A method for ascertaining the state of charge of an electrical energy storage unit is described. In one example, the method includes ascertaining a voltage gradient at least based on a detected first voltage value of the electrical energy storage unit; comparing the ascertained voltage gradient with a predefined voltage gradient threshold value; and ascertaining the state of charge of the electrical energy storage unit depending on the comparison. A corresponding computer program, a corresponding machine-readable storage medium, a corresponding apparatus and a corresponding electrical energy storage system are also described.