Technically, a battery is a collection of cells, usually connected and enclosed inside a single housing, each of which is a device that converts chemical energy into electrical energy. The first battery was invented in 1800 by Italian physicist Allesandro Volta, though there is speculation that earlier units may have existed. Volta created a “pile” of cells to make his first battery, stacking them on top of one another in series to increase the available voltage. (In some parts of Europe, batteries are still known as “piles,” and the schematic symbol used to represent a battery is based on a “pile” of cells.) The term “battery” is credited to Benjamin Franklin, who, in 1748, referred to a series of Leyden jars (early capacitors) as a “battery,” taking the term from a battery or group of cannons.
Early batteries did not produce stable voltage and were limited to producing energy for short durations. By 1836, with the Daniell cell (invented by British chemist John Frederick Daniell), batteries had improved enough to be used for serious applications, such as powering telegraph gear. Though, keeping a battery going back then required refreshing the electrolyte by adding fresh liquid on a regular basis.
The electricity from most batteries is produced by a chemical reaction between an electrolyte and metal electrodes. Early batteries used sulfuric or other acid as the electrolyte and often leaked. Today, batteries are based on alkaline, carbon zinc, zinc chloride, NiMH (nickel metal hydride), NiCd (nickel cadmium), lithium compounds, or other chemistries. The amount of electrolytic material and the size of the electrodes determines the capacity of a cell. Therefore, all other things being equal, a larger cell will always be more powerful than a smaller cell.
There are two main types of batteries: “primary” (use once and throw it away when it is exhausted) and “secondary” (rechargeable). Subcategories include:
- Wet cells — contain a liquid, such as many automotive lead-acid batteries.
- Dry cells — not truly dry, but filled with a moist chemical mixture.
- VRLA (valve-regulated lead-acid, a.k.a. gel cell, absorbed glass mat battery, sealed battery, acid-starved battery…these are used in uninterruptible power supplies and other high-power applications)
- Molten salt — (a.k.a. thermal cell.) These literally must be heated to high temperatures to melt a salt compound that serves as the electrolyte. Invented by the Germans in World War II for use in V-2 rockets, molten salt batteries currently show promise for use in electric cars and other applications.
- Reserve batteries — inactive until activated by immersion in water or some other mechanism; typically these are short duration devices designed to be stored for as long as many years. They are used in oceanographic, military, and other applications.