An electric power unit for a naval vessel, in particular a submarine vessel comprising: an insulating casing suitable for housing a plurality of lithium-ion polymer batteries, each of which comprises a rectangular-shaped flat body that houses the electrochemical components of the battery and is provided with a negative/positive electrode (12 and 13) formed by respective metal blades that extend along a side of the rectangular-shaped flat body, said batteries comprise: a first battery assembly wherein at least a first battery (7-a) and a second battery (7-b) are provided and arranged with the respective flat bodies facing one another; a second battery assembly wherein at least a first battery (7-d) and a second battery (7-e) are provided and arranged with the respective flat bodies facing one another; the first and the second assembly being arranged in series one with the other; characterized by comprising mechanical and electrical interconnection means of the first and second battery assemblies comprising: a first flat element carried by said casing, the blade electrodes of the first batteries (7-a; 7 d) of the first and the second assembly being arranged with their lower face in contact with the opposite sides of the first flat element; a second flat conductor element overlaying the first flat element and arranged in contact with an upper face of the blade electrodes (12 and 13) of the first batteries of the first and the second assembly; the blade electrodes (12 and 13) of the second batteries (7-b; 7 e) of the first and the second assembly being arranged with their lower face in contact with the opposite sides of the second flat conductor element; a third flat conductor element overlaying the first and the second element and arranged in contact with an upper face of the blade electrodes (12 and 13) of the second batteries (7-d; 7 e) of the first and the second assembly; a clamping device able to apply a force to the layered structure formed by the first, second and third flat elements and by the electrodes interposed between the metal conductors. a first battery assembly wherein at least a first battery (7-a) and a second battery (7-b) are provided and arranged with the respective flat bodies facing one another; a second battery assembly wherein at least a first battery (7-d) and a second battery (7-e) are provided and arranged with the respective flat bodies facing one another; the first and the second assembly being arranged in series one with the other; characterized by comprising mechanical and electrical interconnection means of the first and second battery assemblies comprising: a first flat element carried by said casing, the blade electrodes of the first batteries (7-a; 7 d) of the first and the second assembly being arranged with their lower face in contact with the opposite sides of the first flat element; a second flat conductor element overlaying the first flat element and arranged in contact with an upper face of the blade electrodes (12 and 13) of the first batteries of the first and the second assembly; the blade electrodes (12 and 13) of the second batteries (7-b; 7 e) of the first and the second assembly being arranged with their lower face in contact with the opposite sides of the second flat conductor element; a third flat conductor element overlaying the first and the second element and arranged in contact with an upper face of the blade electrodes (12 and 13) of the second batteries (7-d; 7 e) of the first and the second assembly; a clamping device able to apply a force to the layered structure formed by the first, second and third flat elements and by the electrodes interposed between the metal conductors. a first flat element carried by said casing, the blade electrodes of the first batteries (7-a; 7 d) of the first and the second assembly being arranged with their lower face in contact with the opposite sides of the first flat element; a second flat conductor element overlaying the first flat element and arranged in contact with an upper face of the blade electrodes (12 and 13) of the first batteries of the first and the second assembly; the blade electrodes (12 and 13) of the second batteries (7-b; 7 e) of the first and the second assembly being arranged with their lower face in contact with the opposite sides of the second flat conductor element; a third flat conductor element overlaying the first and the second element and arranged in contact with an upper face of the blade electrodes (12 and 13) of the second batteries (7-d; 7 e) of the first and the second assembly; a clamping device able to apply a force to the layered structure formed by the first, second and third flat elements and by the electrodes interposed between the metal conductors.