A power supply device, an electronic apparatus, and a power supply method are provided. The power supply device comprises a first buck circuit used to convert, if a single battery cell in N cells connected in series has a voltage lower than a shutdown voltage of an electronic apparatus, a total voltage of the N cells into a power supply voltage of the electronic apparatus, such that the power supply voltage is higher than or equal to the shutdown voltage, N being an integer greater than or equal to 2; and a first power supply channel used to supply power to a system of the electronic apparatus according to the power supply voltage.