Spare capacitor on mainboard of computer

The utility model discloses a spare capacitor device, which comprises three capacitors with different capacitances and different working voltages, a detecting circuit and a switching circuit, wherein the three capacitors are respectively composed of a first capacitor with 5V output voltage, a second capacitor with 3. 3V output voltage, and a third capacitor with 2V output voltage, a spare capacitor device is arranged on a power provider of a computer system and is respectively connected with the 5V voltage output terminal, the 3. 3V voltage output terminal, and the 12V voltage output terminal of the power provider in parallel. When the basic input and output system (BIOS) of a computer mainboard is renewed, the spare capacitor device can continuously provide power for 30 seconds to make the BIOS complete the renewal promptly once a power breakdown of the computer system happens.