The battery is at present the most practical and widely used means of storing electrical energy. The storage capacity of a battery is usually defined in ampere-hours (Ah); energy is strictly defined in kilowatt-hours (kWh) or joules, but since the voltage of a particular battery system is normally fixed and known, the Ah definition is more convenient.
The terms battery and cell are often interchanged, although strictly a battery is a group of cells built together in a single unit.
Batteries can be classified into primary and secondary types. A primary battery stores electrical energy in a chemical form which is introduced at the manufacturing stage. When it is discharged and this chemically stored energy is depleted, the battery is no longer serviceable.
Applications for primary batteries are generally in the low-cost domestic environment, in portable equipment such as torches, calculators, radios and hearing aids.
A secondary or rechargeable battery absorbs electrical energy, stores this in a chemical form and then releases it when required. Once the battery has been discharged and the chemical energy depleted, it can be recharged with a further intake of electrical energy. Many cycles of charging and discharging can be repeated in a secondary battery.
Applications cover a wide range. In the domestic environment secondary batteries are used in portable hand tools, laptop computers and portable telephones. Higher powered applications in industry include use in road and rail vehicles and in standby power applications.
The capacity of secondary battery systems ranges from 100 mAh to 2000 Ah. Their useful life ranges from 2 to 20 years; this will depend, among other things, upon the number of charge discharge cycles and the type and construction of battery used.
A fuel cell is an energy conversion device that is closely related to a battery. Both are electrochemical devices for the conversion of chemical to electrical energy. In a battery the chemical energy is stored internally, whereas in a fuel cell the chemical energy (fuel and oxidant) is supplied externally and can be continuously replenished.