Method for determining an internal resistance (R i) of an electrical energy store having the following method steps taking place in succession: a. analogue-to-digital conversion of an analogue voltage signal (U analogue) and of an analogue current signal (I analogue) of the electrical energy store in order to obtain digital voltage values and digital current values, b. filtering of the digital voltage values and of the digital current values using a bandpass filter around the zero phase frequency (f zp) in order to obtain filtered voltage values (U filt) and current values (I filt), c. checking of whether the following calculation prerequisites are satisfied: I. the bandpass filter is initialized, II. a current peak value (I p) with its associated time (t p) is ascertained from the filtered current values (I filt), III. the absolute value of the current peak value (I p) is greater than the absolute value of a threshold current value (I th), wherein the absolute value of the threshold current value (I th) is greater than the absolute value of a minimum threshold current value (I th,min) that can be used to calculate a zero phase resistance (R zp) having a prescribed maximum difference from the actual internal resistance (Ri), IV. the filtered voltage value (U filt) at the previously ascertained time (t p) is a voltage peak value (U p), V. the current peak value (I p) and the voltage peak value (U p) have the same arithmetic sign, these calculation prerequisites being checked in succession, but calculation prerequisite I also being able to be checked at any other point in method step c and the method being terminated if one of the calculation prerequisites is not satisfied, d. calculation of the zero phase resistance (R zp) from the voltage peak value (U p) and the current peak value (I p) if the method has not yet been terminated beforehand, e. provision of the zero phase resistance (R zp) as a determinable internal resistance (R i) of the electrical energy store if the method has not yet been terminated beforehand. a. analogue-to-digital conversion of an analogue voltage signal (U analogue) and of an analogue current signal (I analogue) of the electrical energy store in order to obtain digital voltage values and digital current values, b. filtering of the digital voltage values and of the digital current values using a bandpass filter around the zero phase frequency (f zp) in order to obtain filtered voltage values (U filt) and current values (I filt), c. checking of whether the following calculation prerequisites are satisfied: I. the bandpass filter is initialized, II. a current peak value (I p) with its associated time (t p) is ascertained from the filtered current values (I filt), III. the absolute value of the current peak value (I p) is greater than the absolute value of a threshold current value (I th), wherein the absolute value of the threshold current value (I th) is greater than the absolute value of a minimum threshold current value (I th,min) that can be used to calculate a zero phase resistance (R zp) having a prescribed maximum difference from the actual internal resistance (Ri), IV. the filtered voltage value (U filt) at the previously ascertained time (t p) is a voltage peak value (U p), V. the current peak value (I p) and the voltage peak value (U p) have the same arithmetic sign, these calculation prerequisites being checked in succession, but calculation prerequisite I also being able to be checked at any other point in method step c and the method being terminated if one of the calculation prerequisites is not satisfied, I. the bandpass filter is initialized, II. a current peak value (I p) with its associated time (t p) is ascertained from the filtered current values (I filt), III. the absolute value of the current peak value (I p) is greater than the absolute value of a threshold current value (I th), wherein the absolute value of the threshold current value (I th) is greater than the absolute value of a minimum threshold current value (I th,min) that can be used to calculate a zero phase resistance (R zp) having a prescribed maximum difference from the actual internal resistance (Ri), IV. the filtered voltage value (U filt) at the previously ascertained time (t p) is a voltage peak value (U p), V. the current peak value (I p) and the voltage peak value (U p) have the same arithmetic sign, d. calculation of the zero phase resistance (R zp) from the voltage peak value (U p) and the current peak value (I p) if the method has not yet been terminated beforehand, e. provision of the zero phase resistance (R zp) as a determinable internal resistance (R i) of the electrical energy store if the method has not yet been terminated beforehand.