Battery

In Science and Technology

A battery is a device that stores electrochemical or electrostatic energy. A modern battery is usually an array of electrochemical cells, although a single cell is also called, in correct modern usage, a battery.

Benjamin Franklin coined the term battery in 1748 to describe an array of charged glass plates. He adapted the word from its earlier sense meaning a beating, which is what an electric shock from the apparatus felt like. In those days, the entertaining effect of an electric shock was one of the few uses of the technology. Other experimenters made batteries from a number of Leyden jars connected in parallel. The definition was later widened to include an array of electrochemical cells or capacitors. The first chemical battery was the Voltaic pile invented by Alessandro Volta in 1800.

Above: circuit symbol for a battery (+ and - signs are optional)

The cells in a battery can be connected in parallel or in series, or both. A parallel combination of cells has the same voltage as a single cell, but can supply a higher current (the sum of the currents from all the cells). On the other hand, a series combination has the same current rating as a single cell but its voltage is the sum of the voltages of all the cells. Most practical electrochemical batteries, such as 9 volt flashlight (torch) batteries and 12V automobile (car) batteries, have a series structure. In both types, the energy stored in the battery is equal to the sum of the energies stored in all the cells.

A battery can be modelled as a perfect voltage source (i.e. one that can supply infinite current) in series with a resistor. The voltage source depends on the chemistry of the battery, not on whether it is empty or full. When a battery runs down, the value of the imaginary resistor, called the internal resistance, increases. When the battery is connected to a load (e.g. a light bulb), which has its own resistance, the resulting voltage across the load depends on the ratio of the battery's internal resistance to the resistance of the load. When the battery is fresh, its internal resistance is low, so the voltage across the load is almost equal to that of the battery's internal voltage source. As the battery runs down and its internal resistance increases, the proportion of its internal voltage that gets through the internal resistance to appear at the load gets smaller, so the battery's ability to deliver power to the load decreases.

Common battery types

From a user's viewpoint, at least, batteries can be generally divided into two main types - rechargeable and non-rechargeable (disposable). Each is in wide usage.

Disposable batteries, also called primary cells, are intended to be used once, until the chemical changes that induce the electrical current supply are complete, at which point the battery is discarded. These are most commonly used in smaller, portable devices with either low current drain, only used intermittently, or used well away from an alternative power source. (see also waste).

Rechargeable batteries or secondary cells, by contrast, after being drained can be re-used. This is done by applying externally supplied electrical current which causes the chemical changes that occur in use to be reversed. Devices to supply the appropriate current are called rechargers.

The oldest form of rechargeable battery still in modern usage is the 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 deal with the explosive oxygen and hydrogen gases which are vented 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 the ultimate in low weight and ease of handling is not a concern. A more expensive type of lead-acid battery called a gel battery contains a semi-solid electrolyte to prevent spillage. More portable rechargeable batteries include several "dry cell" types, which are sealed units and are therefore useful in appliances like mobile phones and laptops. Cells of this type (in order of increasing power density and cost) include nickel-cadmium (nicad or NiCd), nickel metal hydride (NiMH), and lithium-ion (Li-Ion) cells.

Disposable cells come in a number of standard sizes, so the same battery type can be used in a wide variety of appliances. Some of the major types for portable appliances are listed below:

The relevant European standard is IEC 60086-1 Primary batteries - Part 1: General (BS397 in the UK).

The relevant US standard is ANSI C18.1 American National Standard for Dry Cells and Batteries-Specifications.

An extensive series of articles on many aspects of batteries and their use in portable equipment is available at http://www.buchmann.ca/

In Warfare

A battery is an attack by a group of artillery, such as cannon, or the group of artillery itself. Battery Park, at the south end of Manhattan Island in New York city, is the place where the artillery that defended the city was clustered as a battery.