An electric battery is a source of electric power consisting of one or more
electrochemical cells with external connections[1] for powering electrical devices.
When a battery is supplying power, its positive terminal is the cathode and its
negative terminal is the anode.[2] The terminal marked negative is the source of
electrons. When a battery is connected to an external electric load, those
negatively charged electrons flow through the circuit and reach the positive
terminal, thus causing a redox reaction by attracting positively charged ions, or
cations. Thus, higher energy reactants are converted to lower energy products, and
the free-energy difference is delivered to the external circuit as electrical
energy. 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.[3]
Primary (single-use or "disposable") batteries are used once and discarded, as the
electrode materials are irreversibly changed during discharge; a common example is
the alkaline battery used for flashlights and a multitude of portable electronic
devices. Secondary (rechargeable) batteries can be discharged and recharged
multiple times using an applied electric current; the original composition of the
electrodes can be restored by reverse current. Examples include the lead–acid
batteries used in vehicles and lithium-ion batteries used for portable electronics
such as laptops and mobile phones.
Batteries come in many shapes and sizes, from miniature cells used to power hearing
aids and wristwatches to, at the largest extreme, huge battery banks the size of
rooms that provide standby or emergency power for telephone exchanges and computer
data centers. Batteries have much lower specific energy (energy per unit mass) than
common fuels such as gasoline. In automobiles, this is somewhat offset by the
higher efficiency of electric motors in converting electrical energy to mechanical
work, compared to combustion engines.
