A system, comprising a device to be charged and an adapter, the device to be charged comprising: a plurality of battery cells; a charging interface; a first charging circuit, coupled with the charging interface, configured to receive an output voltage and an output current of the adapter via the charging interface and to directly apply the output voltage and the output current of the adapter to both ends of the plurality of battery cells coupled in series, so as to perform a direct charging on the plurality of battery cells; an equalization circuit, coupled with the plurality of battery cells, and configured to equalize voltages of respective battery cells of the plurality of battery cells; and a second charging circuit, wherein the plurality of battery cells comprise a first battery cell and a second battery cell, and the equalization circuit is configured to transfer electric quantity between the first battery cell and the second battery cell in an electromagnetic coupling manner; the adapter is configured to support a first charging mode and a second charging mode, a charging speed of the adapter in the second charging mode is greater than that of the adapter in the first charging mode, the adapter is configured to charge the plurality of battery cells via the second charging circuit in the first charging mode, and is configured to charge the plurality of battery cells via the first charging circuit in the second charging mode; and the charging interface comprises a data wire, the device to be charged further comprises a control unit, and the control unit is configured to perform a bidirectional communication with the adapter via the data wire to control an output of the adapter in the second charging mode, and wherein when the control unit performs the bidirectional communication with the adapter via the data wire to control the output of the adapter in the second charging mode, the control unit is configured to perform the bidirectional communication with the adapter to negotiate about a charging mode between the adapter and the device to be charged, the control unit is configured to perform the bidirectional communication with the adapter to determine a charging voltage outputted by the adapter in the second charging mode for charging the device to be charged. a plurality of battery cells; a charging interface; a first charging circuit, coupled with the charging interface, configured to receive an output voltage and an output current of the adapter via the charging interface and to directly apply the output voltage and the output current of the adapter to both ends of the plurality of battery cells coupled in series, so as to perform a direct charging on the plurality of battery cells; an equalization circuit, coupled with the plurality of battery cells, and configured to equalize voltages of respective battery cells of the plurality of battery cells; and a second charging circuit, wherein the plurality of battery cells comprise a first battery cell and a second battery cell, and the equalization circuit is configured to transfer electric quantity between the first battery cell and the second battery cell in an electromagnetic coupling manner; the adapter is configured to support a first charging mode and a second charging mode, a charging speed of the adapter in the second charging mode is greater than that of the adapter in the first charging mode, the adapter is configured to charge the plurality of battery cells via the second charging circuit in the first charging mode, and is configured to charge the plurality of battery cells via the first charging circuit in the second charging mode; and the charging interface comprises a data wire, the device to be charged further comprises a control unit, and the control unit is configured to perform a bidirectional communication with the adapter via the data wire to control an output of the adapter in the second charging mode, and wherein when the control unit performs the bidirectional communication with the adapter via the data wire to control the output of the adapter in the second charging mode, the control unit is configured to perform the bidirectional communication with the adapter to negotiate about a charging mode between the adapter and the device to be charged, the control unit is configured to perform the bidirectional communication with the adapter to determine a charging voltage outputted by the adapter in the second charging mode for charging the device to be charged.