A power supply system comprising: a first circuit including a first power supply line connected to a first load , a power supply source connected to the first power supply line , and a first battery connected to the first power supply line ; a second circuit including a second power supply line connected to a second load and a second battery connected to the second power supply line , the second load being configured to perform a function that substitutes for a function performed by the first load ; and a voltage controller including a converter control unit and a direct current to direct current converter , the direct current to direct current converter being connected between the first power supply line and the second power supply line , characterised in that the converter control unit is configured to control the direct current to direct current converter by using an input voltage from the first power supply line such that an output voltage higher than or equal to a voltage of the second battery is output to the second power supply line , the voltage controller includes a first abnormality detection unit configured to detect an abnormality of the first circuit, the converter control unit is configured to, when the abnormality of the first circuit has not been detected by the first abnormality detection unit , control the direct current to direct current converter by using the input voltage from the first power supply line such that the output voltage higher than or equal to the voltage of the second battery is output to the second power supply line from the direct current to direct current converter , and the converter control unit is configured to, when the abnormality of the first circuit has been detected by the first abnormality detection unit , control the direct current to direct current converter such that the first power supply line is interrupted from the second power supply line . a first circuit including a first power supply line connected to a first load , a power supply source connected to the first power supply line , and a first battery connected to the first power supply line ; a second circuit including a second power supply line connected to a second load and a second battery connected to the second power supply line , the second load being configured to perform a function that substitutes for a function performed by the first load ; and a voltage controller including a converter control unit and a direct current to direct current converter , the direct current to direct current converter being connected between the first power supply line and the second power supply line , characterised in that the converter control unit is configured to control the direct current to direct current converter by using an input voltage from the first power supply line such that an output voltage higher than or equal to a voltage of the second battery is output to the second power supply line , the voltage controller includes a first abnormality detection unit configured to detect an abnormality of the first circuit, the converter control unit is configured to, when the abnormality of the first circuit has not been detected by the first abnormality detection unit , control the direct current to direct current converter by using the input voltage from the first power supply line such that the output voltage higher than or equal to the voltage of the second battery is output to the second power supply line from the direct current to direct current converter , and the converter control unit is configured to, when the abnormality of the first circuit has been detected by the first abnormality detection unit , control the direct current to direct current converter such that the first power supply line is interrupted from the second power supply line . <EOS>