Abstract: A device for monitoring battery cells of a battery string under load condition. The device has differential voltage units for each battery cell, wherein a band-pass filter is arranged downstream of each differential voltage unit, wherein the band-pass filters are connected to rectifier circuits with a smoothing circuit arranged downstream, and wherein the device is designed so the output signals of the smoothing circuits are supplied to an evaluation unit and are compared for deviations in relation to a standardized output signal.

Abstract: The invention relates to a method for determining a state of at least one cell of a battery, wherein the battery has a plurality of cells, which are connected in series with each other, the method comprising at least the following steps: a) applying an alternating current to the plurality of cells; b) measuring the alternating voltage produced thereby at at least a first cell and a second cell; c) analyzing a phase position of the measured alternating voltage of each cell; wherein a difference at least between a first phase position of a first alternating voltage measured at the first cell and a second phase position of a second alternating voltage measured at the second cell forms a conclusion about a difference between the states of at least the first cell and the second cell.

Abstract: A device for monitoring battery cells of a battery string under load condition. The device has differential voltage units for each battery cell, wherein a band-pass filter is arranged downstream of each differential voltage unit, wherein the band-pass filters are connected to rectifier circuits with a smoothing circuit arranged downstream, and wherein the device is designed so the output signals of the smoothing circuits are supplied to an evaluation unit and are compared for deviations in relation to a standardized output signal.