Robert Llewellyn no less.

Naturally the ‘maths’ (and results) are somewhat explained while doing some insane launches; but do you agree?

Tom Scott (via Boldride)

Categories: Charging, Tesla

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48 Comments on "Tesla Model S P85D: Heavier When Charged? (Video)"

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Car will lose acceleration far faster to voltage sag as the battery is depleted than it gains as the battery ever so slightly loses weight.

IIRC, metal-air batteries actually *increase* in weight as they discharge (as they bring in outside oxygen and oxidize the metals).

I’m calling BS on this one. E=mc2 applies to nuclear reactions where mass is converted to energy. In any chemical battery powered device no electrons leave the device while the battery is being discharged, they just move from one place to another. To move the electrons back to where they can do some useful work takes energy which is supplied by the charger. The charger takes the electrons from where they ended up in the drained battery and puts them back where they are again available to do work.

A useful analogy is a pumped hydro storage system where water is pumped to reservoir at a higher location when excess energy is available, charging the storage as it were. When the stored energy is needed, the water is allowed to drain through a hydroelectric generator to a reservoir at a lower location.
Ignoring evaporation and leaks, a given mass of water could play the role of battery forever since in this case like the electrons in the battery the water is just being moved around!

Thank you. I haven’t watched the video, but I cringed when I read the implication. (For the record, I have a college degree in Physics). E=mc^2 is useful when changing between mass and energy. It does NOT apply when we are simply changing the energy state of matter. I concur with your use of water as an analogy. The electrons and ions in the battery are NOT being created and destroyed. They are simply being moved around between a higher and lower energy state.

First law of thermodynamics in action. Even we engineers know not to mess with that one.

Well, technically the 1st law of thermodynamics (i.e. conservation of energy) *can* be broken, in very extreme cases. Einstein expanded and combined the ideas of conservation of mass, and conservation of energy. Energy *can* be created and destroyed, but mass is destroyed and created in its place – the relationship between the two is the famous equation E=mc^2. In practice, this only really ever happens on earth inside of a nuclear power plant or explosion. It is not how energy is generally exchanged, however.

Anyone with a college degree in physics should know better than to make this assertion. In converting matter to energy, or vice versa, neither matter no energy is “destroyed” in the sense of thermodynamics. They are merely converted from one state to another.

The laws of thermodynamics are immutable, at least in this universe. While it’s possible on the quantum scale over very short time periods to violate (for example) the conservation of energy, on a longer time scale the Laws of Thermodynamics still rule in every case, without exception.

In the traditional sense, strictly of conservation of energy, I stand by my statement – the first law of thermodynamics is incomplete, and therefore can be “violated”. What Einstein did was to expand and combine the concepts of conservation of mass and conservation of energy. Taken on its own, energy can be created or destroyed. Just like matter can be created or destroyed. But by destroying one, you are creating the other and vice-versa. In a sense, you and I are saying the same thing, so I find it amusing that you frame your statement as if you disagree with me.

But to say that “energy has mass” is wrong. It’s basically backwards. Rather, you can think of mass as another form of energy (in addition to kinetic, potential, etc). Transferring to mass “energy” is done with the conversion factor of c^2.

Brian said:

“But to say that ‘energy has mass’ is wrong. It’s basically backwards. Rather, you can think of mass as another form of energy (in addition to kinetic, potential, etc). Transferring to mass ‘energy’ is done with the conversion factor of c^2.”

To quote Wikipedia:

In physics, mass is a property of a physical body which determines the strength of its mutual gravitational attraction to other bodies, its resistance to being accelerated by a force, and in the theory of relativity gives the mass–energy content of a system.

Brian, I have noted over the years that there are a surprising number of people on the Internet who claim to have degrees in science, but only fool the scientifically illiterate. I don’t know you well enough to say that you’re one of them, but your posts certainly don’t exhibit a good grasp of science fundamentals.

Brian said: “Energy *can* be created and destroyed, but mass is destroyed and created in its place”

Lensman said: “In converting matter to energy, or vice versa, neither matter no energy is “destroyed” in the sense of thermodynamics. They are merely converted from one state to another.”

I think you guys are saying the same thing just using the terms in different perspective…

Yes, it’s a semantic argument… that is, an argument over how ideas are expressed and/or word definitions. That part of our disagreement isn’t scientific in nature.

It is weird by the e=mc^2 is an absolute relation not a single way from mass to energy. So, the video is correct when a battery is charged it weight, even so little, more than when it is discharged and that weight difference is exactly the weight corresponding to the energy.

I think they were just using e=mc2 to establish an upper limit to the loss of mass of it were nuclear. They were vague on the details, but I don’t think they meant to imply that e=mc2 really does apply here. The commenter even says words to the effect that they can just check the upper limit using the equation.

Of course it applies here. E=MC^2, or “energy equals mass times the speed of light squared”, gives you the equation for figuring the mass (usually expressed as weight here on planet Earth) of a specified amount of energy. Since we can figure the difference in energy between a Model S battery pack that’s fully charged and one that’s fully depleted, it’s a straightforward math problem to figure the difference in math. No “upper limit” need apply here. This isn’t nebulous theory, it’s fairly simple, straightforward physics.

That’s all great, though needlessly argumentative. But a Model S doesn’t actually CONVERT energy to mass or vice versa. It would need a “Mr. Fusion” ™ home fusion system for that. That’s why the equation doesn’t actually apply here, but rather just gives us some figures to use for comparison. I too, have a degree in physics. A real one. Like from an accredited University. Do you, Lensman? Or are you one of the posers you keep referring to here?

If you have a degree in physics, then you ought to be familiar with the concept of mass-energy equivalence, and why Einstein’s famous equation is central to the question of how much mass is contained in the energy which enters and leaves a battery pack as it’s charged and discharged.

See Wikipedia quote in my post below.

FSJ said:

“I too, have a degree in physics. A real one. Like from an accredited University. Do you, Lensman?”

No, I have no science degree; I’m an autodidact. Yet it would appear that I understand the fundamentals of physics better than two people here with actual degrees in physics. 😀

I think what several people posting here are missing is the concept of mass-energy equivalence. If that concept had been given a mention in the article, there would likely be a lot less argument over the point. Wikipedia has an excellent article on that which is very relevant to the discussion here: ~~~~~~~~~~~~~~~~~~~~ In physics, mass–energy equivalence is the concept that the mass of an object or system is a measure of its energy content. For instance, adding 25 kilowatt-hours (90 megajoules) of any form of energy to any object increases its mass by 1 microgram (and, accordingly, its inertia and weight) even though no matter has been added. A physical system has a property called energy and a corresponding property called mass; the two properties are equivalent in that they are always both present in the same (i.e. constant) proportion to one another. Mass–energy equivalence arose originally from special relativity as a paradox described by Henri Poincaré. The equivalence of energy E and mass m is reliant on the speed of light c and is described by the famous equation: E = mc^2 Thus, this mass–energy relation states that the universal proportionality factor between equivalent amounts of energy and… Read more »

Dude, do you even physics?

I really do not understand why the battery would lose weight. Inside a gas car, the liquid fuel expands into gas and is moved out the tailpipe. In a battery the electrons are just moving from one electrode to the other. They are just re-arranging, not being expelled.

How would it _not_ lose mass? You have to move _something_ from the battery to the motors to make them move and if you do so then there is less energy in the battery and therefore it must have less mass.

Just compare the weight of the chemical fuel, gas, which is converted into heat and mechanical energy as opposed to electrons, which weigh next to nothing.

And next to nothing is not nothing.

Excellent fun way to get publicity !

The real question is how much weigh one electron and how much electrons are in one kwh and one battery.
And the answer will probably be something around nano/pico gram per charge.

That has nothing to do with it. The electrons are present in the battery whether it’s charged or discharged. The difference in question is the gain or loss of mass (not matter) as a battery is charged or discharged.

* * * * *

Reminds me of the filksong (sung to the tune of “Johnny’s So Long at the Fair”):

Oh, dear, what can the matter be?
When it’s converted to energy
There’s a slight loss of parity.
E equals M C squared.

E=mc^2 does apply. The electrons change mass as their orbits change. As the video points out, this is inconsequential in a chemical battery.

I am sure that a few grams of weight are lost, off the tires.

Ah, finally someone addresses the reality. Actually, despite a large number of posts saying otherwise above, the Model S with a depleted battery pack will weigh less than one with a full pack. Energy has mass, and a depleted pack has less energy than a full one, even if the number of electrons is the same. But as perhaps Red HHR was suggesting, other losses of weight from driving the car some distance would be so much greater than the loss in mass due to depleted energy that the latter would be lost in experimental error. You could come a lot closer to answering the question by removing the battery pack, resting it on a scale, charging it, and then depleting it without moving it. But even then, I question there is any scale in the world sensitive enough to measure such an infinitesimal difference in mass. Laboratory scales that sensitive aren’t engineered to hold 1200 pound weights. A better approach would be to take just one cell of the type Tesla uses in the battery pack, and use an extremely sensitive scale to measure the before and after weight. If the scale was sensitive enough, perhaps it could measure… Read more »

“If the scale was sensitive enough, perhaps it could measure the tiny difference in mass.”

Let us say you can find a scale that is sensitive enough (which is a statement of precision). Can you find a scale with that kind of precision to have measurement that is repeatable enough or within its measurement uncertainty to accurately measure the intended weight difference? (assuming you have ways to remove the effect of temperature, pressure and humidity on the measurement for this type of accuracy).

I think that’s an interesting question, but altho I’m willing to discuss the subject, I won’t attempt to answer this question in an authoritative manner, since I’ve never worked in a laboratory that specializes in measuring this type of measurement or analysis.

I’ve seen a lot of articles about clever methods and instruments which can measure objects on tiny microscopic scales, approaching the nanoscale. But with something as large as an 18650 battery cell, I think the caveat in the post I quoted below is relevant: “One can’t really measure it this precisely because pieces of the battery evaporate, the battery may absorb some dust, humidity etc.” That caveat applies to a single battery cell as well as the entire pack.

Bottom line: My guess is that the experimental error would be greater than the change in mass, so that no instrument could actually measure the mass gain or loss in an object as large as a 18650 battery cell… let alone the entire ~1200 pound Tesla Model S battery pack.

But that’s just a guess.

Indeed! And that’s the important point here.

Maybe not. Maybe gained weight by picking more stuff up off the road than they lost.

Sorry Tesla fans, but E=MC^2 does figure into the answer. It gives the formula for figuring out how much mass is gained or lost when a battery is charged or discharged, based on energy gain or loss, which is entirely relevant to the question. (As already pointed out, there is little or no net gain or loss of electrons.)

Here’s what appears to me to be a good analysis:


And here’s the gist:

For example, Chevrolet Volt has batteries that may store 16 kWh. Multiply it by 1,000 and 3,600 to get the value in Joules; divide it by 1017 which is (approximately) the squared speed of light and you get the mass difference in kilograms. It’s about


That’s half a microgram – for this huge Chevrolet Volt battery. One can’t really measure it this precisely because pieces of the battery evaporate, the battery may absorb some dust, humidity etc. The mass difference above is comparable to the mass of a water droplet of diameter 0.1 mm or so.

That’s a very good and comprehensive link you found. Thanks.

“half a microgram”

Can you prove that to me with an actual measurement device accurate enough and with less uncertainty than the half a microgram?


That is where physics major is looking an engineering majors for idea and then engineering majors just look back thinking those physics majors are just “crazy”… =)

If your point is that this is nothing but a theoretical question, having absolutely no engineering value or practical application, then I entirely agree. It’s basically just a thought experiment.

But as Einstein’s thought experiments showed, sometimes a thought experiment can lead to something with practical application… like a nuclear power plant.

Actually the approx. half a microgram claim was for a Volt battery pack. The guy in the video said two micrograms, which is roughly the same proportion, considering the difference in kWh between the Volt and an 85 kWh Model S.

Five times the difference. So there!

Why are you addressing Tesla fans specifically? This concerns any battery.

Because the headline of this article is going to attract Tesla fans.

I doubt anyone needs to have it pointed out that this concept applies to all batteries, not just Tesla battery packs.

Perhaps the weight distribution in the car will change? :evil grin:

BTW, in the video, the guy said that battery pack would get lighter, not necessarily the car would get lighter….

Because you can pick up just some little dirty from the track to more than offset the weight you lost in the battery…

Yeah. Even picking up a slight amount of dust would likely weigh more. But then, that would be offset by loss of tire particles… 🙂

It seems funny to me that people in a pedantic fashion always quote that simple formula but never include the units.

Which are Joules, Kilograms, and 300,000,000.

But to answer Jay’s question, of course the car is heavier after charging. By 2 donuts and 2 cups of coffee, minus the last cup of coffee from the last stop.

Speaking of units, I was charging my car on the way back from a trip east at the AAA in Clarence which is near by the new Supercharger there.

Doesn’t seem to get much use for an 8 stall installation since I’ve only ever seen one car from NJ there.

But the energy usage for 2 months (including heating losses from the charger fans) has been 8400 kwh, and the demands have been 122 kw last month and 88 kw this month.

So does that mean only a 60 kw model S has stopped to charge this month so far?

It is ambiguous to measure energy in kW. I’m guessing that is an average rate, so one half a car on average this month.

Your point is irrelevant. I’m measuring power not energy. Apparently, this month there hasn’t been an 85 kwh S charging since the demand on the meter for the whole joint is only 88 kw.

They tell me the 60 kwh S’s don’t grab juice quite as fast as a dead 85kwh S.

And as far as averages go, the 8400 odd kwh used to date by the station over 6 weeks would be an average of 8 1/3 kw all the time.

So assuming the average S charges at an average 83 kw (in this case including losses in the charger bays – the real ‘charge’ rate is somewhere around 70-75 kw), it means that there has been 1 car charging there around 10% of the time.

There is not enough displayed granularity, (at least for the amount of time I chose to stick around waiting for the meter to change) in Nyseg’ revenue meter (digital devices are much harder for these eyes to ‘view’ than the old analog ones) to hazzard a guess as to the quiescent draw of the station, but my gut feeling is it is low, and can be elmininated from the analysis without much accuracy loss.

Yes, a charged battery weighs slightly more than a discharged one. Einstein’s equation applied to all forms of energy, including information! A formatted hard disk weighs more than a blank one. A HD written with alternating 1’s and 0’s weighs more than one filled with all 1’s or 0’s, and one filled with all the entries in Wikipedia weighs more than that.

Why? Entropy! Complete disorder is maximum entropy. Creating order takes energy, thus an object’s non-maximum entropic state “stores” energy. Since information is represented by objects with low entropy, information also is a form of energy storage.

Too esoteric? In discussing mass gains/losses, certainly. However, the conversion of entropy to energy is EXACTLY why electric cars are so efficient. 33 kWh of AC electricity is NOT the same as the 33 kWh “energy equivalent” of a gallon of gasoline. Electricity is more highly ordered so, with clever things like inverters and motors, that order can be used in ways that gasoline-energy cannot.

And yes, I’m a physicist.

from a gallon of gasoline. Electri

A Hard disk filled with alternate 1’s and 0’s weighs more than a hard disk filled with all 0’s or all 1’s? (Assuming that 0’s and 1’s have the same mass, which to me since it is the magnetism sense, seems to be a safe assumption).

What an idiotic statement.

The idiocy is the assumption that all 0’s or all 1’s are less ordered than alternate 1’s and 0’s.