Complex physics and long computation time hinder the adoption of computer aided engineering models in the design of largeformat battery cells and systems. A modular, efficient battery simulation model-the multiscale multidomain (MSMD) model-was previously introduced to aid the scale-up of Li-ion material & electrode designs to complete cell and pack designs, capturing electrochemical interplay with 3-D electronic current pathways and thermal response. This paper enhances the computational efficiency of the MSMD model using a separation of time-scales principle to decompose model field variables. The decomposition provides a quasi-explicit linkage between the multiple length-scale domains and thus reduces time-consuming nested iteration when solving model equations across multiple domains. In addition to particle-, electrode-and cell-length scales treated in the previous work, the present formulation extends to bus bar-and multi-cell module-length scales. In cutting-edge industries such as automotive and aviation, computer models are valuable tools for reducing the cost of product development, improving manufacturing processes, optimizing designs, and implementing advanced controls. Although the global electricdrive-vehicle market is growing rapidly, the lack of a model that can accurately predict a battery's behavior is recognized as a threat to the automotive industry that has been enhancing its dependence on computer models. In a lithium-ion battery, which is the preeminent candidate powering electric-drive vehicles, physiochemical processes take place in intricate geometries over a wide range of time and length scales. The device response of a battery results from complex nonlinear interplays among material characteristics, design variables, and environmental and operational conditions. The multiscale nonlinear nature of battery physics even more critically affects the device behavior as the size of a battery increases. Without understanding the interplays among the interdisciplinary physicochemical processes occurring across varied scales, it is costly to design long-lasting, highperforming, safe, large batteries.The U.S. Department of Energy's Computer Aided Engineering for Electric Drive Vehicle Battery (CAEBAT) program has supported development of modeling capabilities to help industries accelerate mass-market adoption of electric-drive vehicles and their batteries. In support of the U.S. Department of Energy, National Renewable Energy Laboratory developed the multiscale multidomain (MSMD) model, overcoming challenges in modeling the highly nonlinear multiscale response of battery systems.1,2 The MSMD model introduces separate model domains at particle, electrode, and cell levels, while tightly coupling the physics across the scales. The separation of a model domain and the adoption of local homogeneity assumption are enabled by the intrinsic nature of typical battery systems where substantial time-and length-scale segregation occurs. The MSMD particle-domain models (PDMs) solve collective response of ele...