A replacement method is provided for replacing a to-be-replaced battery module with a replacing fresh battery module in a battery assembly formed by electrically connecting a plurality of battery modules in series or parallel. The method includes the step (S110) of charging the fresh battery module so that the quantity of charged electricity of the fresh battery module is 5 to 20% less than the quantity of charged electricity of a battery module other than the to-be-replaced battery module, and the step (S120) of replacing the to-be-replaced battery module with the charged fresh battery module.