Weight gain of a fully charged Model S

[scottm]

My son is a physicist so we did the math.
My model S weighs about 2 - 3 micrograms more when fully charged. The range of weight here acknowledges there is somewhat less than 85kWh battery and somewhat more than 70 available to me.
I don't have a scale to prove it... but interesting nonetheless.
Imagine that! 3 micrograms energy mass can move a car 400km.

 

[Gouldness]

Yep, i believe Jason Hughes "hacked" this and found the 85kw batteries have 81 but only 77 usable: Tesla’s hacked Battery Management System exposes the real usable capacity of its battery packs

 

[scottm]

Ya, (yawn) it's well known the usable range of S 85 is not based on a physical 85 kWh pack... that's old news. They could have called the car a S77 (or anything else) and I would have still bought it. So long as it continues to deliver a promised 691 horsepower --- snicker. I understand car marketing when I see it.

What matters is how far I get to go on a charge for my driving style and environment... I don't really care anymore what the total battery capacity is... I care more about usable range and battery degradation over time - it has to stay above that "acceptable" line.

The new news is: did you know a charged pack of this size has added weight!?

Blew me away when I learned it. Just the concept of "pure energy" and it's frickin' density is amazing.

In the future, when we can contain just 1 gram of pure energy.. that gives my car up to a half million complete "charges". But would never really have to stop and charge for the life of the car.

 

[UCIBtheG]

Can you expand on how exactly you determined this 2-3 microgram weight difference? I'm curious what kind of scale can measure something as heavy as a car (I'm assuming you didn't pull the battery from the car) with that accuracy. How did you know that a fly didn't land on the object when it was weighed? Or poop on it for that matter?

EDIT: Sorry, missed the spot where you said you didn't have a scale to prove it, so I guess this is all just a theoretical thing your son figured out based on the mass of the battery. For a minute there, I was reminded of the "21 Grams" movie.

 

[Lloyd]

My son is a physicist so we did the math.
My model S weighs about 2 - 3 micrograms more when fully charged. The range of weight here acknowledges there is somewhat less than 85kWh battery and somewhat more than 70 available to me.
I don't have a scale to prove it... but interesting nonetheless.
Imagine that! 3 micrograms energy mass can move a car 400km.

Actually there is NO weight gain as electrons are just moved in a battery from one potential state to another.

 

[McRat]

Technically, the mass of a EV should be very slightly less when fully charged. Why? Tires do not hold air perfectly. Let's say you lose 1 psi per month. Find the volume of the tires, and how long the charge takes, assume mfr pressure at the start, and voila. That's not counting the outgassing of plastics in contact with components that heat up during the charging. A hot car weighs less than a cold car.

 

[Brass Guy]

I have a decent grasp of electricity, am no EE, but I thought charging a battery is more like moving electrons from one pole to back to the other; so there'd be no actual weight change. Not even a fly's poop.
Oops, I spilled some electrons filling my Tesla - do electrons stain? Can I clean it with soda water?

Edit - Wow, I guess I'm typing exceptionally slowly today.

 

[Gouldness]

In the future, when we can contain just 1 gram of pure energy.. that gives my car up to a half million complete "charges". But would never really have to stop and charge for the life of the car.

Mind numbingly cool. Probably not within the realm of physics though. I hear Isaac Arthur chiming in...

 

[scottm]

Actually there is NO weight gain as electrons are just moved in a battery from one potential state to another.

.. whip out your physics texts again... !!

What you're saying is "common sense" and is practically zero, but is not the limits of the physical truth.

As the electrons move from one plate to the other, through chemical means, they generate electric power. The distance the electrons move through the chemical is spent energy. Energy has a mass equivalence. (This tests most people's limits of understanding, Einstein got it!). The mass of that energy expelled from the pack (externally from its contact points) carries that weight away from the pack. Ta-dah!

2 to 3 micrograms to be exact.

 

[scottm]

Technically, the mass of a EV should be very slightly less when fully charged.

..you're assuming driving is needed to deplete a Tesla, with a hot air / tire thing going on. Everybody knows the car discharges itself without even driving... vampire and battery warming to name a couple ways.

So a stone cold Tesla depleted of charge, and then charged, with no other environmental factors coming into play weighs more - even after the cable is disconnected

And then, if you flick off that spec of fly poop on the windshield, it will weigh about the same as before. Which is what makes this conversation interesting. Because ... do flies really poop?

 

[Max*]

Because ... do flies really poop?

Yes, yes they do.

Flies: They Poop, They Die

 

[evp]

Actually there is NO weight gain as electrons are just moved in a battery from one potential state to another.

True, there are the same number of electrons in the charged battery, but we're talking about relativistic mass gain here, you know -- E = MC**2. The electric field and chemical energy in the electrolyte both have a mass equivalence which could be measured with a sensitive enough scale. Wolfram Alpha will tell you that 85kWh/C^2 = 3.4E-6 grams.

 

[f-stop]

 

[gregd]

So, being totally pedantic about this, the mass of the car should change by 3.4 micrograms, but the weight will change in addition depending on pack temperature. Mass is the inherent property of the car, while weight depends on the environment the car is in, specifically in this case, the volume of air the car is displacing. As the pack gets warmer with charging (or driving, for that matter), it expands ever so slightly, which increases the volume of air it displaces. That additional volume of air displaced will reduce the weight of the car by the same amount. The car, in effect, is being turned ever so slightly more into a balloon. So, depending on how much extra buoyancy the warmer pack provides, one may not actually see all of that 3.4 micrograms of mass increase reflected in the car's weight. Until it cools off, of course. This gives us a new view into the term "Land Yacht".

Now, back to your regularly scheduled physics programming...

 

[OBX John]

I love that there are people in the world besides me that care about this stuff.

 

[McRat]

Yes, yes they do.

Flies: They Poop, They Die

 

[Hugh Mannity]

[nerd ON]

"They" say the earth's rotation is slowing and it also collects space dust continuously which both contribute to gravity increasing on an ongoing basis, (yes I realise it's minute and probably not even measureable by any of our known instruments); however technically that means our Model S's are heavier after each charge.



[nerd OFF]

 

[rjcbox]

The OP is correct.

BTW, you become relatively younger than your friends, when you drive your Tesla really fast

 

[Pando]

In the future, when we can contain just 1 gram of pure energy.. that gives my car up to a half million complete "charges". But would never really have to stop and charge for the life of the car.

Great Scott!

 

[Joe F]

So the Tesla Grin becomes 2-3 nanometres wider on a full charge! And here I thought it couldn't get any better!