A battery energy storage system, comprising: a rechargeable battery; a converter for converting a DC current from the rechargeable battery into a current to be supplied to a load (22, 26) and for converting a current from a source (20, 22, 26) into a DC current to be supplied to the rechargeable battery; a closed cooling circuit for cooling the rechargeable battery and the converter with a cooling fluid adapted for performing a phase change, when heated by at least one of the rechargeable battery and the converter; a controller for controlling the battery storage system; wherein the cooling circuit interconnects a cooling component for the rechargeable battery and a cooling component for the converter; wherein the rechargeable battery is composed of a plurality of battery cells; wherein the cooling component for the rechargeable battery is a sealed housing, in which the battery cells and their electrical interconnections are accommodated; wherein the battery cells and the electrical interconnections are at least partially immersed in the cooling fluid in the sealed housing for two-phase cooling; wherein the controller is adapted for: determining a temperature of the rechargeable battery and a temperature of the converter; controlling a cooling fluid flow through the rechargeable battery and the converter based on the temperature of the rechargeable battery and the temperature of the converter; actively heating the cooling fluid, when the temperature of the rechargeable battery is below a threshold temperature, wherein the cooling fluid is heated by flowing through the converter, which is controlled to generate waste heat. a rechargeable battery; a converter for converting a DC current from the rechargeable battery into a current to be supplied to a load (22, 26) and for converting a current from a source (20, 22, 26) into a DC current to be supplied to the rechargeable battery; a closed cooling circuit for cooling the rechargeable battery and the converter with a cooling fluid adapted for performing a phase change, when heated by at least one of the rechargeable battery and the converter; a controller for controlling the battery storage system; wherein the cooling circuit interconnects a cooling component for the rechargeable battery and a cooling component for the converter; wherein the rechargeable battery is composed of a plurality of battery cells; wherein the cooling component for the rechargeable battery is a sealed housing, in which the battery cells and their electrical interconnections are accommodated; wherein the battery cells and the electrical interconnections are at least partially immersed in the cooling fluid in the sealed housing for two-phase cooling; wherein the controller is adapted for: determining a temperature of the rechargeable battery and a temperature of the converter; controlling a cooling fluid flow through the rechargeable battery and the converter based on the temperature of the rechargeable battery and the temperature of the converter; actively heating the cooling fluid, when the temperature of the rechargeable battery is below a threshold temperature, wherein the cooling fluid is heated by flowing through the converter, which is controlled to generate waste heat. determining a temperature of the rechargeable battery and a temperature of the converter; controlling a cooling fluid flow through the rechargeable battery and the converter based on the temperature of the rechargeable battery and the temperature of the converter; actively heating the cooling fluid, when the temperature of the rechargeable battery is below a threshold temperature, wherein the cooling fluid is heated by flowing through the converter, which is controlled to generate waste heat.