An accumulator is an apparatus by means of which energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 can be stored, such as a rechargeable battery or a hydraulic accumulator

Hydraulic accumulator

A 'hydraulic accumulator' is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. That external source can be a spring, a raised weight, or a compressed gas...

. Such devices may be electrical, fluidic

Thermal energy storage

Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above or below that of the...

 or mechanical

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

 and are sometimes used to convert a small continuous power source into a short surge of energy or vice versa. Other examples of accumulators include capacitor

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s, compulsator

Compulsator

A compensated pulsed alternator, also known by the portmanteau compulsator, is a form of power supply.As the name suggests, it is an alternator that is "compensated" to make it better at delivering pulses of electrical energy than a normal alternator.- Description and operation:The principle is...

s, steam accumulator

Steam accumulator

A Steam accumulator is an insulated steel pressure tank containing hot water and steam under pressure. It is a type of energy storage device. It can be used to smooth out peaks and troughs in demand for steam. Steam accumulators may take on a significance for energy storage in solar thermal...

, wave energy machines, pumped-storage hydroelectric plants.

In general usage in an electrical context the word accumulator would normally mean a lead-acid battery

Lead-acid battery

Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large...

.