A battery is a device that converts chemical energy directly to
electrical energy. It consists of one or more voltaic cells.
Each voltaic cell consists of two half cells connected in series
by a conductive electrolyte. One half-cell is the positive
electrode and the other is the negative electrode. The
electrodes do not touch each other but are electrically
connected by the electrolyte, which can be either solid or
liquid. In many cells, the materials are enclosed in a container,
and a separator, which is porous to the electrolyte, which
prevents the electrodes from coming into contact.
Each half
cell has an electromotive force (or emf), determined by its
ability to drive electric current from the interior to the
exterior of the cell. The net emf of the battery is the
difference between the emfs of its half-cells, as first
recognized by Volta. Thus, if the electrodes have emfs
and
,
then the net emf is
.
(Hence, two identical electrodes and a common electrolyte give a
zero net emf.)
The electrical potential difference, or
across the terminals of a battery is known as terminal
voltage and is measured in volts. The terminal voltage of a
battery that is neither charging nor discharging is called the
open-circuit voltage and equals the emf of the battery. Because
of internal resistance, 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 that is charging
exceeds the open-circuit voltage.
An ideal battery has negligible internal resistance, so it would
always have a terminal voltage of
.
This means that to produce a potential difference of 1.5 V,
chemical reactions inside would perform 1.5
J
of work for a charge of 1
C.
The voltage developed across a cell's terminals depends on
the chemicals used in it and their respective concentrations.
For example, alkaline and carbon-zinc cells both measure
approximately 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.
Classification of batteries
Disposable and rechargeable
Batteries are usually divided into two broad classes:
-
Primary batteries irreversibly (within limits of
practicality) transform chemical energy to electrical
energy. When the initial supply of reactants is exhausted,
energy cannot be readily restored to the battery by
electrical means.
-
Secondary batteries can be recharged; that is,
they can have their chemical reactions reversed by supplying
electrical energy to the cell, restoring their original
composition.
Historically, some types of primary batteries used, for
example, for telegraph circuits, were restored to operation by
replacing the components of the battery consumed by the chemical
reaction. Secondary batteries are not indefinitely rechargeable
due to dissipation of the active materials, loss of electrolyte
and internal corrosion.
From a user's viewpoint, at least, batteries can be generally
divided into two main types: non-rechargeable (disposable)
and rechargeable. Each type is in wide usage, as each has
its own advantages and disadvantages.
Disposable batteries, also called primary cells, are
intended to be used once and discarded. These are most commonly
used in portable devices with either low current drain, are only
used intermittently, or are used well away from an alternative
power source. Primary cells were also commonly used for alarm
and communication circuits where other electric power was only
intermittently available. Primary cells cannot be reliably
recharged, since the chemical reactions are not easily
reversible and active materials may not return to their original
forms. Battery manufacturers recommend against attempting to
recharge primary cells, although some electronics enthusiasts
claim it is possible to do so using special types of chargers.
By contrast, rechargeable batteries or secondary cells
can be recharged by applying electrical current, which reverses
the chemical reactions that occur during its use. Devices to
supply the appropriate current are called chargers or
rechargers.
The oldest form of rechargeable battery still in modern usage
is the "wet cell" lead-acid battery. This battery is notable in
that it contains a liquid in an unsealed container, requiring
that the battery be kept upright and the area be well ventilated
to ensure safe dispersal of the hydrogen gas produced by these
batteries during overcharging. The lead-acid battery is also
very heavy for the amount of electrical energy it can supply.
Despite this, its low manufacturing cost and its high surge
current levels make its use common where a large capacity (over
approximately 10Ah) is required or where the weight and ease of
handling are not concerns.
A common form of lead-acid battery is the modern wet-cell car
battery. This can deliver approximately 10,000 watts of power
over a short period and has a peak current output that varies
from 450 to 1100 amperes. An improved type of liquid electrolyte
battery is the sealed valve regulated lead acid (VRLA) battery,
popular in the automotive industry as a replacement for the
lead-acid wet cell, as well as in many lower capacity roles
including smaller vehicles and stationary applications such as
emergency lighting and alarm systems. The one-way pressure
activated valve eliminates electrolyte evaporation while
allowing out-gassing to prevent rupture. This greatly improves
resistance to damage from vibration and heat. VRLA batteries
have the electrolyte immobilized, usually by one of two means:
Other portable rechargeable batteries include several "dry
cell" types, which are sealed units and are therefore useful in
appliances such as mobile phones and laptop computers. Cells of
this type (in order of increasing power density and cost)
include nickel-cadmium (NiCd), nickel metal hydride (NiMH) and
lithium-ion (Li-Ion) cells.
Recent developments include batteries with embedded
functionality such as USBCELL, with a built-in charger and USB
connector within the AA format, enabling the battery to be
charged by plugging into a USB port without a charger, and low
self-discharge (LSD) mix chemistries such as Hybrio, ReCyko, and
Eneloop, where cells are pre-charged prior to shipping.
