A battery is a device that
converts chemical energy directly to electrical energy.[12]
It consists of one or more voltaic cells. Each voltaic cell
consists of two
half cells connected in series by a conductive electrolyte.
Each cell has a positive electrode (cathode), and a negative
electrode (anode). These do not touch each other but are
immersed in a solid or liquid electrolyte.[13]
In a practical cell the materials are enclosed in a container,
and a separator between the electrodes prevents the electrodes
from coming into contact.
Each half cell has a net
electromotive force (or emf), with the net emf of the
battery being the difference between the emfs of the half-cells,
a fact first recognized by Volta. Each half cell emf is due to a
charge-transferring (or faradaic) chemical reaction at the
electrode-electrolyte interface, which transfers charge across
the interface. The reaction stops when the charge transfer is
enough to cancel out the tendency to the reaction to occur.
Non-charge-transferring, or nonfaradaic, reactions (also called
parasitic or "side-reactions") can also occur at the interface.
These are undesirable, using up the chemicals without producing
current (which is the rate of charge transfer).
The electrical potential
difference across the terminals of a battery is known as its
terminal voltage, measured in
volts.
The terminal voltage of a battery that is neither charging nor
discharging is called the
open-circuit voltage, and gives the emf of the battery. The
terminal voltage of a battery that is discharging is smaller in
magnitude than the open-circuit voltage, and the terminal
voltage of a battery being charged is greater than the
open-circuit voltage.
[14]
The voltage developed across a
cell's terminals depends on the chemicals used in it and their
concentrations. For example, alkaline and carbon-zinc cells both
measure about 1.5 volts, due to the energy release of the
associated chemical reactions. Because of the high
electrochemical potential changes in the reactions of
lithium compounds, lithium cells can provide as much as 3
volts or more.