Method for battery condition determination in a dual battery system of a vehicle, said dual battery system including: - power battery means being connected to an electrical system including a starter motor, and - starter battery means being connectable in parallel with said power battery means by a circuit breaker, whereby, when said circuit breaker is closed, said power battery means and said starter battery means provide a dual system voltage to the electrical system, characterized by the following steps being performed during a time period when a starter motor is inactive: - determining, when said circuit breaker is open, at least one of an open circuit power battery voltage and an open circuit starter battery voltage, - determining, when said dual circuit breaker is closed, said dual system voltage, and - utilizing said dual system voltage and the determined at least one of said open circuit power battery voltage and said open circuit starter battery voltage for determining a relative state of health for said power battery means, said relative state of health being related over time to a state of health for said starter battery means, where the state of health for said starter battery means is presumed to be good, and wherein a variation in size over time of a first difference is utilized for determining said relative state of health for said power battery means, said first difference being a difference between said dual system voltage and said open circuit power battery voltage, and wherein said state of health for said power battery means is considered to deteriorate relatively to said state of health for said starter battery means if said first difference varies such that a difference between said dual system voltage and said open circuit starter battery voltage decreases over time. - power battery means being connected to an electrical system including a starter motor, and - starter battery means being connectable in parallel with said power battery means by a circuit breaker, whereby, when said circuit breaker is closed, said power battery means and said starter battery means provide a dual system voltage to the electrical system, - determining, when said circuit breaker is open, at least one of an open circuit power battery voltage and an open circuit starter battery voltage, - determining, when said dual circuit breaker is closed, said dual system voltage, and - utilizing said dual system voltage and the determined at least one of said open circuit power battery voltage and said open circuit starter battery voltage for determining a relative state of health for said power battery means, said relative state of health being related over time to a state of health for said starter battery means, where the state of health for said starter battery means is presumed to be good, and wherein a variation in size over time of a first difference is utilized for determining said relative state of health for said power battery means, said first difference being a difference between said dual system voltage and said open circuit power battery voltage, and wherein said state of health for said power battery means is considered to deteriorate relatively to said state of health for said starter battery means if said first difference varies such that a difference between said dual system voltage and said open circuit starter battery voltage decreases over time.