Power supply system for electrically powered vehicle and method for controlling the same

A method for controlling a power supply system for an electrically powered vehicle equipped with a motor generating power to drive the vehicle, said power supply system including a main power storage device, an electric power feeding line supplying electric power to an inverter that controls and drives said motor, a first voltage converter provided between said electric power feeding line and said main power storage device for converting voltage there between bidirectionally, a plurality of sub power storage devices provided in parallel to each other, a second voltage converter provided between said plurality of sub power storage devices and said electric power feeding line, for converting voltage between one of said plurality of sub power storage devices and said electric power feeding line bidirectionally, a connection unit provided between said plurality of sub power storage devices and said second voltage converter, for selectively connecting a sub power storage device selected from said plurality of sub power storage devices to said second voltage converter, and a switching control device controlling selective connection between said plurality of sub power storage devices and said second voltage converter, said method for controlling comprising the steps of:

• determining, with the switching control device, whether or not said selected sub power storage device should be switched based on states of charge of said plurality of sub power storage devices;
• instructing, with the switching control device, said first voltage converter to provide a voltage on said electric power feeding line to be a first voltage higher than a voltage output from said main power storage device and a voltage output from a sub power storage device to be connected to said second voltage converter after switching when said step of determining determines that it is necessary to switch said selected sub power storage device;
• decreasing, with the switching control device, values of upper limits on electric power input/output to/from said selected sub power storage device gradually to zero after said voltage on said electric power feeding line has reached said first voltage;
• switching, with the switching control device, connection between said plurality of sub power storage devices and said second voltage converter when said step of decreasing sets said values of said upper limits on electric power input/output to zero;
• and
• increasing, with the switching control device, said values of said upper limits on electric power input/output gradually to values corresponding to a state of charge of the sub power storage device newly connected to said second voltage converter after said step of switching switches connection between said plurality of sub power storage devices and said second voltage converter.