How Do Batteries Actually Work?

A battery is a sealed chemical reaction that has been given an escape route for its electrons. Two materials that want to swap electrons are kept apart, and the only easy path between them runs through your phone, your flashlight, or your car's motor. Alessandro Volta showed in 1800 that this could be done on demand. Today the same idea, refined into lithium-ion cells, powers phones, laptops, and electric vehicles, and the 2024 global average price of a battery pack was $115 per kilowatt-hour.

Key takeaways

• A battery converts stored chemical energy into electrical energy through redox (reduction-oxidation) reactions, in which one material gives up electrons and another accepts them.
• Every cell has two electrodes, an electrolyte that carries ions, and usually a separator that keeps the electrodes from touching.
• Electrons travel through the outside circuit, and ions travel through the electrolyte. Neither path alone makes a battery work.
• Lithium-ion cells dominate because lithium is the lightest metal and has the highest electrochemical potential, which gives high cell voltage and high energy per kilogram.
• Cells age through predictable mechanisms, and heat and high voltage speed all of them up.

The basic principle: a controlled electron escape

Everyday chemistry already involves electrons changing hands. Iron rusts when it gives electrons to oxygen. A battery arranges the same kind of exchange so that the electrons take a detour you control.

Consider two metals, zinc and copper. Zinc atoms give up electrons more readily than copper atoms do. If you put a zinc strip in one solution and a copper strip in another, the zinc side tends to push electrons out while the copper side tends to take them in. The reaction can only proceed if electrons have somewhere to travel, and if the charge that builds up on each side is neutralized by ions moving through the liquid between them.

A zinc and copper cell, step by step

The half-reactions are standard textbook chemistry. At the zinc electrode, oxidation occurs: Zn → Zn²⁺ + 2e⁻. At the copper electrode, reduction occurs: Cu²⁺ + 2e⁻ → Cu. Connect the two electrodes with a wire and electrons flow from the zinc to the copper. That flow is the electric current.

Inside the cell, the ions do their own job. Zinc ions (Zn²⁺) enter the solution at one electrode, and copper ions leave the solution at the other. To keep each side electrically neutral, other ions migrate across the liquid junction or through a salt bridge, which is a conductive link that closes the circuit. Chemistry teaching materials describe it plainly: the salt bridge must be present to complete the circuit, with negatively charged ions drifting toward the anode and positively charged ions toward the cathode.

This arrangement was refined by John Frederic Daniell in 1836, and the Daniell cell kept the zinc and copper pairing but used separate solutions of zinc sulfate and copper sulfate. A review of the cell reports an open-circuit potential of about 1.07 volts, which is why textbooks round it to roughly 1.1 V. That voltage is set by the chemistry of the two materials, not by the size of the cell. A bigger zinc strip can supply current for longer, but it cannot push electrons harder.

The analogy: a dam and a corridor

Two analogies help. First, voltage behaves like the height of a dam. The higher the water sits, the more energy each liter releases as it falls, and the higher the cell voltage, the more energy each electron delivers as it moves through your device. Current behaves like the flow rate through the turbine.

Second, picture a stadium with two seating sections and a one-way turnstile arrangement. People (electrons) want to move from one section to the other, but the turnstile only lets them pass through the wire outside the building. Meanwhile, a service tunnel inside (the electrolyte) lets staff (ions) walk between sections to keep the crowds balanced. Block either route and nothing moves. This is why a battery whose electrolyte has dried out stops working even though its electrodes are fine.

When a battery is "dead"

A battery is not empty of charge in the way a bucket is empty of water. It is a chemical system that has been driven toward equilibrium. As the reaction proceeds, the reactants that supplied the energy are used up, and the difference in chemical potential between the electrodes shrinks. When the difference is gone, so is the voltage. In a rechargeable cell, an outside power source can push the reaction backward and rebuild the difference. In most single-use cells, the reactions are not reversible enough to try.

Where it started: frogs, piles, and the first continuous current

The story of the battery begins with a dissected frog. In the 1780s, Luigi Galvani saw frog legs twitch when touched by two different metals, and he concluded the tissue contained "animal electricity." Volta disagreed, arguing the metals themselves were the source. His answer was the voltaic pile, a stack of zinc and copper discs separated by brine-soaked cloth or card. In a letter to the Royal Society in March 1800, he announced the device, and it gave the world its first source of steady electric current. The frog-leg story is told in Galvani's frog leg experiment, and the first cell's chemistry is covered in Volta's battery and the electrochemistry behind it.

Volta's piles had a problem called polarization, where bubbles of hydrogen built up on the copper and the current weakened. Daniell's 1836 cell, with separate solutions for each electrode, largely fixed that problem. From there the history runs through lead-acid cells, the dry cell, and nickel-based chemistries to the lithium-ion cell, a path summarized in Battery Invention: From Volta Pile to Lithium-Ion.

The four parts of every cell

Every battery, from a coin cell to a grid-scale storage container, has the same four functional parts.

Anode

The anode is the electrode where oxidation happens when the cell is discharging. It releases electrons into the outside circuit. In an alkaline AA cell, the anode is zinc. In a lithium-ion cell, it is typically graphite holding lithium.

Cathode

The cathode is where reduction happens during discharge. It accepts electrons from the outside circuit. In an alkaline cell, the cathode is manganese dioxide. In a lithium-ion cell, it is a metal oxide or phosphate that lithium ions can slip into.

A common confusion is that the labels "anode" and "cathode" are defined by what happens during discharge. In a rechargeable cell, the reactions reverse during charging, so the electrode that was the anode technically acts as a cathode while charging. Battery makers keep the discharge labels for simplicity.

Electrolyte

The electrolyte is the medium that lets ions move between electrodes while refusing to let electrons take the same shortcut. It can be a liquid (alkaline solution, sulfuric acid, an organic solvent with dissolved lithium salt), a gel, or a solid. Because electrons cannot cross it, they are forced through the outside circuit, which is where the work gets done.

Separator

The separator is a thin, porous sheet, usually a polymer, between the electrodes. It keeps them from touching, since direct contact would create an internal short circuit, but it lets ions pass through its pores. A damaged or flawed separator is one of the routes to battery failure, which is why separator quality is a major design focus.

The main battery types and what they trade off

Different applications weigh cost, weight, safety, lifetime, and power in different ways, so several chemistries survive side by side. The voltages below are nominal values, meaning typical operating voltage, and not guarantees.

Chemistry	Nominal cell voltage	Rechargeable	Typical use	Main trade-off
Alkaline (zinc and manganese dioxide)	About 1.5 V	Generally no	Remote controls, flashlights	Cheap and long shelf life, but modest energy and single use
Lead-acid	About 2 V (six cells make a 12 V car battery)	Yes	Engine starting, backup power	Rugged and cheap per unit energy, but heavy, and lead is toxic
Nickel-cadmium (NiCd)	About 1.2 V	Yes	Older power tools, legacy devices	Tough, but cadmium is toxic and the cells can develop a "memory effect"
Nickel-metal hydride (NiMH)	About 1.2 V	Yes	Rechargeable AAs, some hybrid cars	Less toxic than NiCd, but lower energy density than lithium-ion
Lithium-ion	About 3.6 V	Yes	Phones, laptops, EVs, storage	High energy density, but needs electronic management and can fail dangerously if damaged
Lithium iron phosphate (LFP)	About 3.2 V	Yes	EVs, stationary storage	Long life and good stability, with somewhat lower energy per kilogram

Battery University's reference material gives the common nominal voltage for nickel-based cells as 1.20 V and for lithium-ion as 3.60 V, and says the multi-metal lithium-ion chemistries reach specific energy of up to 250 watt-hours per kilogram (Wh/kg). Specific energy, the energy stored per unit of mass, is the number that decides how heavy a battery must be to do a given job.

Why voltage matters more than it looks

A 3.6-volt lithium-ion cell delivers about the same voltage as three NiMH cells in series. That sounds like a minor convenience, but it means a phone can be built with one cell where a nickel-based design needed three. Fewer cells means fewer connections, less casing, and less weight. Battery University calls the high cell voltage the key to lithium-ion's superior specific energy.

Why lithium-ion won

Lithium has three properties that suit batteries. It is the lightest metal, so each atom of lithium that carries a charge adds little mass. It has the highest electrochemical potential among the metals, which translates into high voltage. And lithium ions are small, so they can fit into the gaps in other materials.

Early lithium batteries used lithium metal for one electrode, which provided excellent energy but grew needle-like deposits called dendrites during recharging. Dendrites can pierce the separator, cause short circuits, and trigger thermal runaway, the self-heating chain reaction that is the worst failure mode of a lithium cell. Researchers solved the problem by changing the approach: instead of using lithium metal, they used lithium ions that shuttle in and out of host materials. Sony commercialized that design in 1991, according to Battery University.

The 2019 Nobel Prize in Chemistry went to John Goodenough, M. Stanley Whittingham, and Akira Yoshino "for the development of lithium-ion batteries." Whittingham's work in the 1970s laid the foundation, Goodenough's work on cathode materials raised the voltage, and Yoshino built the first commercially viable cell. The longer story is in The Lithium-Ion Battery: How Three Scientists Powered the Wireless Age.

How a lithium-ion cell works, step by step

The best way to picture a lithium-ion cell is as a pair of bookshelves with a library between them. Graphite in the anode is built of stacked layers, and lithium ions can slot between the layers like books on a shelf. The cathode has its own shelves, in the crystal structure of a metal oxide or phosphate. Inserting ions into such a structure without destroying it is called intercalation, and it is why the cell can be recharged thousands of times without the electrodes dissolving.

When discharging, lithium ions leave the graphite and travel through the electrolyte and separator to the cathode. For each ion that moves, an electron travels through the outside circuit to meet it. The anode is oxidized and the cathode is reduced. When charging, a charger applies a voltage that pushes everything in reverse: ions go back to the graphite, electrons go back to the anode, and the cell's chemical potential is rebuilt.

During normal operation, there is no bulk lithium metal in the cell. All the lithium is in ionic form or sitting inside electrode structures. That is why the name "lithium-ion" matters: it signals a design that avoids the dendrite problem of lithium-metal cells.

The cell's nominal voltage of 3.6 V sits in a usable window of roughly 3.7 to 2.8 V per cell, according to Battery University, and protection circuits typically cut off the cell below about 2.2 V and above 4.3 V. That narrow window is why a lithium-ion pack always includes a battery management system, a small computer that monitors every cell group and cuts the power if anything strays outside safe limits. The technology is walked through at greater depth in How Lithium-Ion Batteries Work.

Energy, power, and charging speed

People often use "capacity," "energy," and "power" interchangeably, but they measure different things.

• Capacity is usually stated in ampere-hours (Ah) or milliampere-hours (mAh) and describes how much charge the cell holds.
• Energy is capacity multiplied by voltage, stated in watt-hours (Wh) or kilowatt-hours (kWh).
• Power is how quickly energy can be delivered, stated in watts or kilowatts.

A worked example: a phone battery rated 5,000 mAh at a nominal 3.7 V holds 5 Ah × 3.7 V = 18.5 Wh of energy. If the phone draws an average of 2 W, the battery lasts about 9 hours (18.5 ÷ 2). The same logic scales up. A hypothetical electric car with a 75 kWh pack that draws an average of 15 kW moves for about five hours.

What the C-rate means

Battery engineers describe charging and discharging speed with the C-rate. A rate of 1C means the full capacity is charged or discharged in one hour. A rate of 2C means 30 minutes, and 0.5C means two hours. Using the 75 kWh example, charging at 1C would require 75 kW. A 250 kW peak would be about 3.3C.

Faster charging is not free. Pushing ions into the anode faster than the graphite can accept them creates the conditions for lithium plating, where metallic lithium deposits on the surface of the anode, a mechanism Battery University's cycling tests suggest as a likely cause of the extra capacity loss seen at higher C-rates. Battery University advises charging energy-focused cells at 1C or less where possible and notes that very fast chargers trade lifetime for convenience.

Energy density versus power density

A cell optimized for energy has thick electrodes and stores a lot but delivers it gently, which suits a phone or a long-range EV. A cell optimized for power has thin electrodes and delivers energy fast but holds less, which suits a power tool. Supercapacitors take the power end to an extreme, trading energy storage for the ability to charge and discharge almost instantly, as described in The Invention of the Supercapacitor.

Why batteries age

No lithium-ion cell lasts forever, and the reasons are specific. Battery University's reference article on what causes lithium-ion cells to die lists several mechanisms that work together.

Solid electrolyte interphase (SEI) growth. When a lithium-ion cell charges for the first time, some lithium is consumed in forming a thin film on the graphite called the SEI. The film is useful at first, because it protects the anode from further reaction with the electrolyte. But it keeps growing, and as it thickens it traps lithium that can no longer carry charge and raises the cell's internal resistance. Think of limescale in a kettle: a thin layer is harmless, but it accumulates, and eventually it affects performance. Battery University identifies SEI growth as a main cause of capacity loss in most graphite-based cells when the charge voltage stays below 3.92 V per cell.

Lithium plating. As described above, charging too fast can deposit metallic lithium on the anode surface instead of intercalating it. Plated lithium is partly lost for good and can form dendrites.

Cathode degradation. At high voltages, especially when warm, the electrolyte oxidizes at the cathode and forms a restrictive layer. Battery University attributes this to charge voltages above roughly 4.10 V per cell at elevated temperature. The cathode's crystal structure can also coarsen and crack with repeated cycling.

Calendar aging. Even an unused cell loses capacity over time because the parasitic reactions never fully stop. Battery University reports that lithium-ion cells lose about 2% per month in storage at 0°C and 50% state of charge, and up to 35% per month at 60°C when fully charged.

For more on this topic, see Why Lithium Ion Batteries Age.

What the testing data suggest about longevity

Battery University collects numbers from laboratory tests, and they show how sensitive a cell is to how it is used. These are test-based figures, and the article itself cautions that real-world life tends to be shorter than lab models predict, and that tables conflict in places because of differences in cell quality and test methods.

Condition	Reported effect
Depth of discharge, NMC cells	About 300 cycles at 100% depth, about 1,000 at 40%, and about 6,000 at 10% (to 70% remaining capacity)
Depth of discharge, LFP cells	Roughly double the NMC figures
Peak charge voltage	Each 0.10 V reduction roughly doubles cycle life; 4.20 V per cell gives about 300 to 500 cycles, 4.10 V gives about 600 to 1,000
Storage temperature and charge, one year	At 25°C, about 80% capacity retained at full charge versus 96% at 40% charge; at 40°C, about 65% versus 85%

The pattern is consistent. High charge, high voltage, and high temperature all stress the cell, and shallow cycles are gentler than deep ones. The price of gentleness is usable energy: charging to a lower voltage extends life but gives up capacity.

What this means for everyday use

None of this requires fanatical habits. Several points follow directly from the data above:

• Heat is among the biggest stresses. Battery University says environmental conditions, not cycling alone, govern longevity. Leaving a phone or laptop in a hot car works against the cell.
• Partial charging is fine. Battery University states that lithium-ion has no memory and does not need periodic full discharges, apart from an occasional full cycle to recalibrate a smart battery's fuel gauge.
• Sitting at full charge for long periods adds stress, which is why many phones and EVs offer charge limits and smarter charging schedules.
• Storing a battery partway charged in a cool place is better than storing it full or empty.

For a deeper look at the phone case, see how to optimize your iPhone battery life.

Safety: what thermal runaway is

A lithium-ion cell packs a lot of energy into a small, sealed volume and contains a flammable liquid electrolyte. If a cell is damaged, overheated, overcharged, or has an internal defect, it can short internally. The short produces heat, the heat accelerates reactions that produce more heat, and the temperature rises until the cell vents or burns. This self-reinforcing process is called thermal runaway, and in a pack it can spread from cell to cell.

A well-documented example is the January 7, 2013 fire in the auxiliary power unit battery of a Japan Airlines Boeing 787 parked at Boston Logan International Airport. The US National Transportation Safety Board found that an internal short circuit in one cell led to thermal runaway, which spread to adjacent cells. It attributed the event to two failures: Boeing had not included design requirements to mitigate the most severe effects of an internal short in a cell, and the FAA had not identified that gap during certification. After a second battery incident days later, regulators grounded the global 787 fleet for nearly four months, and it returned to service in April 2013 after a fix was approved.

The lesson is not that lithium-ion batteries are inherently unsafe. Billions of cells operate without incident. The lesson is that safety comes from design, manufacturing quality, and electronic management, not from the chemistry alone, and that it needs to be engineered around the failure mode that cannot be eliminated.

The economics: price, supply, and who makes them

Batteries have become cheaper at a pace few technologies match. BloombergNEF's annual survey found the average lithium-ion battery pack price fell 20% in 2024 to $115 per kilowatt-hour, a record low and the largest annual drop since 2017. The firm reported that packs for battery electric vehicles averaged $97 per kilowatt-hour, the first time they dropped below $100. It also reported global cell manufacturing capacity of 3.1 terawatt-hours, more than 2.5 times 2024 demand, which pushes prices down.

The International Energy Agency's Global EV Outlook 2025 adds context. EV battery demand exceeded 950 gigawatt-hours in 2024, up 25% from 2023, and including storage, energy-sector battery demand reached 1 terawatt-hour. LFP batteries accounted for nearly half of the global EV battery market. China produced 80% of global battery cells in 2024, and the IEA reports it supplied about 85% of cathode active materials and over 90% of anode active materials, so the supply chain is concentrated.

Where the lithium comes from

The US Geological Survey's Mineral Commodity Summaries 2025 estimates that global lithium production in 2024 was about 240,000 metric tons of lithium content, up 18% from 204,000 tons in 2023, and that batteries account for 87% of lithium end uses.

Country	2024 mine production (estimated, metric tons)	Reserves (metric tons)
Australia	88,000	7,000,000
Chile	49,000	9,300,000
China	41,000	3,000,000
Zimbabwe	22,000	480,000
Argentina	18,000	4,000,000
World total (excluding US production)	240,000	30,000,000

Source: USGS Mineral Commodity Summaries 2025. Totals include countries not listed.

Reserves of 30 million tons divided by 240,000 tons of annual production gives about 125 years at 2024 rates. That is arithmetic, not a forecast: demand is growing, and reserve estimates are revised as exploration continues. USGS also reports that measured and indicated lithium resources worldwide total about 115 million tons, far above the reserve figure. The practical constraint is how fast mines and processing plants can be built, not a near-term shortage of lithium in the ground. Price swings are large: USGS notes the annual average US lithium carbonate price in 2024 was $14,000 per ton, a drop of 66% from 2023.

Recycling is a growing part of the answer, and the USGS notes that automobile companies and recyclers partnered in 2024 to supply the industry with recovered material, with the US Department of Energy announcing $44.8 million in funding for eight recycling projects.

What comes next

The chemistry is still moving, though the pace of breakthroughs is slower than headlines suggest.

Sodium-ion. These cells use sodium, which is far more abundant than lithium, in a similar rocking-chair design. The IEA notes that CATL announced a second-generation sodium-ion product in 2025 and that HiNa launched an improved cell in March 2025, and says analyses suggest sodium-ion needs higher energy density or higher lithium prices to compete with LFP on cost per kilowatt-hour. Reports on CATL's Naxtra cell cite up to 175 Wh/kg, which is below the best lithium-ion figures but in the range of LFP, and say it may do well in cold weather. These are company claims.

Solid-state. Replacing the liquid electrolyte with a solid could allow lithium-metal anodes, higher energy density, and less flammable cells. Companies including Toyota, Samsung SDI, and QuantumScape have publicly pointed to production windows around 2027 to 2028, but industry coverage notes earlier target dates have repeatedly slipped, and the main obstacles are manufacturing rather than chemistry. Semi-solid designs are already in some vehicles. Treat any specific energy-density number for all-solid-state cells as a target until it is demonstrated in production.

Lithium-sulfur and lithium-air. These remain research directions. They promise high theoretical energy density but face cycle-life and engineering problems, and there is no dependable commercial date to cite.

Grid storage. Batteries are also moving onto the grid to smooth renewable power, where cost and cycle life matter more than weight. Why that is harder than it looks is explained in Why Grid Storage Is Hard, and the car side is covered in How Do Electric Cars Actually Work?.

Common misconceptions

"Batteries store electricity." They store chemical energy, and electricity is what you get when the chemistry runs through a circuit. This matters because it explains why batteries lose capacity when their chemistry degrades, even if nothing looks wrong on the outside.

"Lithium-ion batteries have a memory effect." Nickel-cadmium cells could develop one. Battery University says lithium-ion has no memory, so you can charge whenever it is convenient.

"You should drain a battery fully before recharging." For lithium-ion, deep discharge is a stress, not a benefit. The test data above show life improving as the depth of discharge falls.

"Fast charging ruins a battery." Frequent very fast charging adds stress, and the effect depends on temperature and cell design. Occasional fast charging on a road trip is a different matter from using it every day.

"Lithium will run out soon." The reserve and resource numbers above do not point to near-term exhaustion. The more realistic risks are price volatility, supply concentration in a few countries, and the pace of new mine and processing capacity.

Frequently asked questions

Q: Should I unplug my phone at 100%? A: It does not matter much for a single night. The more important factors are heat and time spent at high voltage, and the data show that holding a cell at full charge adds stress. Many phones now include optimized charging that holds the charge at a lower level for part of the night, which works around the problem without effort from you.

Q: Why does my battery seem worse in the cold? A: Chemical reactions and ion movement slow down at low temperatures, which reduces the power the cell can deliver and the capacity you can access. Much of the loss is temporary and returns when the battery warms up. Charging a very cold lithium-ion cell quickly is a separate problem, because it raises the risk of lithium plating.

Q: What does mAh actually mean? A: Milliampere-hours measure charge capacity. To compare batteries at different voltages, convert to energy: multiply the capacity in amp-hours by the voltage to get watt-hours. A 5,000 mAh cell at 3.7 V holds 18.5 Wh, the same energy as a 2,500 mAh cell at 7.4 V.

Q: Why can't most alkaline batteries be recharged? A: The reactions in an alkaline cell are not reversible enough for a charger to restore the original chemistry reliably, and manufacturers generally advise against trying. Rechargeable chemistries are designed so that charging and discharging run the same reactions in both directions.

Q: Can an EV battery be replaced? A: Technically yes, but it is expensive, and whether it is needed depends on the vehicle, the chemistry, and how it was used. Packs are designed to last a long time, and the aging data above show that temperature and charging habits matter. Used packs with reduced capacity can sometimes be repurposed for stationary storage.

Q: What is the difference between a battery and a supercapacitor? A: A battery stores energy in chemical reactions, which gives high energy but slower delivery. A supercapacitor stores energy in an electric field at the surface of its electrodes, which gives much faster charge and discharge but much less energy per kilogram. Many designs combine both.

The takeaway

A battery works because a chemical reaction has been split into two halves, with electrons forced to take the long way round through your device while ions complete the loop inside. That one design principle explains the rest: why voltage depends on chemistry rather than size, why lithium's low mass and high potential made it the default, why heat and high voltage wear cells down, and why safety is a systems problem more than a chemistry problem. The next decade is likely to bring a wider choice of chemistries, such as LFP and sodium-ion for low-cost uses, and perhaps solid-state for the highest-end applications, rather than a single winner. The practical edge, for engineers and users alike, belongs to understanding which trade-off a given battery was built to make.

Sources

• BloombergNEF, Lithium-ion battery pack prices see largest drop since 2017, falling to $115 per kilowatt-hour (December 2024)
• International Energy Agency, Global EV Outlook 2025: Electric vehicle batteries
• U.S. Geological Survey, Mineral Commodity Summaries 2025: Lithium
• U.S. National Transportation Safety Board, Investigation DCA13IA037: Boston Logan 787 battery fire
• Battery University, BU-204: How do lithium batteries work?
• Battery University, BU-808: How to prolong lithium-based batteries
• Battery University, BU-808b: What causes Li-ion to die?
• Battery University, BU-107: Comparison table of secondary batteries
• OpenStax Chemistry via LibreTexts, Galvanic cells
• The Nobel Prize, Chemistry 2019: summary

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