A method for monitoring a battery state without interruption of functioning of a battery system using a battery twin wherein the battery system comprises a plurality of battery cells (3, 3') connected to each other, wherein the battery twin comprises at least one battery twin cell that is identical to at least one of the battery cells (3, 3') of the battery system the method comprises: a step of measuring at least one parameter of the at least one battery cell of the battery system, wherein such measured at least one parameter describes working conditions of the at least one battery cell 3' the method characterized in that it further comprises an offline measurement part, that comprises: a step of reproducing the working conditions described with the measured at least one parameter on the battery twin in such way that the at least one parameter of the battery twin under the reproduced working conditions is equal to the measured at least one parameter of the at least one battery cell of the battery system, a step of measuring at least one further parameter of the battery twin while the battery twin is working under the reproduced working conditions, after the offline measurement part the method further comprises a step of making judgment about the battery state based on the measured at least one further parameter of the battery twin. wherein the battery system comprises a plurality of battery cells (3, 3') connected to each other, wherein the battery twin comprises at least one battery twin cell that is identical to at least one of the battery cells (3, 3') of the battery system the method comprises: a step of measuring at least one parameter of the at least one battery cell of the battery system, wherein such measured at least one parameter describes working conditions of the at least one battery cell 3' the method characterized in that it further comprises an offline measurement part, that comprises: a step of reproducing the working conditions described with the measured at least one parameter on the battery twin in such way that the at least one parameter of the battery twin under the reproduced working conditions is equal to the measured at least one parameter of the at least one battery cell of the battery system, a step of measuring at least one further parameter of the battery twin while the battery twin is working under the reproduced working conditions, after the offline measurement part the method further comprises a step of making judgment about the battery state based on the measured at least one further parameter of the battery twin. a step of measuring at least one parameter of the at least one battery cell of the battery system, wherein such measured at least one parameter describes working conditions of the at least one battery cell 3' the method characterized in that it further comprises an offline measurement part, that comprises: a step of reproducing the working conditions described with the measured at least one parameter on the battery twin in such way that the at least one parameter of the battery twin under the reproduced working conditions is equal to the measured at least one parameter of the at least one battery cell of the battery system, a step of measuring at least one further parameter of the battery twin while the battery twin is working under the reproduced working conditions, a step of reproducing the working conditions described with the measured at least one parameter on the battery twin in such way that the at least one parameter of the battery twin under the reproduced working conditions is equal to the measured at least one parameter of the at least one battery cell of the battery system, a step of measuring at least one further parameter of the battery twin while the battery twin is working under the reproduced working conditions, after the offline measurement part the method further comprises a step of making judgment about the battery state based on the measured at least one further parameter of the battery twin.