A two-step charger includes an AC/DC power supply, a transformer, a compensator, a controller, a comparator and an isolating controller. The comparator detects whether a battery is connected to the transformer. If so, a command voltage of the charger is set at a high level. Since the battery voltage is lower than the command voltage, the controller orders the compensator to send out a compensating current. The current entering the primary side of the transformer is increased to promote the output current from the secondary side of the transformer. Once the comparator detects that the battery reaches as high as the charger, the command voltage is adjusted to a low level. The controller orders the compensator to stop outputting the compensating current. Therefore, a larger current can speed up battery charging. Once the battery is fully charged, the charging voltage is lowered to avoid high temperature.