1. Attention of H9 Note that the positive and negative values of the batteries are not in the same direction. 2. 4 Kinds of Adjustable Backlight With attached power adapter, the backlight is adjustable: from HIGH to OFF. 3. USB or Battery, which is recommend? 1) When the base unit is powered by adapter, the backlight is adjustable from OFF to HIGH, bright enough at night to see. 2) The battery is mainly used to back up data, such as date, time, and alarm. When the led clock only uses batteries, the screen will turn off after 10s and you need to press the SNZ/LIGHT button to activate it.