A battery is a device that converts chemical energy contained within its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. It is a source of electric power consisting of one or more electrochemical cells with external connections for powering electrical devices. The terminal marked negative is the source of electrons that will flow through an external electric circuit to the positive terminal. When a battery is connected to an external electric load, a redox reaction converts high-energy reactants to lower-energy products, and the free-energy difference is delivered to the external circuit as electrical energy.
A cell refers to a single anode and cathode separated by electrolyte used to produce a voltage and current. A battery can be made up of one or more cells. Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit. All batteries are made up of three basic components: an anode (the negative electrode), a cathode (the positive electrode), and an electrolyte (the chemical medium that allows the flow of ions between the cathode and anode) .
Historically, the term "battery" specifically referred to a device composed of multiple cells; however, the usage has evolved to include devices composed of a single cell. The word "battery" was used to describe a "series of similar objects grouped together to perform a function," as in a battery of artillery. In 1749, Benjamin Franklin first used the term to describe a series of capacitors he had linked together for his electricity experiments. Later, the term would be used for any electrochemical cells linked together for the purpose of providing electric power.
Batteries come in several styles, including single-use alkaline batteries and rechargeable nickel-cadmium or nickel-hydride batteries. The amount of energy a device can hold per unit volume is known as energy density.
