A hybrid drive system for a harvester comprising: - a main drive; - a reservoir of hydraulic fluid; - a hydraulic power storage for storing hydraulic fluid at a pressure higher than that of the reservoir, the hydraulic power storage comprising a plurality of hydraulic power storage units; - a hydraulic power unit being: - mechanically connected to the main drive; - hydraulically connected to the hydraulic power storage and to the reservoir; - an intermittent auxiliary drive configured to be powered intermittently by the main drive during an operating period such that the main drive powers the intermittent auxiliary drive during an activated time period and the main drive does not power the intermittent auxiliary drive during a deactivated time period; - a power exchange control system coupled to the hydraulic power unit and the hydraulic power storage, the power exchange control system being configured to selectively connect a first subset and at least one different, other subset (64, 66) of one or more of the hydraulic power storage units to the hydraulic power unit in function of a desired bidirectional power exchange between the main drive and the hydraulic power storage in function of the operating period, characterized in that the power exchange control system is further configured to: - determine for the activated time period: - a peak load time period during which the average power required by the intermittent auxiliary drive is higher than the average power required by the intermittent auxiliary drive during the activated time period; and - a nominal load time period during the remaining part of the activated time period; - determine for the deactivated time period an initial deactivated time period followed by a subsequent deactivated time period; and - connect the first subset during the peak load time period and during the initial deactivated time period; - connect the at least one other subset (64, 66) during the nominal load time period and during the subsequent deactivated time period. - a main drive; - a reservoir of hydraulic fluid; - a hydraulic power storage for storing hydraulic fluid at a pressure higher than that of the reservoir, the hydraulic power storage comprising a plurality of hydraulic power storage units; - a hydraulic power unit being: - mechanically connected to the main drive; - hydraulically connected to the hydraulic power storage and to the reservoir; - mechanically connected to the main drive; - hydraulically connected to the hydraulic power storage and to the reservoir; - an intermittent auxiliary drive configured to be powered intermittently by the main drive during an operating period such that the main drive powers the intermittent auxiliary drive during an activated time period and the main drive does not power the intermittent auxiliary drive during a deactivated time period; - a power exchange control system coupled to the hydraulic power unit and the hydraulic power storage, the power exchange control system being configured to selectively connect a first subset and at least one different, other subset (64, 66) of one or more of the hydraulic power storage units to the hydraulic power unit in function of a desired bidirectional power exchange between the main drive and the hydraulic power storage in function of the operating period, characterized in that the power exchange control system is further configured to: - determine for the activated time period: - a peak load time period during which the average power required by the intermittent auxiliary drive is higher than the average power required by the intermittent auxiliary drive during the activated time period; and - a nominal load time period during the remaining part of the activated time period; - a peak load time period during which the average power required by the intermittent auxiliary drive is higher than the average power required by the intermittent auxiliary drive during the activated time period; and - a nominal load time period during the remaining part of the activated time period; - determine for the deactivated time period an initial deactivated time period followed by a subsequent deactivated time period; and - connect the first subset during the peak load time period and during the initial deactivated time period; - connect the at least one other subset (64, 66) during the nominal load time period and during the subsequent deactivated time period.