Provided are an apparatus using a battery and a method of detecting a time to change/recharge the battery in the apparatus. The apparatus includes: a battery for applying a battery voltage; a function unit for receiving the battery voltage to perform an intrinsic operation and cutting off the battery voltage; a voltage divider for dividing the battery voltage to generate a divided voltage; an analog-to-digital converter (ADC) for receiving the battery voltage to convert the divided voltage into a digital voltage signal in response to a measurement control signal before and after a low-power mode is finished; and a controller for receiving the battery voltage to generate a low-power signal for entering the low-power mode, enabling the measurement control signal in response to the low-power signal before and after the low-power mode is finished, and informing a user of a time to change/recharge the battery by detecting the remaining power of the battery using the digital voltage signals generated before and after the low-power mode is finished.