A rechargeable battery is disclosed. The rechargeable battery of the invention includes a high density capacitor and an integrated circuit. The high density capacitor is connected to a ground terminal and a first node carrying a first voltage. The integrated circuit includes a band gap circuit, a first detecting unit, a voltage divider, a second detecting unit and at least one low dropout voltage regulator. The band gap circuit generates a band gap voltage according to the first voltage. The first detecting unit measures the first voltage and determines whether to apply an input charging voltage to the high density capacitor. The voltage divider is connected in parallel with the high density capacitor and has a second node carrying a second voltage. The second detecting unit measures the second voltage according to the band gap voltage and determines whether to connect a third node to the first node. Each low dropout voltage regulator is connected to the third node and generates a specified voltage output and a specified current output according to the band gap voltage and the first voltage.