What is a battery? A battery is one or more electrochemical cells that store energy in chemical form and convert it into electrical energy when connected to a suitable circuit. Inside a cell, chemical reactions move electrons through an external circuit while ions move within the cell through an electrolyte. This movement allows a battery to power lights, phones, vehicles, tools and many other devices.
A battery does not create energy from nothing. It converts stored chemical energy into electrical energy, with some energy also transferred as heat and other losses. Rechargeable batteries use an external electrical supply to drive chemical changes in the opposite direction, restoring much of the cell’s ability to deliver energy. A rechargeable battery still has a finite life and must be used within its specifications.
Quick notes: A cell is an electrochemical unit; a battery may contain one or several cells. Voltage measures electrical potential difference. Capacity is commonly quoted in ampere-hours (Ah) or milliampere-hours (mAh). Energy is measured in watt-hours (Wh) or kilowatt-hours (kWh). Primary batteries are designed mainly for one-time use; secondary batteries are rechargeable. Never puncture, crush, heat or dismantle a battery.
How a battery produces electricity
A typical battery cell contains two electrodes and an electrolyte. One electrode participates in an oxidation reaction, releasing electrons into the external circuit. At the other electrode, a reduction reaction accepts electrons. Ions move through the electrolyte to maintain charge balance within the cell.
- Chemical energy is stored: the cell contains materials with the capacity to undergo an electrochemical reaction.
- The circuit is connected: a conductive path links the cell’s terminals through a device.
- Electrons move externally: electrons flow through the wire and the connected device, transferring energy.
- Ions move internally: charged particles move through the electrolyte to support the chemical reactions and maintain charge balance.
- The device operates: electrical energy is converted into light, sound, movement, computation or heat.
The exact chemistry differs between alkaline, lithium-ion, lead-acid and other batteries. The simple two-electrode picture is a useful starting point, but real cells include separators, current collectors, protective structures and carefully controlled materials.
Cell, battery, electrode and electrolyte: the key terms
- Cell: one electrochemical unit that converts chemical energy and electrical energy.
- Battery: one or more cells arranged to provide a desired voltage and capacity.
- Anode: the electrode where oxidation occurs during discharge.
- Cathode: the electrode where reduction occurs during discharge.
- Electrolyte: a medium that allows ions to move between the electrodes.
- Separator: a component that prevents direct electrical contact between electrodes while allowing suitable ion movement.
The terms anode and cathode are defined by the reactions occurring, not simply by a permanent plus or minus label in every operating mode. In a rechargeable battery, the direction of reactions changes between charging and discharging, so diagrams should state which mode they describe.
What is the difference between primary and rechargeable batteries?
Primary batteries
Primary batteries are designed mainly for one-time use. Common household alkaline cells are familiar examples. They are useful when a device needs modest power for a long period or when recharging is inconvenient. Once the reactants are used up, the battery should be replaced and disposed of through an appropriate collection route.
Secondary batteries
Secondary batteries can be recharged by supplying electrical energy to drive the cell reactions in reverse. Lithium-ion, nickel-metal hydride and lead-acid batteries are examples. Rechargeability makes them useful for phones, laptops, power tools, electric vehicles and backup systems, but repeated cycling gradually changes their capacity and performance.
What is voltage, capacity and energy?
Voltage measures potential difference between two points. A cell’s nominal voltage depends on its chemistry and design. Connecting cells in series can increase the total voltage, while parallel arrangements can increase available capacity, subject to the battery pack’s design and protection system.
Capacity is often quoted in ampere-hours. It describes how much electric charge a battery can deliver under stated test conditions. Capacity alone does not tell you the total energy unless voltage is also considered.
Approximate energy (Wh) = voltage (V) × capacity (Ah)
Example: 3.7 V × 5 Ah ≈ 18.5 WhA 5,000 mAh label is the same as 5 Ah, but manufacturers may quote capacity at the internal cell voltage rather than the USB output voltage. Conversion losses and the power bank’s electronics mean the usable energy delivered to a phone is lower than a simple mAh comparison might suggest.
How a rechargeable battery charges
Charging applies an external voltage and current so that the cell’s electrochemical reactions are driven in the direction that stores energy. The charger and battery-management system control the process according to the battery chemistry and design. Lithium-ion systems commonly use carefully controlled current and voltage stages and protection against overcharging, over-discharging, overcurrent and excessive temperature.
A battery should be charged using compatible equipment and within the manufacturer’s instructions. A charger with the same plug does not necessarily have the correct electrical characteristics. Damaged, swollen, unusually hot or leaking batteries should not be charged or used.
What is a lithium-ion battery?
A lithium-ion battery stores and releases energy through the movement of lithium ions between electrodes during charge and discharge. It is widely used because it can provide relatively high energy density and can be recharged many times. Different lithium-ion chemistries offer different trade-offs in energy density, power, cost, lifespan and thermal stability.
Lithium-ion does not describe one identical recipe. Variants such as lithium iron phosphate (LFP) and nickel-manganese-cobalt (NMC) have different properties and use cases. Pack design, cooling, electronics, charging limits and operating conditions are also important to performance and safety.
Why batteries lose capacity over time
Battery ageing occurs through chemical and physical changes inside the cell. Repeated charge-discharge cycles, high temperatures, prolonged storage at high charge, very deep discharges and high charging rates can accelerate some ageing mechanisms. The effect varies by chemistry and design.
A phone that lasts fewer hours than it did when new may have reduced battery capacity, but background apps, screen brightness, poor signal and software changes can also increase energy use. Battery health estimates are useful indicators, not perfect measurements of every future runtime.
What affects battery performance?
- Temperature: very hot or very cold conditions can reduce performance or accelerate ageing, depending on chemistry.
- Discharge rate: drawing high current can reduce usable capacity and generate heat.
- Age and cycle history: chemical changes accumulate over time and use.
- Storage conditions: long storage at unsuitable charge levels or high temperatures can shorten life.
- Cell balance: in multi-cell packs, differences between cells can affect usable pack capacity and safety.
- Battery management: monitoring and protection systems help keep cells within operating limits.
Battery safety: what users must never do
- Do not puncture or crush batteries. Internal damage can cause short circuits, heat and fire.
- Do not expose batteries to fire or excessive heat. Heat can trigger dangerous chemical reactions.
- Do not use swollen or damaged batteries. Stop using the device and follow manufacturer and local disposal guidance.
- Use compatible chargers and cables. Avoid damaged or counterfeit charging equipment.
- Do not mix unknown battery types. Use only the combinations and replacements specified by the device maker.
- Keep batteries away from children and metal objects. Loose cells can be shorted by coins, keys or other conductors.
If a lithium-ion battery is smoking, hissing, swelling rapidly or burning, move away, avoid breathing fumes and call local emergency services. Do not attempt to carry a dangerous battery through a building. Follow fire-service instructions.
How to choose a battery for a device
Start with the device manufacturer’s required chemistry, voltage, size and connector. Then check capacity, discharge rating, temperature limits, charging compatibility, certifications and warranty. A physically fitting battery is not necessarily electrically or thermally suitable.
For a power bank, compare watt-hours when possible, not just the large mAh figure. Check the supported output power, USB-C Power Delivery compatibility where relevant, airline rules for carrying spare batteries and the seller’s safety information. For an existing practical guide, read How Power Banks Work.
Battery recycling and disposal
Batteries contain useful materials and should not automatically be placed in ordinary rubbish. Damaged or improperly disposed batteries can cause fires in waste collection and processing facilities. Follow your local authority’s instructions and use approved collection points for the battery type.
Before recycling, protect exposed terminals where the collection guidance recommends it, especially for loose lithium batteries. Do not dismantle a pack to recover materials. The safe route is to use recognised collection and recycling systems.
Frequently asked questions about batteries
What is a battery in simple words?
A battery is one or more cells that convert stored chemical energy into electrical energy for a circuit.
What is the difference between a cell and a battery?
A cell is one electrochemical unit. A battery can contain one or several cells connected together.
What is the difference between a primary and secondary battery?
A primary battery is designed mainly for one-time use; a secondary battery can be recharged.
Does a higher mAh rating always mean a better battery?
Not by itself. Voltage, energy in watt-hours, device compatibility, discharge capability, quality and safety also matter.
Why does a phone battery wear out?
Ageing reactions and operating conditions gradually reduce a cell’s ability to store and deliver energy.
Can a swollen battery still be used?
No. Stop using or charging it and follow the device maker’s safety and disposal instructions.
Continue learning at eduKateSG
For an everyday application, read How Power Banks Work. To understand the electricity that a battery supplies to a circuit, continue with What Is Electricity?. For how solar energy can be stored and used at home, see How Solar Panels Work.
Sources and further reading
The US Department of Energy battery resources explain battery operation and energy storage. The US Environmental Protection Agency’s household battery guidance explains safe handling and recycling in its jurisdiction. Always follow the battery and device manufacturer’s safety instructions and the local authority’s disposal rules.