A method of assembling and electrically interconnecting an energy storage system, the system includes a cabinet and a plurality of energy storage modules which may be connected together in the cabinet, each energy storage module has a plurality of energy storage devices. The method includes carrying out an electrical interconnection step by providing a removable current monitoring device at one pole of a circuit of the cabinet during the electrical interconnection step; connecting a first module to the pole; monitoring current flow after connecting the first module and, if no current flow is detected, connecting a further module in series with the first module. The monitoring and connecting steps are repeated for each subsequent module until all modules of the cabinet have been electrically interconnected. The final module is connected to the other pole of the circuit and the current monitoring device is disconnected and removed from the circuit.