A transport refrigeration system comprising: a refrigeration unit configured to circulate refrigerant through a refrigeration circuit, the refrigeration unit comprising: a refrigerant compression device having an inlet and an outlet; a refrigerant heat rejection heat exchanger fluidly connected to the outlet; a flash tank fluidly connected to the refrigerant heat rejection heat exchanger, the flash tank being configured to separate the refrigerant into a liquefied refrigerant and a vaporized refrigerant; and a refrigerant heat absorption heat exchanger fluidly connected to the flash tank and configured to receive the liquefied refrigerant from the flash tank, the refrigerant heat absorption heat exchanger being fluidly connected to the inlet, the transport refrigeration system further comprising: a battery system configured to power the refrigeration unit, wherein, in use, the battery system powers the refrigeration unit, and wherein the battery system comprises a plurality of battery modules; and a heat transfer system thermally connected to the battery system, the heat transfer system being configured to transfer heat from the flash tank to the battery system through the vaporized refrigerant to heat up the battery system, the heat transfer system comprising a vapor inlet, a vapor outlet, and an interconnected network of tubes, wherein the heat transfer system further comprises a plurality of heat transfer plates, wherein each of plurality of the battery modules is interposed between at least two heat transfer plates, the heat transfer plates being thermally connected with the battery modules, and wherein the heat transfer system is configured so that the vaporized refrigerant enters the heat transfer system through the vapor inlet, travels through the plurality of heat transfer plates through the interconnected network of tubes, and then exits the heat transfer system through the vapor outlet. a refrigeration unit configured to circulate refrigerant through a refrigeration circuit, the refrigeration unit comprising: a refrigerant compression device having an inlet and an outlet; a refrigerant heat rejection heat exchanger fluidly connected to the outlet; a flash tank fluidly connected to the refrigerant heat rejection heat exchanger, the flash tank being configured to separate the refrigerant into a liquefied refrigerant and a vaporized refrigerant; and a refrigerant heat absorption heat exchanger fluidly connected to the flash tank and configured to receive the liquefied refrigerant from the flash tank, the refrigerant heat absorption heat exchanger being fluidly connected to the inlet, a refrigerant compression device having an inlet and an outlet; a refrigerant heat rejection heat exchanger fluidly connected to the outlet; a flash tank fluidly connected to the refrigerant heat rejection heat exchanger, the flash tank being configured to separate the refrigerant into a liquefied refrigerant and a vaporized refrigerant; and a refrigerant heat absorption heat exchanger fluidly connected to the flash tank and configured to receive the liquefied refrigerant from the flash tank, the refrigerant heat absorption heat exchanger being fluidly connected to the inlet, the transport refrigeration system further comprising: a battery system configured to power the refrigeration unit, wherein, in use, the battery system powers the refrigeration unit, and wherein the battery system comprises a plurality of battery modules; and a heat transfer system thermally connected to the battery system, the heat transfer system being configured to transfer heat from the flash tank to the battery system through the vaporized refrigerant to heat up the battery system, the heat transfer system comprising a vapor inlet, a vapor outlet, and an interconnected network of tubes, wherein the heat transfer system further comprises a plurality of heat transfer plates, wherein each of plurality of the battery modules is interposed between at least two heat transfer plates, the heat transfer plates being thermally connected with the battery modules, and wherein the heat transfer system is configured so that the vaporized refrigerant enters the heat transfer system through the vapor inlet, travels through the plurality of heat transfer plates through the interconnected network of tubes, and then exits the heat transfer system through the vapor outlet. a battery system configured to power the refrigeration unit, wherein, in use, the battery system powers the refrigeration unit, and wherein the battery system comprises a plurality of battery modules; and a heat transfer system thermally connected to the battery system, the heat transfer system being configured to transfer heat from the flash tank to the battery system through the vaporized refrigerant to heat up the battery system, the heat transfer system comprising a vapor inlet, a vapor outlet, and an interconnected network of tubes, wherein the heat transfer system further comprises a plurality of heat transfer plates, wherein each of plurality of the battery modules is interposed between at least two heat transfer plates, the heat transfer plates being thermally connected with the battery modules, and wherein the heat transfer system is configured so that the vaporized refrigerant enters the heat transfer system through the vapor inlet, travels through the plurality of heat transfer plates through the interconnected network of tubes, and then exits the heat transfer system through the vapor outlet.