Inhomogeneous heat generation and temperature distribution in association with non-uniform electrochemical reaction is deemed as a serious problem in large-format lithium-ion batteries. To address this challenge, it requires a thorough understanding of the battery’s coupled electrochemical-thermal performance. Therefore, in this paper, we would like to investigate the relevant physical phenomena observed in the battery electrical and thermal responses via a reduce-order 3D electrochemical-thermal model. More importantly, the dominant physical mechanisms of the battery performance are to be discussed. The model has been developed to parameterize large-format Li-ion prismatic pouch cells, and it can be further applied to simulate the irregular temperature distribution on the battery due to the non-uniform electrochemical reaction and heat generation. A scaling analysis of the model is carried out to determine the governing dimensionless numbers of the battery during charging and discharging, based on which the model complexity can be dramatically reduced. The key dimensionless number found in this study are known to be related to the material properties of the battery and the control parameters in the tests.