Introduction
A battery stores energy in chemical form and releases it as electrical energy when a circuit is connected between its terminals. Inside the battery, chemical reactions move charged particles called ions through an electrolyte. At the same time, electrons travel through the external circuit, where they can operate a lamp, radio, motor, computer, or other load.
Two broad families
| Family | What it means | Common examples |
|---|---|---|
| Primary | Designed to be used until depleted and then replaced. | Zinc-carbon, alkaline, primary lithium coin and cylindrical cells. |
| Secondary | Designed so that an appropriate charger can reverse much of the chemical change. | Lead-acid, nickel-metal hydride, nickel-cadmium, lithium-ion. |
What a battery label tells you
- Voltage: the electrical pressure available at the terminals.
- Capacity: how much charge the battery can deliver, commonly stated in ampere-hours or milliampere-hours.
- Energy: the useful combination of voltage and capacity, commonly stated in watt-hours.
- Chemistry: the materials and reactions used inside the cell.
- Polarity: which terminal is positive and which is negative.
- Current limits: how much current may be drawn or supplied during charging.
Batteries are not ideal voltage sources
A fresh battery has a characteristic open-circuit voltage, but its terminal voltage usually falls when current is drawn. The amount of drop depends on the battery chemistry, size, temperature, state of charge, age, and internal resistance. For this reason, two batteries with the same printed voltage may perform very differently in a demanding load.
Common applications
Small primary cells are convenient for clocks, remote controls, smoke alarms, meters, and other devices that use modest current. Rechargeable cells are favored where the battery is used repeatedly, such as in vehicles, cordless tools, portable electronics, emergency power, and energy-storage systems. The best chemistry depends on cost, weight, required current, operating temperature, shelf life, cycle life, and safety.
Continue through the battery pages
The Basics page explains voltage, current, capacity, series and parallel connections, and simple testing. The History page follows the development from Volta's pile to modern rechargeable cells. The Advanced page introduces electrochemical potential, internal models, charging, state estimation, pack design, and the mathematics used by engineers.