Hydraulic accumulators, most simply put, are energy storage devices. They are similar to rechargeable batteries within electrical systems, except that they store and discharge energy in the form of pressurized fluid. The accumulator itself is a pressure vessel that holds hydraulic fluid and a compressible gas, most commonly nitrogen. The shell or housing of the accumulator is made from materials such as steel, stainless steel, aluminum, titanium, and fiber-reinforced composites. Within the device a flexible barrier, such as a piston or rubber bladder, separating the oil from the gas.