A typical phone battery stores about 15 to 20 watt-hours of energy. That's enough to light a 10-watt bulb for about two hours, but a careful phone can stretch it over a whole day.

The biggest energy users are the screen and the radios. Bright screens, weak signal and video calls all drain the battery faster than reading text.

Phones also sip power while they sleep. Checking for messages and keeping time uses tiny amounts, which is why a phone left alone for a week will still slowly run flat.

Electrons are far too small to see. Even a thin wire carrying a small current has billions of billions of them drifting along each second.

Electrons only flow if there's a full loop, called a circuit. Break the loop anywhere and the flow stops, which is how a switch works.

The charging cable is part of a loop too. While charging, the charger pushes electrons into the battery's minus end, the opposite way to normal use.

Lithium is the lightest metal there is. Being light and very eager to give up an electron makes it brilliant for packing lots of energy into a small, light battery.

The 'ion' in lithium-ion means a charged atom. We'll meet lithium ions properly once we shrink down to the size of atoms.

Because the same lithium is moved back and forth, the battery can be charged hundreds of times. Older one-use batteries burned their chemicals up once and were done.

The pouch wall is a sandwich itself: plastic on the outside, a thin aluminium foil in the middle to block water and air, and another plastic on the inside that melts to make the seal.

Pouches are light and can be made almost any flat shape, which is why phones moved to them. Laptops and electric cars often use metal cans instead.

If a battery is damaged or very old, gas can build up inside. The pouch then puffs up like a pillow. A swollen battery should be replaced, never pressed or punctured.

The minus tab is usually nickel or nickel-coated copper, and the plus tab is aluminium. Each one matches the foil it's welded to inside.

The tabs carry all the current the phone draws, up to a few amps when it's busy. That's why they're thicker and wider than the paper-thin foils they gather up.

A phone battery is just one cell. Laptops and electric cars join many cells together, tab to tab, to get more power and more energy.

The guard chip stops charging at about 4.2 to 4.4 volts and stops draining at about 3 volts. Going past either limit damages the battery from inside.

It also counts how much charge goes in and out. That's how your phone can show an exact percentage instead of just 'full' or 'empty'.

There's a temperature sensor too. If the battery gets too hot, the phone slows charging or stops altogether and shows a warning.

Graphite is made of pure carbon. Its atoms sit in flat sheets stacked on top of each other, with gaps just the right size for lithium to slip into.

The copper foil is about 6 to 10 thousandths of a millimetre thick, roughly ten times thinner than a human hair.

Copper is used because it carries electricity very well and doesn't react with lithium at the minus side's low voltage.

Most phone batteries use lithium cobalt oxide. Electric cars often use oxides with nickel and manganese, or a cheaper iron phosphate.

Aluminium is used on this side because it's light, carries electricity well, and copper would slowly dissolve at the plus side's high voltage.

John Goodenough found the cobalt oxide cathode in 1980. It roughly doubled the voltage of earlier lithium batteries, to almost 4 volts.

A phone battery has around 20 to 40 pairs of layers. Unfolded, the coated foils would cover an area about the size of a kitchen table.

Thin layers matter because lithium only has to travel a short way. Thicker layers would hold more, but the battery would be slower to charge and give out power.

Every gadget battery makes this same trade-off between packing in energy and giving it out fast.

Separators are usually made of polyethylene or polypropylene, the same plastics as shopping bags and food tubs, stretched until they're full of holes.

A typical one is about 10 to 20 thousandths of a millimetre thick. About half of it is empty space.

It has to be strong as well as thin. It must not tear while the layers are wound or stacked, or while they swell and shrink during charging.

The liquid isn't water. Water would split apart at the battery's voltage, so it's a carefully chosen mix of solvents that can handle about 4 volts.

There's very little of it, just a few drops spread through all the layers. It soaks into the tiny holes in the separator and the electrodes.

The liquid can burn if the battery overheats. That's one reason battery makers keep looking for solid electrolytes that won't catch fire.

This safety trick is called a shutdown. Once the holes close, lithium can't get through either, so the battery stops working before it gets hotter.

A bent, crushed or punctured battery is dangerous because the layers might touch. That's why you should never pierce or squash one.

Some separators have a thin ceramic coating too. Ceramic doesn't melt, so the wall keeps standing even if the plastic starts to soften.

Atoms are so small that about 10 million of them would fit across the full stop at the end of this sentence.

In graphite, each carbon atom holds on tightly to three neighbours in its own sheet, but the sheets themselves only hold each other loosely.

We've drawn the atoms as balls and sticks so you can see them. Real atoms are fuzzy clouds, with no hard edges.

An ion is any atom that has lost or gained electrons, so it carries an electric charge. The 'ion' in lithium-ion means these charged lithium atoms.

Lithium gives up its electron more easily than almost any other atom. That's what makes the battery's voltage high.

Lithium is also tiny and light, the third-lightest of all atoms, so a battery can hold a lot of it without being heavy.

Graphite swells by about a tenth when it's full of lithium, then shrinks back when the lithium leaves.

Earlier batteries used pure lithium metal for the minus side. It grew sharp spikes when recharged and could catch fire. Parking lithium inside graphite fixed that.

Stanley Whittingham first showed in the 1970s that lithium could slip in and out of layered materials this way. He shared the 2019 Nobel Prize for it.

The ions don't travel in a straight line. They bump and wander through the liquid, but overall they drift steadily from minus to plus.

Each trip is only a few hundredths of a millimetre long, but every second billions of billions of ions make it.

When lithium leaves graphite and settles into the oxide, energy is released. The battery hands that energy to the electrons, and they carry it out to the phone.

Voltage is how hard the battery pushes each electron. A bigger difference between the two sides' pull on lithium means a bigger voltage.

Current is how many electrons flow each second. A phone running a game draws more current than one showing the lock screen.

Power is the two together: voltage times current. A busy phone uses around 2 to 5 watts, and the battery supplies it all.

The ions and electrons must always arrive in step. If either path is blocked, the other stops too. That's why an open switch stops the lithium as well as the current.

As the oxide fills up with lithium, its pull gets weaker, so the voltage slowly drops from about 4.2 volts to about 3 volts.

Your phone's guard chip stops the battery at about 3 volts, even though there's a little lithium left, because draining further would damage the oxide.

The charger has to push a bit harder than the battery's own voltage, a little over 4 volts, or nothing would flow backwards.

A wall charger turns the high voltage from the socket into the low voltage a phone needs. Your phone then controls exactly how much goes into the battery.

Some energy is always lost as heat while charging. That's why phones and chargers get a little warm.

Nothing new goes into the battery when you charge it. The same lithium just goes back to the graphite side, for the hundredth or thousandth time.

Charging is slower than you might think because lithium has to squeeze between the sheets. Push too fast and it piles up at the surface instead.

That's why fast charging is fast at first, then slows down near the end. The last spots in the graphite are the hardest to fill.

Older rechargeable batteries changed their materials completely each time. That wore them out faster and made them heavier.

This rocking back and forth is why lithium-ion batteries are sometimes called 'rocking chair' batteries.

Each full empty-and-refill is called a cycle. Charging from 50% to 100% twice counts as one cycle, not two.

SEI stands for solid electrolyte interphase. It's made of bits of broken-down liquid stuck to the graphite surface.

The first crust forms at the factory, on the battery's very first charge. That's planned, and the first few per cent of lithium are spent making it.

Heat speeds up the crust growing. A phone left in a hot car ages much faster than one kept cool.

Spikes were the reason early lithium-metal batteries caught fire. Parking lithium safely inside graphite was the fix that made phone batteries possible.

Charging in the cold is risky because the lithium moves sluggishly. That's why phones limit charging when they're freezing.

Good chargers watch the temperature and slow down just enough to keep lithium slotting in neatly.

This runaway heating is called thermal runaway. It's rare, roughly one in tens of millions of cells, but it's why airlines ask you to keep batteries in the cabin, not in checked bags.

Most batteries are happiest at room temperature. Using them in the heat of summer sun, or charging under a pillow, ages them fastest.

Many phones now offer to stop charging at 80% overnight. A battery that isn't kept completely full stays cooler and gentler inside.