A battery pack for a vehicle, in particular for a hybrid vehicle or an electric vehicle, comprising: - a casing, - a group of battery cells, arranged within the casing, - a control circuit, also arranged within the casing, - an autonomous cooling system arranged within the casing and configured to cool the battery cells, said battery pack being characterized in that said cooling system which is arranged within the casing of the battery pack includes a circuit for a refrigerant fluid, comprising: - a compressor, actuated by an electric motor, - a condenser, which receives compressed fluid from the compressor, - an expansion device, to expand the fluid coming from the condenser, and - an evaporator, which receives expanded fluid from said expansion device and supplies the fluid to said compressor, wherein - said condenser and said evaporator forming part of the refrigerant fluid circuit consist of two hollow panels arranged adjacent to two opposite major faces of said group of battery cells, each hollow panel having a single inner chamber communicating with an inlet (14B,16B) and an outlet (14B,16B) of the panel, arranged on two opposite sides of the panel, - the hollow panel forming the condenser of said cooling system is placed in contact with a bottom wall of said casing of the battery pack, below the group of battery cells, - a wall of thermally insulating material is interposed between the condenser panel and the lower side of the battery cell group, - the hollow panel forming the evaporator of said cooling system is arranged above the group of battery cells, - walls of thermally insulating material are arranged above the evaporator panel and above the battery cell group, and around the side walls of the battery cell group. - a casing, - a group of battery cells, arranged within the casing, - a control circuit, also arranged within the casing, - an autonomous cooling system arranged within the casing and configured to cool the battery cells, - a compressor, actuated by an electric motor, - a condenser, which receives compressed fluid from the compressor, - an expansion device, to expand the fluid coming from the condenser, and - an evaporator, which receives expanded fluid from said expansion device and supplies the fluid to said compressor, wherein - said condenser and said evaporator forming part of the refrigerant fluid circuit consist of two hollow panels arranged adjacent to two opposite major faces of said group of battery cells, each hollow panel having a single inner chamber communicating with an inlet (14B,16B) and an outlet (14B,16B) of the panel, arranged on two opposite sides of the panel, - the hollow panel forming the condenser of said cooling system is placed in contact with a bottom wall of said casing of the battery pack, below the group of battery cells, - a wall of thermally insulating material is interposed between the condenser panel and the lower side of the battery cell group, - the hollow panel forming the evaporator of said cooling system is arranged above the group of battery cells, - walls of thermally insulating material are arranged above the evaporator panel and above the battery cell group, and around the side walls of the battery cell group.