A method for determining a state value of a traction battery of an electric vehicle characterises the ageing state, preferably an SoH value. The traction battery is charged or discharged by a test load and a respective output voltage and load current value pair is acquired. An ohmic internal resistance is established on the basis of the acquired value pair. The state value is established on the basis of the established ohmic internal resistance. At least one normalisation variable characterizing the traction battery is established. On the basis of the established ohmic internal resistance and the normalisation variable, a normalised internal resistance based on a reference value of the normalisation variable is established. The state value is established on the basis of the normalised internal resistance. A diagnostics device has an evaluation unit which is directly or indirectly couplable to the traction battery and carries out the method.