Method for maintaining a predetermined vehicle electrical system voltage in a battery-assisted vehicle electrical system during operation of an electric starter motor, comprising the following steps of: - operating the electric starter motor from the vehicle electrical system during a first phase in series with a limiting resistor in order to limit the current flowing through the starter motor; and - operating the electric starter motor from the vehicle electrical system during a second phase with a reduced limiting resistor in order to increase power converted by the starter motor, wherein a transition from the first phase to the second phase is controlled on the basis of electrical characteristic variables (U_in, U_out) sensed at the limiting resistor during the first phase, - determining a threshold value during the first phase; and - initiating the second phase if the current flowing through the limiting resistor undershoots the determined threshold value, wherein the threshold value is determined to be a value which is at least as large as I 1 R K + R 2 R K + R 1; where I 1 is the current flowing during the first phase; R K is the sum of the non-reactive resistances in a circuit comprising a battery of the vehicle electrical system, a first line from the battery to the limiting resistor, a second line from the limiting resistor to the starter motor and the starter motor; R 1 is the limiting resistor; and R 2 is the reduced limiting resistor, characterized in that R K is determined as follows: - determining voltages at both connections of the limiting resistor at a time t 1; - determining the sum of the resistances of the battery and of the line leading from the battery to the limiting resistor on the basis of the voltages determined at the time t 1; - determining voltages at both connections of the limiting resistor at mutually different times t 2 and t 3 after inductive effects have decayed, and - determining the sum of the resistances of the second line leading from the limiting resistor to the starter motor and of the starter motor on the basis of the voltages determined at the times t 2 and t 3. - operating the electric starter motor from the vehicle electrical system during a first phase in series with a limiting resistor in order to limit the current flowing through the starter motor; and - operating the electric starter motor from the vehicle electrical system during a second phase with a reduced limiting resistor in order to increase power converted by the starter motor, wherein a transition from the first phase to the second phase is controlled on the basis of electrical characteristic variables (U_in, U_out) sensed at the limiting resistor during the first phase, - determining a threshold value during the first phase; and - initiating the second phase if the current flowing through the limiting resistor undershoots the determined threshold value, wherein the threshold value is determined to be a value which is at least as large as I 1 R K + R 2 R K + R 1; where I 1 is the current flowing during the first phase; R K is the sum of the non-reactive resistances in a circuit comprising a battery of the vehicle electrical system, a first line from the battery to the limiting resistor, a second line from the limiting resistor to the starter motor and the starter motor; R 1 is the limiting resistor; and R 2 is the reduced limiting resistor, characterized in that R K is determined as follows: - determining voltages at both connections of the limiting resistor at a time t 1; - determining the sum of the resistances of the battery and of the line leading from the battery to the limiting resistor on the basis of the voltages determined at the time t 1; - determining voltages at both connections of the limiting resistor at mutually different times t 2 and t 3 after inductive effects have decayed, and - determining the sum of the resistances of the second line leading from the limiting resistor to the starter motor and of the starter motor on the basis of the voltages determined at the times t 2 and t 3.