A method for evaluating a state of charge of a rechargeable battery of an electrical system, said electrical system comprising at least one primary power source (1, 2) and a rechargeable battery as a secondary power source, a DC/DC converter and an electrical network comprising a primary electrical network that connects the primary power source (1,2) to the DC/DC converter and a secondary electrical network that connects the rechargeable battery to at least the DC/DC converter; the DC/DC converter converting an input DC voltage supplied by the at least one primary power source (1, 2) into an output DC voltage supplied to the secondary electrical network, the at least one primary power source (1, 2) and the rechargeable battery being connected in parallel to at least one electrical load (7, 8, 9) and being configured to supply, via the electrical network (10, 11), DC voltage and DC current to said at least one electrical load (7, 8, 9), said electrical system further comprising sensors to measure an output DC current and the output DC voltage supplied by the DC/DC converter, and further comprising at least one control unit connected to the sensors and able to control at least the DC/DC converter, characterized in that said method comprising the following steps: a. controlling the DC/DC converter to reduce the output DC voltage from an actual voltage down to a target DC voltage; b. maintaining the target DC voltage during a first predetermined period of time; c. measuring the output DC current during the first predetermined period of time; d. determining, within the first predetermined period of time, a time during which the output DC current is remained lower than a predetermined threshold current value; e. comparing the time during which the output DC current value is remained lower than the predetermined threshold current value with a second predetermined period of time; in that the state of the rechargeable battery is considered operational if the output DC current has been lower than the predetermined threshold current value during a time period that is longer than the second predetermined period of time; in that, when operational, the rechargeable battery can be used to restart the primary electrical network after the primary power source (1, 2) has been shutdown; and in that the rechargeable battery is determined as not operational if the time period during which, the output DC current value is remained lower than the predetermined threshold current value, is shorter than the second predetermined period of time. a. controlling the DC/DC converter to reduce the output DC voltage from an actual voltage down to a target DC voltage; b. maintaining the target DC voltage during a first predetermined period of time; c. measuring the output DC current during the first predetermined period of time; d. determining, within the first predetermined period of time, a time during which the output DC current is remained lower than a predetermined threshold current value; e. comparing the time during which the output DC current value is remained lower than the predetermined threshold current value with a second predetermined period of time; in that the state of the rechargeable battery is considered operational if the output DC current has been lower than the predetermined threshold current value during a time period that is longer than the second predetermined period of time; in that, when operational, the rechargeable battery can be used to restart the primary electrical network after the primary power source (1, 2) has been shutdown; and in that the rechargeable battery is determined as not operational if the time period during which, the output DC current value is remained lower than the predetermined threshold current value, is shorter than the second predetermined period of time.