A method for controlling the charging process of a vehicle's batteries by means of a charging system, which batteries comprise a starting battery set with first parameters which characterise this first battery set and a service battery set with second parameters which characterise this second battery set, wherein the method comprises - catering (A1, A2) for the first and second parameters of the battery sets, in which the first parameters comprise the starting battery set's battery type, state of charge, state of health and temperature, and the second parameters comprise the service battery set's battery type, state of charge, state of health and temperature and where said service battery set is always connected to an electrical system including a generator, and - controlling the charging process on the basis of these parameters in which the method for controlling the charging process comprises regulating the magnitude of charging voltage applied from the electrical system to the battery sets (11, 12) and the amount of time for which it is applied in order to put the charging system into different charging situations: - a first charging situation in which both the starting battery set and the service battery set are connected to the electrical system by a breaker being put into a closed state and charging the starting battery set is of higher priority than charging the service battery set; - a second charging situation in which both the starting battery set and the service battery set are connected to the electrical system by the breaker being put into the closed state and charging the service battery set is of higher priority than charging the starting battery set; - a third charging situation in which the breaker is put into an opened state so that the starting battery set is disconnected from the electrical system and the service battery set is connected to the electrical system; and - a fourth charging situation in which both the starting battery set and the service battery set are connected to the electrical system by the breaker being put into the closed state and the charging of both sets is of equal priority. - catering (A1, A2) for the first and second parameters of the battery sets, in which the first parameters comprise the starting battery set's battery type, state of charge, state of health and temperature, and the second parameters comprise the service battery set's battery type, state of charge, state of health and temperature and where said service battery set is always connected to an electrical system including a generator, and - controlling the charging process on the basis of these parameters in which the method for controlling the charging process comprises regulating the magnitude of charging voltage applied from the electrical system to the battery sets (11, 12) and the amount of time for which it is applied in order to put the charging system into different charging situations: - a first charging situation in which both the starting battery set and the service battery set are connected to the electrical system by a breaker being put into a closed state and charging the starting battery set is of higher priority than charging the service battery set; - a second charging situation in which both the starting battery set and the service battery set are connected to the electrical system by the breaker being put into the closed state and charging the service battery set is of higher priority than charging the starting battery set; - a third charging situation in which the breaker is put into an opened state so that the starting battery set is disconnected from the electrical system and the service battery set is connected to the electrical system; and - a fourth charging situation in which both the starting battery set and the service battery set are connected to the electrical system by the breaker being put into the closed state and the charging of both sets is of equal priority. - a first charging situation in which both the starting battery set and the service battery set are connected to the electrical system by a breaker being put into a closed state and charging the starting battery set is of higher priority than charging the service battery set; - a second charging situation in which both the starting battery set and the service battery set are connected to the electrical system by the breaker being put into the closed state and charging the service battery set is of higher priority than charging the starting battery set; - a third charging situation in which the breaker is put into an opened state so that the starting battery set is disconnected from the electrical system and the service battery set is connected to the electrical system; and - a fourth charging situation in which both the starting battery set and the service battery set are connected to the electrical system by the breaker being put into the closed state and the charging of both sets is of equal priority.