A method of charging a hydro-pneumatic energy storage system is described. The system has a first hydro-pneumatic accumulator with a first hollow vessel. Disposed within the first hollow vessel is a first compressible volume containing a first amount of gas. The system has a second hydro-pneumatic accumulator with a second hollow vessel. Disposed within the second hollow vessel is a second compressible volume containing a second amount of gas. The gas contained in the first volume is pre-pressurized to a first hydrostatic pre-charge pressure and the gas contained in the second volume is pre-pressurized to a second hydrostatic pre-charge pressure. The second pre-charge pressure is higher than the first pre-charge pressure. In addition, the gas in the first volume is pressurized by discharging a non-compressible hydraulic fluid into the first vessel while keeping a quantity of non-compressible hydraulic fluid contained in the second vessel constant to keep the pressure of the gas contained in the second volume at the second pre-charge pressure.