Batteries power our phones, laptops, watches, cars, toys and even the electric grid. They let us carry energy in our pockets and drive hundreds of miles without gasoline. Yet a battery seems mysterious: a sealed package that somehow produces electricity, slowly loses its strength and occasionally, in rare cases, catches fire. At its heart, a battery is a clever chemical device that controls a reaction so that electrons flow through a wire instead of directly between chemicals. This explainer describes how batteries work, why lithium-ion batteries dominate modern electronics and why batteries wear out over time.
Electricity is moving electrons
Electricity in a circuit is the flow of electrons, tiny negatively charged particles, through a conductor such as a copper wire. To make electrons flow, something has to push them. That push is called voltage, measured in volts. The amount of charge flowing per second is the current, measured in amperes.
A battery provides this push by using chemical reactions that release energy when electrons move from one substance to another.
The basic parts of a battery
Every battery cell has three main parts:
- Anode: The negative electrode, where a chemical reaction releases electrons during discharge.
- Cathode: The positive electrode, where a chemical reaction absorbs electrons during discharge.
- Electrolyte: A substance between the electrodes that allows charged atoms, called ions, to move between them, but blocks electrons.
A thin, porous material called a separator usually sits between the electrodes to keep them from touching and short-circuiting, while allowing ions to pass.
Strictly speaking, a "cell" is a single unit with one anode and one cathode, and a "battery" is a group of cells connected together. In everyday language, both are called batteries.
How a battery produces electricity
When you connect a device to a battery, you complete a circuit. Here is what happens:
- At the anode, a chemical reaction releases electrons.
- The electrons cannot travel through the electrolyte, so they flow through the external circuit, the wires and your device, toward the cathode. Along the way, they power your device.
- At the same time, ions move through the electrolyte inside the battery, balancing the charge.
- At the cathode, electrons and ions combine in another chemical reaction.
The key is the electrolyte: by blocking electrons while allowing ions to pass, it forces electrons to take the long way around, through your device. If the electrodes touched directly, the reaction would happen all at once inside the battery, releasing its energy as heat.
The voltage of a cell depends on the chemicals used for the electrodes. A standard alkaline cell produces about 1.5 volts. A lithium-ion cell produces about 3.6 to 3.7 volts on average. Cells can be connected in series to increase voltage or in parallel to increase capacity.
A brief history
In 1800, the Italian scientist Alessandro Volta built the first true battery, called the voltaic pile, by stacking discs of zinc and copper separated by cloth soaked in salt water. The unit of voltage, the volt, is named after him. Since then, chemists have developed many battery types with different materials, voltages and properties.
Primary and rechargeable batteries
Primary batteries are designed to be used once. Their chemical reactions cannot be easily reversed. Common examples include alkaline batteries in remote controls and flashlights, and lithium coin cells in watches.
Rechargeable batteries, also called secondary batteries, can be recharged many times. When you plug them into a charger, an external power source pushes electrons back in the opposite direction, reversing the chemical reactions and restoring the battery's energy.
| Battery type | Typical use | Notes |
|---|---|---|
| Alkaline | remote controls, toys, flashlights | inexpensive, single use |
| Lead-acid | car starter batteries, backup power | heavy, cheap, reliable for high bursts of current |
| Nickel-metal hydride (NiMH) | rechargeable AA batteries, older hybrids | safer chemistry, moderate energy density |
| Lithium-ion | phones, laptops, electric vehicles, grid storage | high energy density, rechargeable |
| Lithium iron phosphate (LFP) | many electric vehicles, home storage | a lithium-ion type; long life, very stable, slightly lower energy density |
How lithium-ion batteries work
Lithium-ion batteries power most of today's portable electronics and electric cars. They store a lot of energy for their weight and can be recharged hundreds or thousands of times.
The parts
- Anode: Usually graphite, a form of carbon arranged in layers.
- Cathode: A metal oxide containing lithium, such as lithium cobalt oxide, lithium nickel manganese cobalt oxide or lithium iron phosphate.
- Electrolyte: A lithium salt dissolved in an organic solvent.
- Separator: A thin plastic membrane with tiny pores.
Charging and discharging
Lithium-ion batteries work by moving lithium ions back and forth between the electrodes, a design sometimes called a "rocking chair" battery.
- During charging, the charger pushes electrons into the anode. Lithium ions move through the electrolyte from the cathode to the anode, where they slip between the graphite layers, a process called intercalation.
- During discharge, lithium ions move back from the anode to the cathode through the electrolyte, while electrons flow through the external circuit, powering your device.
The 2019 Nobel Prize in Chemistry was awarded to John Goodenough, Stanley Whittingham and Akira Yoshino for developing the lithium-ion battery.
Capacity and energy
A battery's capacity is often given in milliampere-hours (mAh) for small devices, or watt-hours (Wh) and kilowatt-hours (kWh) for larger ones. Watt-hours measure energy, combining capacity and voltage. A typical smartphone battery holds about 15 to 20 watt-hours, a laptop battery about 50 to 100 watt-hours and an electric car battery about 50 to 100 kilowatt-hours or more.
Energy density describes how much energy a battery stores for its weight or volume. Lithium-ion batteries have much higher energy density than older types, which is why they made smartphones and electric cars practical.
Why batteries wear out
Over time, rechargeable batteries lose capacity. A phone that once lasted all day may need charging by the afternoon after a few years. This is caused by several processes:
- Solid electrolyte interphase (SEI): A thin layer forms on the anode during the first charges. It protects the battery but slowly grows, consuming lithium and electrolyte.
- Lithium plating: Charging very fast or at low temperatures can cause lithium metal to deposit on the anode surface instead of entering the graphite, reducing capacity and potentially creating safety risks.
- Structural changes: Repeated expansion and contraction of electrode materials during charging cycles can cause cracks and degrade them.
- Heat: High temperatures speed up the unwanted chemical reactions that degrade a battery.
Tips for longer battery life
- Avoid extreme heat. Do not leave devices in hot cars or in direct sunlight.
- Avoid staying at 100 percent or 0 percent for long periods. Lithium-ion batteries age faster when kept fully charged or fully discharged for long times. Many devices offer optimized or limited charging features that stop at around 80 percent.
- Avoid deep discharges when possible. Frequent partial charges are fine for lithium-ion batteries.
- Use good-quality chargers designed for your device.
- Store batteries partly charged, around 40 to 60 percent, in a cool place if you will not use them for a while.
Older advice about "memory effect," fully draining batteries before recharging, applied to older nickel-cadmium batteries, not to modern lithium-ion batteries.
Battery safety
Lithium-ion batteries are generally safe, but because they store a lot of energy in a small space and contain flammable electrolytes, they can be dangerous if damaged or defective. Problems can cause thermal runaway, a chain reaction in which heat triggers reactions that produce more heat, possibly leading to fire or explosion.
Causes include manufacturing defects, physical damage like punctures or crushing, overcharging with faulty chargers, and exposure to high temperatures. Built-in battery management systems monitor voltage, current and temperature to keep batteries within safe limits.
To stay safe: do not use swollen or damaged batteries, avoid cheap uncertified chargers, do not charge devices under pillows or on beds, and recycle old batteries properly rather than throwing them in household trash, where they can cause fires in garbage trucks and recycling facilities.
The future of batteries
Researchers are developing new battery technologies:
- Solid-state batteries replace the liquid electrolyte with a solid material, potentially offering higher energy density and improved safety.
- Sodium-ion batteries use abundant sodium instead of lithium, potentially reducing costs, and are beginning to appear in some vehicles and storage systems.
- Silicon anodes can store more lithium than graphite, increasing capacity.
- Grid-scale storage, including large lithium-ion installations and flow batteries, helps store renewable energy from solar and wind for use when needed.
Questions readers ask
What is inside a battery? An anode, a cathode, an electrolyte and a separator. Chemical reactions at the electrodes release and absorb electrons, which flow through an external circuit.
Why do batteries lose capacity over time? Unwanted chemical reactions, structural changes in the electrodes and heat gradually reduce the amount of lithium and active material available.
Is it bad to charge my phone overnight? Modern phones manage charging to reduce stress, and many have optimized charging features. Avoiding heat matters more than the time of day you charge.
Why do lithium-ion batteries sometimes catch fire? Damage, defects or overheating can cause thermal runaway, a chain reaction that releases heat and can ignite the flammable electrolyte.
Can batteries be recycled? Yes. Many materials in batteries, including lithium, cobalt, nickel and copper, can be recovered. Use designated battery recycling points.
In short
A battery converts chemical energy into electrical energy. At the anode, reactions release electrons that flow through your device to the cathode, while ions move through the electrolyte inside the battery to balance the charge. Rechargeable lithium-ion batteries shuttle lithium ions between a graphite anode and a metal oxide cathode, offering high energy density for phones and electric cars. Over time, side reactions, structural changes and heat reduce capacity, and damaged batteries can overheat. Gentle charging habits, avoiding heat and recycling old batteries keep them safer and longer-lasting, while new chemistries promise even better batteries in the future.