A lightweight, optimally efficient, easily serviced, piston-in-sleeve high pressure accumulator is provided. The accumulator includes one or more cylindrical composite pressure vessel separate end cap manifolds. A piston slidably disposed in a thin impermeable internal sleeve in the accumulator separates two chambers, one adapted for containing a working fluid and the other adapted for containing gas under pressure. Gas is provided in a volume between the impermeable internal sleeve and the composite pressure vessel wall. Additional gas is optionally provided in gas cylinders. Further components are provided for withstanding harmful effects of radial flexing of the composite vessel wall under high pressures, and from stresses present in use in mobile applications such as with a hydraulic power system for a hydraulic hybrid motor vehicle.