A method for monitoring a battery state without interruption of functioning a battery system using a battery twin, wherein the battery system includes a plurality of interconnected battery cells, and wherein the battery twin includes a battery twin cell that is identical to one battery cell of the battery system, where the method includes measuring a parameter of the battery cell that describes working conditions of either the battery cell and/or the entire battery system, where the method includes an offline measurement part that includes reproduction of the working conditions described with the measured parameter on the battery twin, such that while the battery twin operate under the reproduced working conditions, at least one further parameter of the battery twin is measured, and after the offline measurement part, the method further includes judging the battery state based on the measured at least one further parameter of the battery twin.