the uncharged stages fig4 a illustrates charging of stage 44d thus hydraulic fluid from the reservoir 24 is fed via an inlet 52 through conduit line 54 to the first stage valves 50a b and c for the other illustrated stages are closed while the inlet valve 50d for the first stage is open the outlet gas flow valves 48 from each of the stages are also open as the fluid flows into stage 44d gas will flow from that stage into the remaining uncharged accumulator stages when substantially all of the gas in stage 44c has been forced into the other stages 44a 44b and 44c stage 44d will be turned off completely by closing the inlet and outlet valves 50d and 48d so that it will remain at the maximum pressure it has attained during this portion of the charging phase this pressure is much lower than the maximum accumulator pressure for example in the case of a single stage conventional accumulator this permits stage 44d to be designed for a maximum pressure that is substantially below the accumulator maximum pressure which permits lesser side wall construction i e thinner than normally required referring now to fig4 b the charging of stage 44c is illustrated again the maximum pressure experienced by stage 44c is substantially below the maximum accumulator system pressure this type of multi stage compression can continue until.