A hydraulic hybrid system for implementation in a machine. The system includes a hydraulic system, an energy source, an output, and a transmission. The hydraulic system includes a primary hydraulic pump/motor (primary motor) that is hydraulically coupled to a reservoir and a variable-volume accumulator assembly. The primary motor is configured to charge an accumulator of the variable-volume accumulator assembly with a working fluid when mechanically driven. The storage volume of variable-volume accumulator assembly varies based on a kinetic output condition of the machine. The energy source is configured to produce primary kinetic energy. The output is configured to receive at least a first portion of the primary kinetic energy. The transmission is coupled between the energy source and the output and selectively coupled to the primary motor. The transmission is configured to mechanically drive the primary motor using a second portion of the primary kinetic energy.