Disclosed is a lithium-ion secondary battery system, comprising an assembled battery, further comprising a plurality of lithium-ion secondary batteries; an SOC measuring unit that measures the SOC of the lithium-ion secondary batteries; a temperature detector unit that detects the temperature thereof; and a heater unit for heating the lithium-ion secondary batteries. When the SOC measured by the SOC measuring unit is lower than a set SOC that is associated with the discharge rate and preset, and the temperature measured by the temperature detection unit is lower than a set temperature that is associated with the discharge rate and preset, an instruction is issued from a control unit to the heater unit to supply heat such that the lithium-ion secondary battery reaches a prescribed temperature.