An embodiment is directed to a module-to-module power connector configured to form connections between battery modules installed in a battery housing of an energy storage system. The module-to-module power connector includes electrical interfaces and busbar(s) configured to form one or more electrical connections terminals of adjacent battery modules. The busbar(s) is flexibly configured to permit a defined range of movement of the electrical interfaces during insertion of the respective battery modules into respective battery module compartments. The module-to-module power connector may further be arranged inside in a tunnel space, whereby holes are defined in a battery module mounting area housing the battery modules that open into the tunnel space.