Method for determining a resulting charging duration, the time actually being used for charging the batteries, to be used for charging at least one starter battery in a dual battery system, said dual battery system comprising: - at least one power battery being connected to an electrical system comprising a starter motor and battery charging means, and - said at least one starter battery, being connectable in parallel with said at least one power battery, characterized by the steps of: - determining an initial charging duration, the time period which initially is believed to be sufficient for charging the one or more starter batteries, based on a temperature of said at least one starter battery and on a measure of a behaviour of a starter motor, when said starter motor is activated, and - determining, during charging of said at least one starter battery, said resulting charging duration based on said initial charging duration and on a relation between a nominal charging voltage and an actual charging voltage being used for charging said at least one starter battery, wherein said resulting charging duration is determined by: - first setting a remaining charging duration to a value being equal to said initial charging duration, and - performing, during a time corresponding to said resulting charging duration, at least one reduction of said remaining charging duration until zero time units remains of said remaining charging duration, wherein each one of said at least one reduction is related to a difference between said actual charging voltage and said nominal charging voltage, such that the reduction has a relatively higher value if said difference is relatively small, and such that said reduction has a relatively lower value if said difference is relatively large. - at least one power battery being connected to an electrical system comprising a starter motor and battery charging means, and - said at least one starter battery, being connectable in parallel with said at least one power battery, - determining an initial charging duration, the time period which initially is believed to be sufficient for charging the one or more starter batteries, based on a temperature of said at least one starter battery and on a measure of a behaviour of a starter motor, when said starter motor is activated, and - determining, during charging of said at least one starter battery, said resulting charging duration based on said initial charging duration and on a relation between a nominal charging voltage and an actual charging voltage being used for charging said at least one starter battery, wherein said resulting charging duration is determined by: - first setting a remaining charging duration to a value being equal to said initial charging duration, and - performing, during a time corresponding to said resulting charging duration, at least one reduction of said remaining charging duration until zero time units remains of said remaining charging duration, wherein each one of said at least one reduction is related to a difference between said actual charging voltage and said nominal charging voltage, such that the reduction has a relatively higher value if said difference is relatively small, and such that said reduction has a relatively lower value if said difference is relatively large.