NOVELTY - The power conversion device (1) has a converter (120) that includes a first circuit (21) coupled to a battery, a second circuit (22) coupled to a DC link (30), and a transformer (50) that is provided between the first and second circuits. A controller (70) controls the converter to repeatedly perform charging and discharging operations for the battery in a battery heating mode. A temperature sensor measures the temperature of the battery.    USE - Power conversion device used in energy storage system such as new energy and renewable energy generation system and for generating and supplying electrical energy using renewable energy such as sunlight, water, subterranean heat, rainfall and living organism to load such as home and industrial facility, and solar generation system for converting solar energy such as solar heat and sunlight into electrical energy through solar cells, and wind power generation system for converting wind power into electrical energy, subterranean heat generation system for converting subterranean heat into electrical energy, hydraulic power generation system, ocean power generation system, AC power system provided from electric power company or power generation company, and electrical link formed in wide area including power station, transformer substation, and power transmission line.    ADVANTAGE - The controller heats the battery by controlling the bidirectional converter during a battery heating mode, so that battery is normally operated. The controller performs turn-on and turn-off operations of the first and fourth switches while maintaining the second and third switches in the turn-on state, so that discharging operation of the battery is effectively performed during the battery heating mode. The controller controls heating of the battery by repeatedly performing the charging operation and the discharging operation. The controller controls the bidirectional converter when the power conversion device enters into the battery heating mode, so that the battery repeats charging and discharging during the battery heating mode. The battery monitoring system calculates the state of charge and/or state of health of the battery, so that cell balancing of equalizing the voltage or capacity of the battery is performed. The power conversion device performs a battery heating function without using a separate heating device. The discharging operation raises the temperature of the battery to allow for general or normal driving mode operation.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of an energy storage system employing a power conversion device.    Power conversion device (1)    First circuit (21)    Second circuit (22)    Direct current link (30)    Transformer (50)    Controller (70)    Converter (120)