NOVELTY - The modular assembly comprises a module (10) comprising a support frame for receiving two electrochemical units (50). The support frame comprises main fluidic connectors juxtaposed between coupling modules, and auxiliary fluidic connectors connected with the electrochemical units carried by the support frame. The support frame has hooks (20) for receiving the electrochemical units, and a column (11) that carries the plates and the main and auxiliary fluid connectors. The assembly further comprises plates. The main fluid connectors are arranged on opposite sides of the column.    USE - The modular assembly is useful for coupling electrochemical units such as electrolysis units in an electrochemical system (all claimed), where the electrochemical units are fuel cell units or rechargeable batteries.    ADVANTAGE - The modular assembly allows easy adjustment of the power of the electrochemical system, and allows easy replacement of the electrochemical units for maintenance operations.    DETAILED DESCRIPTION - The modular assembly comprises a module (10) comprising a support frame for receiving two electrochemical units (50). The support frame comprises main fluidic connectors juxtaposed between coupling modules, and auxiliary fluidic connectors connected with the electrochemical units carried by the support frame. The support frame has hooks (20) for receiving the electrochemical units, and a column (11) that carries the plates and the main and auxiliary fluid connectors. The assembly further comprises plates. The main fluid connectors are arranged on opposite sides of the column. The plates are arranged in pairs on opposite sides of the column. The support frame carries power supply connectors (116) that are common to the electrochemical units. The common power supply connector is arranged on top of the column. The module has a relay for selecting electrochemical units. The auxiliary fluidic connectors are equipped with self-closing valves in case of disconnection. The valves of the auxiliary fluidic connectors comprise a check valve. The electric power of the module is greater than or equal to 20 kW. An INDEPENDENT CLAIM is included for an electrochemical system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic perspective view of a modular assembly.    Module (10)    Column (11)    Hooks (20)    Electrochemical units (50)    Power supply connectors. (116)