Wouldn't it be cool if you never had to charge your cell phone? I'm sure that's what a lot of people were thinking recently, when a company called BetaVolt said it had developed a coin-sized “nuclear battery” that would last for 50 years. Is it for real? Yes it is. Will you be able to buy one of these forever phones anytime soon? Probably not, unfortunately, because—well, physics. Let's see why.

All batteries do the same thing: They produce an electric current to do some kind of work. But energy isn't free. If that work is blasting music on your Bluetooth speakers, there has to be something that decreases in energy. In a good old AA, there's a chemical reaction to produce the current. That chemical reaction doesn't last forever, so the battery will eventually die.

In a nuclear battery, the power source is a piece of radioactive material, and it will keep on going like the Energizer bunny until the source is no longer radioactive—which isn't forever, but it's a heck of a lot longer. These aren't actually new. The Voyager 1 space probe, launched in 1977, has a nuclear battery. It's now over 15 billion miles away, and it still has a little juice. That's pretty good mileage!

The specific type on Voyager is called a radioisotope thermoelectric generator, which is a big name for what is basically a hunk of plutonium in a box. As the plutonium decays, it converts mass to energy, producing heat. If you stick a solid-state device on it, the difference in temperature between the hot and cold metals produces voltage and causes an electric current to flow.

It's kind of crazy that a temperature difference alone can generate electricity, but you can test this out at home using some copper wire and a paper clip (without the plutonium), by sticking one end in ice water and the other in hot water. This type of power source is great for space probes because it has no moving parts, so it won't break down, and it lasts for decades.

Now, this new battery announced by BetaVolt uses a different technology called betavoltaic generation. Instead of tapping thermal energy, it captures the ejected electrons, known as beta particles, from a radioactive isotope of nickel to form an electric circuit. It's made up of several layers of nickel sandwiched between plates of diamond, which serve as a semiconductor. There's a bunch of cool stuff to go over here, so let's dive in.

Nickel-63 is an isotope of the stable version of the element, nickel-58. That number is the atomic weight—the total number of protons and neutrons in the nucleus of the atom. Nickel-63 has five extra neutrons, which makes it unstable. Over time, one of those extra neutrons will decay into a proton and produce a new electron. With an extra proton, the atom will now be copper-63, the next element in the periodic table. This nuclear reaction produces energy, shooting the electron out of the atom at high speed.

It's important to know that the rate of radioactive decay isn't constant; it depends on the number of atoms of the material present, so the production of electrons declines exponentially over time. In the case of nickel-63, half of the atoms will decay in about 96 years—we say it has a “half-life” of 96 years.

One other thing we need to be clear on: You get one electron from one atom of nickel and that's it—that atom is done emitting forever. After that it's just dead-weight copper.

Yes it is, since it emits particles through radioactive decay. We usually classify radioactive decay into three types: alpha, beta, and gamma. These labels are based on the type of stuff that is radiated. Alpha particles are just the nuclei of helium atoms, beta particles are electrons, and gamma rays are a type of very high-frequency electromagnetic radiation.

But that doesn't necessarily mean it's dangerous. We're constantly exposed to background radiation just by living on Earth. (Even bananas are slightly radioactive, if you want to be paranoid.) The BetaVolt battery has only a small amount of material, and it probably has some shielding built in. Plus, beta radiation isn't as harmful as, say, gamma rays. So it would probably be safe to use.

All right, let's talk about electricity. First, we need some basic terms. An electrical current is just a flow of electrons in a circuit. The rate of flow is measured in amperes: 1 amp = 6.24 x 1018 electrons flowing through a certain point per second. Got that?

OK, the power supplied (P), measured in watts, can be calculated by multiplying the electrical current (I) by the voltage (V). One watt equals 1 joule of energy per second. Multiplying power by a period of time (∆t) gives you the total energy used (∆E), usually measured in kilowatt-hours.

Now, the BetaVolt battery is a 3-volt battery with a stated power output of 100 microwatts. You can see from the equation above that if we divide power by volts, that will tell us how much current is flowing per second. That gives us an electric current of 0.000033 amps. That's really tiny—for comparison, it's three times less current than you can get from of a stack of pennies. (It doesn't seem quite fair that a nickel battery is weaker than a penny battery.)

Remember, this electric current is just the stream of electrons from the decay of nickel atoms. Using the definition of amperes above, 0.000033 amps would mean we have 2.08 x 1014 electrons per second.

Why do we care about that? Because it tells us how much radioactive material we're using up. It means we'd need to convert 2.08 x 1014 nickel atoms to copper every second to produce that current. If we used that much current for 50 years, we'd consume 34.3 grams of nickel-63, which would be about 3.8 cm3 in volume, or roughly the size of a sugar cube. Sounds reasonable.

But remember, we can't maintain a constant level of current, because the rate of decay—and so the rate of energy production—declines exponentially over time. So yes, a small nuclear battery could last for 50 years. But it will be very weak, and it will get weaker the longer you use it.

No matter, you say! You'll probably want to upgrade your phone before then. Let's just aim for a 10-year phone. But there's a problem: Your phone requires way more than 100 microwatts of power. As an example, the iPhone 13 battery has a capacity of 3,240 mAh (milliamp-hours). This means it can produce an output of 3.24 amps for one hour. For one charge of the battery, that equates to 2.08 x 1019 electrons. Whether you spread this out over an hour or a day, that's how many electrons you'd use to drain the battery.

Of course, when it's in your pocket or sitting on a counter at night, it hardly draws any power. But that wouldn't be true with a betavoltaic battery. The rate of current flow would be determined solely by the decay rate of nickel-63 and how much of it you have—in other words, it's always on. And it would have to produce current at a rate that can power your most intensive tasks.

Your phone probably draws a current between 0.5 and 2 amps, depending on what you're doing. Let's say you want a betavoltaic battery that produces 1.5 amps for 10 years so you can play Pokémon Go whenever you want. That would require a grand total of 2.9 x 1027 electrons, which means you'd need to use up 309,000 grams of nickel-63. Yes, that's 680 pounds. Just to be clear, this battery would weigh as much as a female yak. In fact, you'd need more than that, because the decay rate shrinks over time. Maybe if you gave up games and streaming and just used the phone for basic stuff, you could get by with a goat's worth.

So, yes. Nuclear batteries are real, and they last for ages. But unless phones become massively more efficient, these batteries aren't going to be in the newest smartphones. And in general, they're not really suited to applications that have variable power needs. But I'm sure they'll find specialized use cases that require long life and low power—maybe things like remote sensors?