Actually,How the World Really Works

Actually,
You Can’t “Fill” a Battery

When you charge a phone, it feels like electricity is piling up in the battery. But the number of tiny bits of electricity inside is exactly the same at 0% and at 100%. Let’s look inside and see for ourselves!

Let’s go

1First, what’s “electricity”?

Electricity is made of tiny bits you can’t see. These bits are called “electrons,” and each one is a minus bit. When electrons move along a wire, we say “electricity is flowing.”

Inside a phone battery, these minus electrons and plus bits called “lithium ions” come in equal numbers. Plus and minus pull on each other. But inside a battery they don’t stick together. Each one sits in its own seat.

lithium ion (plus) electron (minus)

Grab either one with your finger and try to pull them apart.

2Two rooms inside a battery

A battery has two rooms. On the left is the carbon room (−), and on the right is the cobalt room (+). Between them is a liquid called the “electrolyte.” Each room has seats for lithium ions (in the gaps between sheets) and seats for electrons.

Pick a room and compare how the electrons look.

100%

Wow! The carbon room is made of the same kind of carbon that’s in your pencil lead.

3What does charging really do?

The charger pulls electrons out of their comfortable cobalt seats and pushes them into the uncomfortable carbon seats. The electrons travel along the wire outside, and lithium ions follow through the electrolyte to keep the numbers even.

The electricity from the wall outlet is used to do this pushing. The outlet’s own electrons don’t go into the battery and pile up there.

Tap the USB plug or “Charge,” and keep an eye on the electrons inside the battery.

Wow! A battery at 100% and a battery at 0% hold exactly the same number of electrons. Charging doesn’t store up electricity. It moves electrons into uncomfortable seats.

4Why take the outside path?

Electrons want to go home to the comfortable cobalt seats. But the electrolyte is a wall that only lets lithium ions through. So the only way home for an electron is the outside path: the wires inside your phone.

Tap the switch to turn it on and off.

Wow! This is why a phone you’re not using doesn’t drain fast. If the outside path isn’t connected, neither electrons nor lithium ions can move.

5What really runs your phone

Along the outside path are the screen and the phone’s tiny computer. On their way home to the comfortable seats, electrons pass through them, and that trip is what lights up and runs your phone.

Pick a way to use the phone and compare how fast the battery goes down. When it’s low, tap the USB plug or “Charge” to charge it.

Wow! Electrons in a wire are actually slow. Most of the time they move less than a millimeter a second. Your phone still turns on right away, because the whole line of electrons in the wire gets pushed at once.

6Summary and quiz

Summary

Electricity is made of tiny bits called “electrons.”
Inside a battery, lithium ions and electrons pull on each other but don’t stick. Each sits in its own seat.
Charging doesn’t add any electrons to a battery. One goes out for every one that comes in.
Charging moves electrons into the uncomfortable carbon seats. Lithium ions follow.
When you use the phone, electrons go home to the comfortable cobalt seats along the outside path, and lithium ions follow through the electrolyte. That trip home is what runs your phone.

Quiz

Actually,

For parents (a note on the physics and chemistry)

For young readers, this page first introduces electrons as “tiny bits of electricity” and uses “electricity” to mean electric charge (the number of electrons). “When electrons move along a wire, we say electricity is flowing” refers to electric current; by convention, current is defined as flowing opposite to the electrons’ motion. The title means that charging doesn’t change a battery’s total charge: for every electron pushed in, one leaves. Energy, on the other hand, really is stored.

Inside the battery, lithium ions and electrons are not bound together into lithium metal. Lithium ions slip between the carbon layers of the graphite anode (the carbon room) and between the cobalt oxide layers of the lithium cobalt oxide cathode (the cobalt room), staying as ions (intercalation). The electrons are spread through the graphite layers in the anode, and are taken up by cobalt in the cathode (cobalt’s oxidation state changes). The page’s “separate seats” stand for this. Because plus and minus attract, each electrode holds equal numbers of ions and electrons, but they don’t stick together. If they actually combine and lithium metal plates out (for example during fast charging or in the cold), it degrades the battery and can cause safety problems.

The stored energy is the difference in chemical energy between lithium in the anode and lithium in the cathode (about 3.7 volts). Most of it can be understood as the difference in where the electron sits: an electron in graphite has higher energy than one taken up by cobalt. That’s what the page means by “cobalt is a comfortable seat for electrons.” What builds up isn’t an amount of electricity, but energy held in how things are arranged.

About chapter 3’s “the outlet’s electrons don’t go into the battery”: inside a charger (AC adapter), the outlet side and the phone side are electrically isolated, so no electrons from the outlet flow into the phone. The energy taken from the outlet is used by the phone’s charging circuit to pull electrons out of the cathode and push them into the anode, and most of it is stored as chemical energy (some becomes heat). The number of electrons in the battery is the same before and after charging.

During discharge, lithium ions and electrons move at the same time. The electrolyte (and separator) only lets ions through, so electrons take the external circuit. If either one got ahead, a charge imbalance would build up and stop it almost immediately. That’s why, when the circuit is open, the ions can’t move either, and the battery doesn’t drain (apart from slight self-discharge). Individual electrons in a wire drift very slowly (typically under a millimeter per second); the energy travels almost instantly through the electric field. The animation shows this as the row of electrons in the wire being nudged along together.

Besides lithium cobalt oxide, cathodes can also contain nickel and manganese, or be made of lithium iron phosphate. And a real “0%” is the point where a protection circuit stops the discharge to keep the battery healthy, so not all of the lithium returns to the cathode. “Comfortable” and “uncomfortable” are friendly stand-ins for lower and higher energy.