A control system suitable for inclusion in a power supply system in which a plurality of power supply units are parallel connected and in which charging and discharging of each of the power supply units is controlled, the control system comprising: a power conditioner that is arranged cooperatively parallel connected to the plurality of power supply units and is further cooperatively connected to an external power system, and is arranged to receive electric power from the power system and is arranged to charge the plurality of power supply units with the received electric power, and arranged to receive electric power from the plurality of power supply units and arranged to discharge the received electric power to the power system; a master controller that is arranged to collect battery information having been output from a unit controller of each of the power supply units; and a system controller that is arranged to communicate to the power conditioner electric power required to the external power supply system as a power command, characterised in that the power conditioner is arranged to determine, according to the power command from the system controller, a working total power, POBall, which is a summation of electric power at which each of the power supply units is charged and discharged, and is arranged to cause charging and discharging of each of the power supply units within working individual power, POB n, based on possible total power, SOPall, at which all the available power supply units are chargeable and dischargeable, the possible total power, SOPall, being determined from a possible individual power, SOP n, of each of the power supply units, determined based on the battery information detected on each of the power supply units, and the working individual power, POB n, being determined based on a power deviation occurring between the power supply units and comprising a ratio relating possible individual powers of each of the power supply units, wherein POB n ≦ SOP n and SOPall ≧ POBall. a power conditioner that is arranged cooperatively parallel connected to the plurality of power supply units and is further cooperatively connected to an external power system, and is arranged to receive electric power from the power system and is arranged to charge the plurality of power supply units with the received electric power, and arranged to receive electric power from the plurality of power supply units and arranged to discharge the received electric power to the power system; a master controller that is arranged to collect battery information having been output from a unit controller of each of the power supply units; and a system controller that is arranged to communicate to the power conditioner electric power required to the external power supply system as a power command, characterised in that the power conditioner is arranged to determine, according to the power command from the system controller, a working total power, POBall, which is a summation of electric power at which each of the power supply units is charged and discharged, and is arranged to cause charging and discharging of each of the power supply units within working individual power, POB n, based on possible total power, SOPall, at which all the available power supply units are chargeable and dischargeable, the possible total power, SOPall, being determined from a possible individual power, SOP n, of each of the power supply units, determined based on the battery information detected on each of the power supply units, and the working individual power, POB n, being determined based on a power deviation occurring between the power supply units and comprising a ratio relating possible individual powers of each of the power supply units, wherein POB n ≦ SOP n and SOPall ≧ POBall.