Abstract: A modular battery system is provided that includes two primary components: a master module and multiple battery modules. The master module has two or more master terminals. The battery modules each have two subordinate negative terminals and two subordinate positive terminals. Two of the battery modules are electrically connected with a first electrical connection between the subordinate terminals on each module. Two or more of the subordinate terminals are electrically connected to the master terminals with a second electrical connection. One or both of the first electrical connection and the second electrical connection includes a parallel connection between two terminals of like polarity.