A hydraulic system is powered by a pump designed to provide a certain amount of continuous pressure. A bigger and more powerful pump can pump hydraulic fluid faster, but it also uses a lot more energy. A hydraulic accumulator is a system which stores pressurized hydraulic fluid. That way, the pump does not have to be powerful enough to cope with a sudden surge in demand. Instead, it can keep steadily pumping hydraulic fluid and rely on the accumulator to provide extra hydraulic fluid when it is needed.

Hydraulic accumulators are storage chambers which contain hydraulic fluid. The fluid is pumped into the accumulator by a hydraulic pump with a one-way valve. The accumulator has another valve which can be opened to let the fluid out into the rest of the hydraulic system. The actual accumulator is under constant pressure. In gas accumulators, a pressurized gas bladder presses against a hydraulic bladder. The more the bladder fills up, the more it presses against the gas, increasing the pressure. A spring accumulator works in a similar way, except a large spring or springs press against the bladder to compress it. In a raised weight accumulator, the hydraulic fluid is pumped into a big piston with a weight on top of it. This weight exerts a constant force, pressing down on the fluid and compressing it as it fills and empties.

Hydraulic accumulators are useful in many different types of systems. A large hydraulic system powering loading cranes at a dock will fill up a whole hydraulic tower in order to have a continuous flow of pressure when the machines need it. When a crane moves, each step has to be planned and checked carefully which takes a lot of time. A relatively small pump can fill up a hydraulic tank during the down time. Much smaller machines use hydraulic accumulators as well. One good example is hydraulic regenerative braking. When a car with hydraulic regenerative braking brakes, the motion of the wheels is used to pump hydraulic fluid into an accumulator. This slows down the car and charges up the accumulator. When the car accelerates again, the hydraulic fluid flows back out, using its stored pressure to power the wheels.