A chargeable device, comprising: a charging interface; and a first charging circuit, coupled to the charging interface, configured to receive voltage and current outputted by an adapter through the charging interface, and to apply the voltage and current outputted by the adapter on two terminals of multiple cells coupled in series and built in the chargeable device to charge the multiple cells directly, when the voltage outputted by the adapter is greater than the total voltage of the multiple cells, wherein the multiple cells are configured to be charged as fast as a single cell having a capacity C, a charging current I, and a voltage V, by using a charging current I/N for the multiple cells, the capacity of each cell of the multiple cells is C/N, and the voltage of each cell of the multiple cells is V, where N indicates the number of cells of the multiple cells; a second charging circuit, comprising a voltage boost circuit coupled between the charging interface and the multiple cells, wherein the voltage boost circuit is configured to convert the voltage outputted from the adapter and received by the charging interface into a second voltage, and the second voltage is applied between the two terminals of the multiple cells to charge the multiple cells, when the voltage value outputted from the adapter is less than a total voltage of the multiple cells, and the second voltage value is greater than the total voltage of the multiple cells, wherein the chargeable device and the adapter are configured to be selectively operable in a first charging mode or a second charging mode, wherein a charging rate of the chargeable device operable in the second charging mode is faster than a charging rate of the chargeable device operable in the first charging mode, wherein the multiple cells are configured to be charged through the first charging circuit when the chargeable device and the adapter are selectively operable in the second charging mode; and the multiple cells are configured to be charged through the second charging circuit when the chargeable device and the adapter are selectively operable in the first charging mode. a charging interface; and a first charging circuit, coupled to the charging interface, configured to receive voltage and current outputted by an adapter through the charging interface, and to apply the voltage and current outputted by the adapter on two terminals of multiple cells coupled in series and built in the chargeable device to charge the multiple cells directly, when the voltage outputted by the adapter is greater than the total voltage of the multiple cells, wherein the multiple cells are configured to be charged as fast as a single cell having a capacity C, a charging current I, and a voltage V, by using a charging current I/N for the multiple cells, the capacity of each cell of the multiple cells is C/N, and the voltage of each cell of the multiple cells is V, where N indicates the number of cells of the multiple cells; a second charging circuit, comprising a voltage boost circuit coupled between the charging interface and the multiple cells, wherein the voltage boost circuit is configured to convert the voltage outputted from the adapter and received by the charging interface into a second voltage, and the second voltage is applied between the two terminals of the multiple cells to charge the multiple cells, when the voltage value outputted from the adapter is less than a total voltage of the multiple cells, and the second voltage value is greater than the total voltage of the multiple cells, wherein the chargeable device and the adapter are configured to be selectively operable in a first charging mode or a second charging mode, wherein a charging rate of the chargeable device operable in the second charging mode is faster than a charging rate of the chargeable device operable in the first charging mode, wherein the multiple cells are configured to be charged through the first charging circuit when the chargeable device and the adapter are selectively operable in the second charging mode; and the multiple cells are configured to be charged through the second charging circuit when the chargeable device and the adapter are selectively operable in the first charging mode.