In a method and a device for determining the internal resistance of a battery cell of a battery, the battery is connected to a controllable motor/generator in such a way that a flow of energy can take place from the battery to the motor/generator or from the motor/generator to the battery, including the following steps: modulation of the magnetic field-forming portion of the motor/generator, determination of a change in voltage at the battery cell and of a change of the current flow through the battery cell during the modulation of the magnetic field-forming portion of the motor/generator, and calculation of the internal resistance of the battery cell as the quotient of the change in voltage and the change in current flow.