A modular battery system provides propulsive power to the rotor system of an aircraft. The modular battery system includes an array of battery modules arranged in at least one stack. Each battery module includes a plurality of battery cells, a first side having positive and negative receptacles and a second side, that is opposite of the first side, having positive and negative plugs. The receptacles and plugs are configured such that adjacent battery modules in a side-by-side relationship are electrically coupled together via plug and receptacle connections and such that the battery modules are electrically coupled together in parallel. An interconnection electrically couples each stack of battery modules together via plug and receptacle connections with one of the battery modules in each stack such that the stacks of battery modules are electrically coupled together in parallel.