This paper introduces an original approach of the modeling and simulation of multiphysics systems that exhibit a wide range of time scales. This approach will be illustrated by the simulation of the energy storage unit (ESU) of an all-electric ferry. The implementation of the models was performed using two different modeling approaches (causal and acausal) highlighting the main difficulties of modeling multiphysics systems. In order to optimize the sizing of the ESU considering its ageing, the system should be simulated for a lifetime of 20 years. For this purpose, a multirate method was developed to speed-up the simulation and the optimization by a factor of 100 or more.