Electrification of maritime and IW vessels shows a low progress, since there are several challenges that need to be overcome before battery energy storage systems (BESS) can compete with other solutions towards the decarbonization of vessels. The main challenges are related to the: (i) weight of the BESS, since a high storage capacity represents a significant share of the vessel’s cargo capacity in small/medium sized vessels, (ii) the reliability so that to ensure the trouble-free routing and reduction of operating and maintenance costs, and (iii) the safety that is the key to enable the large-scale adoption and remove any concern of vessel owners. These three aspects constitute the Innovation Pillars of HARPOONERS, and drive the development of a unique modular concept for interfacing with HV on-board grids, characterized by a high compactness due to the integration of key components in a single configuration, thus eliminating the transformer and a cooling system. This “AC battery system” will be developed together with the management systems for a reliable operation in future powertrains of all-electric or hybrid vessels with reduced emissions. The outcome is a reduced weight, reliable and safer BESS that will allow the uptake of the HARPOONERS solution in both maritime and IW vessels, to further increase the battery capacity and achieve a longer autonomy. This is extremely beneficial in small/medium vessels, in which full electrification will be possible, thus eliminating the use of the combustion engine and its associated CO2 and other emissions, but also on large vessels in which the HARPOONERS solution with less use of raw materials will lead to a reduced environmental impact and lower manufacturing and maintenance costs. To achieve this, HARPOONERS has gathered a balanced European-wide partnership with organizations from the key value chains, such as research, battery system and power electronics manufacturers, shipping owners, and a Classification Society.