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Comment by founderling | Hacker News Reader

I mean for batteries in general. Independent of the technology.

For the spinning matter battery, the limit might be when the whole matter of 1 kg spins at the speed of light. I wonder if it matters how big the radius is with which it spins around the center?

For a non-rechargeable battery, I guess the limit is 1kg * c^2. As Einstein postulated that e=mc^2. And that guy was right more often then not. Is that also the upper limit for rechargeable batteries?

How long could a Tesla drive on 1kg * c^2?

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Yeah, the absolute limit is c^2 if you've got 50-50 matter-antimatter. The efficiency however is brought down by the difficulty of _producing_ antimatter in the first place. A more popular hard-sci-fi concept is black hole engines; a 600kt black hole sprays out a hundred petawatts of Hawking radiation, and the fuel is whatever you've got lying around. That still has issues around energy _capture_, since the radiation is going to be ridiculously high-frequency. The best way I've heard of to dodge those issues is the 'Penrose process', where the fuel is the black hole's rotation and you get good ol' kinetic energy out. That can at most get you 20% of the mass-energy though, and there's a bunch more small print.

20% is the energy gain of particles involved in each energy transfer, total energy available is 29% of the black hole's rotational energy according to Wikipedia.

> For a non-rechargeable battery, I guess the limit is 1kg * c^2.

Assuming it is antimatter-based you can double that capacity by consuming air. But that just turns it into a fuel tank for an internal annihilation engine and hardly qualifies as battery anymore.

This is Tesla we’re talking about. They’d probably ship this with a footnote in the owners’ manual: “Any use of this doomsday device for purposes other than a battery is prohibited. Do not operate outside containment field.’

Aha! So the most efficient mass based rechargeable battery would be one that while loading creates antimatter and while being used burns that antimatter with air (or other stuff) it takes from the environment.

> hardly qualifies as battery anymore

Why not? There are plenty of battery technologies that use air as an oxidizer.

It follows from the definitions of a battery and a cell:

> battery: a container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power.

> cell: a device containing electrodes immersed in an electrolyte, used for current-generation or electrolysis.

An antimatter reactor would not be a battery because it would not contain any cells. Additionally, because it converts nuclear energy into useful power, not chemical energy.

You all really took this question and ran with it in a way that totally avoids answering the practical question at the core — how good are these batteries going to get over the next 20 years?

The answer is really, really good. Fast charging, long rang, 1 million mile lifespans.

Add on top of that the ability to connect into a smart grid and earn money by buying energy off-peak and selling on-peak, and the ability to double as an emergency generator for your home.

1kg * c² = 24 TWh.

Supposing a Tesla averages 200Wh/km, then it would drive ~ 10¹¹ km (4 light-days, or around 600 times the earth-sun distance... which is a lot)

If Wolfram Alpha is right, then 10^11 km is 2 million times around the earth:

https://www.wolframalpha.com/input/?i=10%5E11km+%2F+earth+ci...

Shows the potential of batteries. Charge your one kilo batttery to drive around the planet two million times.

It would be a misnomer in common parlance to describe nuclear fuel as a battery (mass > energy conversion). The broader language to describe this is ‘energy storage.’ E.g. oil is stored solar energy over millions of years, batters are stored electrochemical energy, flywheels are stored mechanical energy, etc.

At least in the medium-term future we would at most be able to use is Hydrogen->Helium which would yield 6.45 x 10^14 Joules [1] which is equivalent to 179 GWh which at 200Wh/km would be 8.95*10^8 km.

That's still enough to go 20.000 times around the globe and longer than any car is known to have lasted so far[2]

[1] https://www.phys.ksu.edu/personal/wysin/astro/review9/p4.htm... [2] https://en.wikipedia.org/wiki/Car_longevity

> For the spinning matter battery, the limit might be when the whole matter of 1 kg spins at the speed of light.

Of course, it's theoretically impossible for the matter to spin at the speed of light, and any actual material will tear itself apart at speeds far below c.

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