An energy storage system for vehicles, comprising: a first energy storage system; a range extender configured to be connected between the first energy storage system and an electric load, the range extender connected to the energy storage system, the range extender comprising: a first DC-to-DC converter configured to be selectably coupled between the first energy storage system and the electric load; a second energy storage system; and a second DC-to-DC converter configured to be selectably coupled between the second energy storage system and at least one of an output side of the first DC-to-DC converter and an input side of the electric load; and a controller configured to control the first energy storage system and the range extender to work in at least one of a normal operation mode and a fault protection mode; characterized in that the range extender further comprises a by-pass configured to be selectably coupled between the first energy storage system and at least one of the output side of the first DC-to-DC converter, an output side of the second DC-to-DC converter, and the input side of the electric load; wherein in the normal operation mode, the first and second energy storage systems (110, 220) are configured to be coupled in parallel to the electric load through the first and second DC-to-DC converters (212, 222), respectively, and in the fault protection mode, the first energy storage system is configured to be coupled to the electric load through the by-pass so as to directly couple the first energy storage system to the electric load, in which the second energy storage system, the first DC-to-DC converter, and the second DC-to-DC converter are disabled, and the second energy storage system is configured to be decoupled from the electric load. a first energy storage system; a range extender configured to be connected between the first energy storage system and an electric load, the range extender connected to the energy storage system, the range extender comprising: a first DC-to-DC converter configured to be selectably coupled between the first energy storage system and the electric load; a second energy storage system; and a second DC-to-DC converter configured to be selectably coupled between the second energy storage system and at least one of an output side of the first DC-to-DC converter and an input side of the electric load; and a controller configured to control the first energy storage system and the range extender to work in at least one of a normal operation mode and a fault protection mode; characterized in that the range extender further comprises a by-pass configured to be selectably coupled between the first energy storage system and at least one of the output side of the first DC-to-DC converter, an output side of the second DC-to-DC converter, and the input side of the electric load; wherein in the normal operation mode, the first and second energy storage systems (110, 220) are configured to be coupled in parallel to the electric load through the first and second DC-to-DC converters (212, 222), respectively, and in the fault protection mode, the first energy storage system is configured to be coupled to the electric load through the by-pass so as to directly couple the first energy storage system to the electric load, in which the second energy storage system, the first DC-to-DC converter, and the second DC-to-DC converter are disabled, and the second energy storage system is configured to be decoupled from the electric load. a first DC-to-DC converter configured to be selectably coupled between the first energy storage system and the electric load; a second energy storage system; and a second DC-to-DC converter configured to be selectably coupled between the second energy storage system and at least one of an output side of the first DC-to-DC converter and an input side of the electric load; and a controller configured to control the first energy storage system and the range extender to work in at least one of a normal operation mode and a fault protection mode; characterized in that the range extender further comprises a by-pass configured to be selectably coupled between the first energy storage system and at least one of the output side of the first DC-to-DC converter, an output side of the second DC-to-DC converter, and the input side of the electric load; wherein in the normal operation mode, the first and second energy storage systems (110, 220) are configured to be coupled in parallel to the electric load through the first and second DC-to-DC converters (212, 222), respectively, and in the fault protection mode, the first energy storage system is configured to be coupled to the electric load through the by-pass so as to directly couple the first energy storage system to the electric load, in which the second energy storage system, the first DC-to-DC converter, and the second DC-to-DC converter are disabled, and the second energy storage system is configured to be decoupled from the electric load.