How Lithium-Ion Batteries Work: Charging, Aging and Safety
A lithium-ion battery stores energy in chemical form and can be used again and again. When you use it, it sends electrons out into a circuit. Inside, lithium ions travel through a liquid, back and forth between a graphite negative electrode and a metal oxide positive electrode. It is light and holds a lot of energy. But damage or heat can make it catch fire, so chips watch over it all the time.
What you'll learn
- What a battery does, and what happens inside a lithium-ion battery
- Why it is light yet holds so much, and the difference between mAh and Wh
- How charging works (constant current, then constant voltage), and what the chips that watch the battery do
- Why batteries wear out, and how to use them to avoid thermal runaway
Prerequisites: a rough idea of what voltage and current are (basic school science). There is only one formula, a single multiplication.
Where you see it
Phones, laptops, wireless earbuds and electric cars all use them. Most batteries you recharge and use many times are lithium-ion batteries. A power bank holds the same kind of battery inside.
Power banks catching fire, and the rules for taking them on planes, often make the news. To understand both the benefits and the risks, it helps to start with how they work inside.
What a battery does
A battery does not hold electricity itself in a box. When the materials inside react chemically, they release energy. A battery is a device that takes that energy out as electricity. As the reaction goes on, electrons flow through the circuit outside the battery. This flow of electrons is the current that runs your phone.
Some batteries, like AA alkaline cells, are done once they run out. These are called primary batteries. Others can be reused: push current through them the other way, and the reaction runs backward. These are called secondary (rechargeable) batteries, and lithium-ion batteries are one kind. Taking electricity out is called discharging, and storing it again is called charging.
A capacitor is a similar part, but it uses no chemical reaction. It simply holds electric charge on two facing plates ("What Is a Capacitor?"). A capacitor can take in and give out charge in an instant. But a battery can store far more energy.
How it works 1: the four parts inside
Inside the battery are two plates where electricity goes in and out, called electrodes. The minus side is the negative electrode, and the plus side is the positive electrode. The negative electrode is usually graphite. Graphite is a form of carbon, also used in pencil lead, and is made of thin stacked layers.
The positive electrode is a metal compound that contains lithium. A typical one is a metal oxide: a metal such as cobalt or nickel, bonded with oxygen and lithium. There is also a material made with iron and phosphorus, called lithium iron phosphate.
The space between the two electrodes is filled with a liquid called the electrolyte. The electrolyte lets lithium ions through but not electrons. It is a lithium salt dissolved in a flammable liquid called an organic solvent. This comes from a document by NITE, the Japanese government agency that tests product safety.
If the electrodes touched, a short circuit (a shortcut for electricity) would form inside the battery. A thin plastic film called the separator prevents this. It is full of holes too small to see, and ions can pass through them.
How it works 2: what happens during discharge and charge
In a fully charged battery, most of the lithium sits in the gaps between the graphite layers of the negative electrode. For lithium, this is a place it was forced into. Moving over to the positive side lowers its energy, so it settles down there.
When you connect a phone, lithium at the negative electrode gives up one electron and becomes a lithium ion. The ion passes through the electrolyte and the separator to the positive electrode. The freed electron cannot pass through the electrolyte. So it travels around the outside circuit to the positive electrode. This flow of electrons runs the phone.
At the positive electrode, the ion slips into the gaps in the metal oxide. The metal oxide also takes in the electron that came around the outside. The key point: ions take the short path inside, and electrons take the long way outside. If electrons could pass through the inside, you could not get any current out.
During charging, the charger applies a voltage from outside and pushes this flow backward. Lithium ions return from the positive electrode to the negative one. Electrons also return to the negative electrode through the outside circuit. Ideally, no lithium is lost; it just moves back and forth between the two sides.
Why they are used so widely
The first reason is that they store a lot of energy for their weight. Energy is measured in Wh (watt-hours). 1 Wh is the energy to use 1 watt of power for one hour. How many Wh a battery stores per kilogram is called its energy density by weight.
| Battery type | Energy per kilogram (rough) |
|---|---|
| Lead acid | 30–50 Wh |
| Nickel-cadmium | 45–80 Wh |
| Nickel-metal hydride | 60–120 Wh |
| Lithium-ion | 70–260 Wh (depends on materials; the type common in phones is 150–200 Wh) |
The figures come from tables on Battery University, a battery explainer site. The pages are undated, so they were checked in October 2026. They are values for a single cell, and they vary by product and year.
The second reason is the high voltage of one cell. A nickel-metal hydride cell gives 1.2 V, while a lithium-ion cell gives about 3.6–3.7 V. The exact figure depends on the product label (Battery University). One cell gives about the voltage of three nickel-metal hydride cells, so a phone can run on a single cell.
mAh and Wh: two kinds of "capacity"
Battery specs often show a number like "4,000 mAh." mAh (milliamp-hours) tells you how many milliamps of current can flow for how many hours. 4,000 mAh is enough for 400 mA for 10 hours, or 4,000 mA for one hour.
But mAh is an amount of electric charge, not an amount of energy. The same charge can do more work when it is pushed out at a higher voltage. To compare energy, use Wh.
Converting takes one multiplication. Multiply the capacity in Ah by the voltage, and you get Wh. For example, a 4,000 mAh (4 Ah) battery labeled 3.7 V holds 4 × 3.7 = about 15 Wh.
Now try a power bank. With 10,000 mAh and 3.7 V cells inside, it holds about 37 Wh. That works out to about 2.5 charges of a 15 Wh phone. In practice you get fewer, because circuits such as the one that converts to USB's 5 V turn some energy into heat. Comparing mAh to mAh only works when the voltages are the same.
How charging works: constant current first, constant voltage last
You cannot charge a lithium-ion battery by simply pouring current into it. In most cases, charging has two stages. In the first stage, a current of a fixed size keeps flowing. This is called constant-current (CC) charging.
As current flows, the battery's voltage slowly rises. When it reaches the upper limit (4.20 V for a typical cell), charging switches to the second stage. Now the voltage is held at that value, and the current that flows in is left to fall on its own. This is called constant-voltage (CV) charging (Battery University, Texas Instruments).
Once the current is small enough, charging stops. Keeping current flowing, or pushing past the limit, damages the battery and can also be dangerous (Battery University).
Apple also says it charges iPhone batteries fast up to 80%, then more slowly after that. If charging feels slow at the very end, this second stage is the main reason.
Chips that watch the battery
Semiconductor chips follow these steps and watch the battery's condition. Around a phone's battery, several chips with different jobs are at work.
Protection IC: cuts the power when something goes wrong
Inside the battery pack is a small chip called the protection IC. It watches for trouble: voltage too high, voltage too low, or current too large (Texas Instruments). These are called overcharge, overdischarge and overcurrent.
When it finds a problem, it turns off two switches in the path of the current. The switches are transistors that turn current on and off when a voltage is applied (power MOSFETs). One stops current in the charging direction, and the other stops it in the discharging direction (ABLIC, a maker of protection ICs).
Fuel gauge IC: estimates what is left
A number on the screen like "63% left" is calculated by the fuel gauge IC. It is named after the fuel gauge in a car. The basic method is to count the current going in and out of the battery over time. Counting alone lets errors build up. So the gauge corrects itself using the voltage while the battery is resting (Texas Instruments).
Charger IC: follows the charging steps
The charger IC takes power coming in from USB or another source. It shapes it into the current and voltage the battery needs. Then it charges using the two-stage steps described above. Many charger ICs also slow or stop charging when the battery is hot.
The chip that hands out power, and BMS in electric cars
The power management IC (PMIC) converts power from the battery to the voltage each chip needs and sends it out. "What Chips Are Inside a Smartphone?" explains how power flows through the whole phone.
Electric cars carry batteries made of many cells connected together. They need a system that watches each cell's voltage and temperature and evens out the differences in charge. This is called a BMS (battery management system) (Texas Instruments).
Why batteries wear out
A phone used for several years does not last as long as it used to, even when fully charged. Every time the battery charges and discharges, small unwanted reactions happen on the electrode surfaces. As a result, the lithium that can move back and forth decreases, and resistance to current grows (Battery University). This is simplified, but three main conditions speed up wear.
1. The number of charges and discharges
Apple counts one cycle each time you use 100% of the capacity, and calls this a charge cycle. Using 50% twice adds up to one cycle. iPhone 14 and earlier are designed to keep 80% of their capacity after 500 cycles. The iPhone 15 lineup is designed for the same after 1,000 cycles. These are values under ideal conditions, and the 1,000 figure was published in 2024.
2. Heat
According to Apple, the ideal surrounding temperature for an iPhone and similar devices is 16–22 °C. Leaving one somewhere above 35 °C can damage its capacity for good. In Japan, for example, NITE also warns people not to leave batteries in hot places in summer.
3. Sitting fully charged for a long time
The longer a battery stays near its voltage limit, the faster it wears. Heat makes this go even faster (Battery University). The iPhone's "Optimized Battery Charging" helps here. When it predicts a long connection, it waits at 80% and reaches 100% around the time you unplug. This shortens the time spent fully charged (Apple).
Safety: what is thermal runaway?
When a battery catches fire, a process called thermal runaway is happening. Heat is made inside the battery faster than it can escape, so the temperature keeps rising. Higher temperature speeds up the reactions, which make even more heat. It is a vicious cycle (ULRI, a US safety research organization).
According to ULRI, thermal runaway can go above 300 °C. Gas and smoke can burst out, and fire can follow. The electrolyte inside is a flammable liquid, so once it starts burning, it is hard to stop. In large batteries, heat from one cell can spread to the next, one after another.
The most common trigger is a short circuit inside the battery. A hard hit, crushing or puncturing can tear the separator, so the positive and negative electrodes touch directly. Then the stored energy pours out as heat all at once. Heating from outside, and charging past the limit, can also be triggers (ULRI).
In Japan, for example, NITE asks people not to apply strong shocks or force. It also asks people not to use or leave batteries in hot places. Never press a swollen battery to flatten it. In one case, a fire started when someone did this (Sendai City Consumer Center). If you notice anything wrong, stop using and charging it, and contact the store or the maker.
When you fly
Air travel has its own rules. In Japan, for example, power banks and spare batteries cannot go in checked baggage. From April 24, 2026, each person may carry up to two power banks into the cabin. Each must be 160 Wh or less (Ministry of Land, Infrastructure, Transport and Tourism). On board, charging a power bank, or charging a phone or other device from one, is now banned. Rules can change, so check your airline's guidance before you fly.
The road to the Nobel Prize
In the 1970s, Stanley Whittingham laid the foundation for rechargeable batteries that use lithium. In 1980, John Goodenough showed that a lithium cobalt oxide positive electrode gives about 4 V (Nobel Prize announcement).
In 1985, Akira Yoshino of Asahi Kasei used a carbon material for the negative electrode. He paired it with a lithium cobalt oxide positive electrode. This avoided using lithium metal, which reacts easily, as the negative electrode. It is the basic structure of today's batteries (Asahi Kasei). The first product came out in 1991, when Sony began mass production. In 2019, the three received the Nobel Prize in Chemistry "for the development of lithium-ion batteries."
Common myths
Myth: If you charge before the battery is empty, its capacity shrinks
In fact, this is the "memory effect," known from nickel-cadmium batteries. With repeated top-up charging, the usable amount seems to drop. It also happens, to a smaller degree, in nickel-metal hydride batteries. Lithium-ion batteries have no such effect. Deep discharges that run them empty are what add stress (Battery University). The myth is a leftover from older rechargeable batteries.
Myth: Charging overnight always damages the battery by itself
In fact, the charger IC stops charging once the battery is full, so current is not forced in all night. Still, staying near full for a long time, and heat, do speed up wear. That is why recent phones have a feature that waits at 80% when they predict a long charge. It also matters not to charge where heat builds up, such as under a blanket.
Myth: The battery "%" is a direct measurement of what is inside
In fact, it is an estimate. The fuel gauge IC works it out from the current going in and out and from the voltage. As a battery ages, the amount it can hold when full also drops, so the basis of the calculation changes. The gauge corrects itself as you use it. The number sometimes jumps suddenly because it is an estimate.
By the numbers
| Number | What it is |
|---|---|
| About 3.6–3.7 V / 4.20 V | Standard voltage of one cell / charging limit of a typical cell (Battery University) |
| 500 / 1,000 cycles | Charge cycles a battery is designed to last while keeping 80% of capacity: iPhone 14 and earlier / iPhone 15 lineup (Apple, ideal conditions) |
| Over 300 °C | Temperature thermal runaway can reach (ULRI) |
| 160 Wh | Upper limit for each power bank carried into the cabin, in Japan (Ministry of Land, Infrastructure, Transport and Tourism; rule from April 24, 2026) |
| 1985 / 1991 / 2019 | Basic structure completed / first product / Nobel Prize in Chemistry |
The phone battery in the example above held about 15 Wh. The 160 Wh cabin limit is roughly ten of those.
Learn more
- Related reading: "What Is a Power Semiconductor?" (switches that turn power on and off, inside chargers), "What Is a Capacitor?" (how it differs from a battery), "What Chips Are Inside a Smartphone?" (PMICs and charging)
- Compare in a table: series vs parallel circuits (voltage and capacity when you connect batteries) / LDO vs switching regulator / buck vs boost converter
- Try the numbers: DC-DC converter calculator (intermediate)
- Terms for the next step: DC-DC converter, LDO, quiescent current (affects how long battery-powered devices last on standby)