The present disclosure discloses a charging system, a charging method, and a power adapter. The charging system includes a battery, a first rectifier, a switch unit, a transformer, a second rectifier, a sampling unit, and a control unit. The control unit outputs a control signal to the switch unit, and adjusts a duty ratio of the control signal according to a current sampling value and/or voltage sampling value sampled by the sampling unit, such that a third voltage with a third ripple waveform outputted by the second rectifier meets a charging requirement of the battery.