A universal serial bus battery includes a universal serial bus interface, a battery, a charger, a comparator, a first switch, and a second switch. The comparator is configured for comparing the voltage at the universal serial bus interface and the voltage of the battery, and to produce an ON signal when the voltage at the serial bus interface exceeds the voltage of the battery, or to produce an OFF signal otherwise. The first switch is configured for establishing an electrical connection between the USB interface and the charger upon receiving the ON signal or cutting off the connection upon receiving the OFF signal. The second switch is configured for establishing a connection between the universal serial bus interface and the battery when the connection between the USB interface and the charger is terminated by the first switch.