A printing device includes a battery, a print head and a controller. The battery is charged by a charging power and supplies a power to the print head. The controller has multiple operation modes and is configured to operate in one of the multiple operation modes. The controller consumes an operating power corresponding to the operation mode. An external power source has multiple power profiles and outputs a supplying power corresponding to one of the power profiles to the printing device. The supplying power is used as the operating power for driving the controller. The controller obtains the multiple power profiles from the externa power source. The controller determines whether the supplying power is greater than a sum of the charging power to charge the battery and the operating power, and determine an operation mode when the battery is being charged.