Batteries are a nearly US$50 billion industry worldwide and are a necessary component of gadgets and toys worldwide. At the heart of the battery is the voltaic cell, an electrochemical system that generates an electrical current by taking advantage of the electron movement between different metals submersed in a conductive liquid.
A basic example of this comes in the form of a copper/zinc cell. Zinc loses electrons more readily than copper. Placing zinc and copper in two containers of liquid with appropriate salts, like zinc sulfate and copper sulfate respectively, with a wire running between, generates an electric current as electrons are moved from the zinc to the copper. As zinc loses electrons, it becomes ionized and enters the solution with the zinc sulfate. Over time, all of the zinc is converted and the mass of the zinc electrode is greatly reduced. Over time the zinc electrode will completely dissolve and the cell will lose its ability to generate a current.
In this particular example, the copper electrode is known as a cathode, because it is the source of the reduction reaction where electrons are gained. The zinc electrode, on the other hand, loses the electrons and is known as the anode.
The standard electric potential of the cell can be used to calculate the electromotive force, measured in volts, can be calculated by using the Nernst equation whilst considering the two half-reactions involved in the electron movement process. This will change over time as ions move and the electrolyte concentrations change.
Voltaic cells strung together in a series are known as voltaic piles. The first battery was a crude voltaic pile. By stringing together the cells, the electromotive force can be enhanced, increasing the effectiveness of the battery.
There are two types of batteries in use today. Primary batteries are ones that transform chemical energy directly into electrical energy, while irreversibly exhausting the reactants. These batteries cannot by recharged or restored by electrical means. The second time is known as secondary batteries, which an be recharged by supplying electrical energy, which restores the original composition. These are usually built with materials in the discharged state, which must be recharged initially by introduction of an electrical current.
It is also important to note that the examples mentioned above are known primarily as wet cells, as liquid is a primary part of the setup. However, many dry cells exist and are utilized today. These cells are usually built with pastes and dry elements, with only the minimal amount of moisture necessary to actually carry the current.