Your device runs on power from the mains supply. However, battery is also needed for memory backup of the alarm and time settings in case of power interruption, even a momentary power interruption. On the other hand, using battery only (without connecting the device to the mains supply) is not enough. The battery would run out very fast and the clock would lose its accuracy. To make sure the battery backup is effective: * Use new batteries. Do not mix old batteries with new ones. * Make sure the batteries are properly installed in the battery compartment. Place a small piece of sponge or a piece of facial tissue on top of the battery before closing the battery compartment can prevent the batteries from falling out of their position.