# How a Battery Charges

An anatomy\-style cutaway of a lithium\-ion cell charging: lithium ions cross inside, electrons go round the wire

By Numa

Topic: Energy

[Open the interactive Jig](https://ask.jig.com/j/Q5ZwHzdp8VDX)

## Guide

### 1. Meet the battery

This is a lithium\-ion cell, the kind that powers laptops, e\-bikes and power tools, resting in its charger\. Beside it stands a giant model of one tiny slice of its insides\. The dial shows the cell's voltage: low, because the cell is empty\.

Drag anywhere to turn the scene\. A real cell like this is 18 mm wide and 65 mm long\.

### 2. Inside the steel can

Under the steel can sits a jelly roll: two long, thin electrode strips wound up tightly with a plastic separator between them\. Unrolled, each strip can be up to about a metre long, which gives the lithium a huge area to move across\.

The strong round can holds the roll tight as the electrodes swell and shrink\.

### 3. Two homes for lithium

The model shows one slice of that roll, hugely enlarged\. On the right is the cathode, the positive side, made of a metal oxide\. On the left is the anode, the negative side, made of graphite: stacked layers of carbon\. Right now all the lithium ions sit in the cathode, so the battery is empty\.

Gold beads are lithium ions: lithium atoms that have given away one electron\. Blue beads are electrons\.

### 4. Press to charge

A charger pushes with a slightly higher voltage than the battery has\. Press Start charging, or the round button on the charger, and watch both kinds of bead begin to move\.

The round button on the charger works on every step\.

### 5. Two roads, one trip

Lithium ions leave the cathode and swim through the liquid electrolyte and the tiny pores of the separator\. Electrons cannot cross the separator, so the charger pulls them out of the cathode and pushes them round the wire into the anode\.

Ions travel inside the battery\. Electrons travel outside, through the wire\.

### 6. Tucked into graphite

At the anode each lithium ion slips in between two carbon layers and meets an electron there\. Packing lithium into the graphite is like pumping water uphill: it stores energy that the battery can give back later\.

Count the gold beads on the left: that is how full the battery is\.

### 7. Full: the charger eases off

Near full, the dial reaches 4\.2 volts, the safe limit for this kind of cell\. The charger then holds the voltage steady while the current, and the flow of beads, slows down\. When it has nearly stopped, charging ends\.

Chargers call this constant current, then constant voltage\.

### 8. Ready to power your day

The lithium now waits in the graphite with its stored energy\. When you switch on a device, the ions drift back to the cathode while electrons flow out through the device and do its work\. Then the battery is ready to charge again\.

Using a battery is charging in reverse: the same two roads, in the opposite direction\.