A method for controlling a power management system in a vehicle operated by a combustion engine, wherein said system comprises a control unit, said method comprising: - supplying electric energy to a start device for said combustion engine by means of a start battery unit; - supplying electric energy to a number of electric consumers in the vehicle including components which are crucial for allowing starting of said combustion engine, by means of a second battery unit; and - charging said second battery unit during operation of the combustion engine by means of an alternator which is associated with said combustion engine; - checking, before starting of the combustion engine by means of said start device, whether the state of charge (SOC 1) of said second battery unit is lower than a predetermined value, and, if this is the case, - accepting an input signal for actuation of said start device if the state of charge (SOC 2) of said start battery unit is higher than a predetermined value; and - during actuation of said start device, supplying electric energy to said crucial components from the start battery unit by means of a battery charging unit which is also configured for charging the start battery unit during operation of the vehicle with electric energy from said second battery unit, said battery charging unit being reversed for supplying electric energy from said start battery unit to said crucial components; characterized in that said method further comprises: - maintaining the voltage of the output of the battery charging unit, which voltage is delivered to said crucial components and to said second battery unit, below a charging acceptance voltage of said second battery unit, under which said second battery unit can not be charged; and - thereby inhibiting charging of said second battery unit during actuation of the start device while supplying electric energy to said crucial components. - supplying electric energy to a start device for said combustion engine by means of a start battery unit; - supplying electric energy to a number of electric consumers in the vehicle including components which are crucial for allowing starting of said combustion engine, by means of a second battery unit; and - charging said second battery unit during operation of the combustion engine by means of an alternator which is associated with said combustion engine; - checking, before starting of the combustion engine by means of said start device, whether the state of charge (SOC 1) of said second battery unit is lower than a predetermined value, and, if this is the case, - accepting an input signal for actuation of said start device if the state of charge (SOC 2) of said start battery unit is higher than a predetermined value; and - during actuation of said start device, supplying electric energy to said crucial components from the start battery unit by means of a battery charging unit which is also configured for charging the start battery unit during operation of the vehicle with electric energy from said second battery unit, said battery charging unit being reversed for supplying electric energy from said start battery unit to said crucial components; - maintaining the voltage of the output of the battery charging unit, which voltage is delivered to said crucial components and to said second battery unit, below a charging acceptance voltage of said second battery unit, under which said second battery unit can not be charged; and - thereby inhibiting charging of said second battery unit during actuation of the start device while supplying electric energy to said crucial components.