To provide an uninterruptible power supply in which a secondary battery of any specifications can be employed as a backup power supply with no problem without changing an existing main power supply section even slightly, and the lifetime of the secondary battery is prolonged as much as possible by supplying power depending on the self discharge characteristics of the secondary battery.

When a lithium ion secondary battery 22 of different specification is employed as a backup power supply, the lithium ion secondary battery 22 is connected with a power supply circuit 1 through a voltage conversion unit 31 equipped with a step up/down section 32. The voltage conversion unit 31 steps up or down a voltage VCHG being supplied from the power supply circuit 1 to the lithium ion secondary battery 22 to a level suitable for charging the lithium ion secondary battery 22. In other words, the lithium ion secondary battery 22 is charged with an optimal voltage by simply interposing the voltage conversion unit 31 between the power supply circuit 1 and the lithium ion secondary battery 22.

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