Kidde Nighthawk CO Alarm: * Battery operated: Provides best protection during power outages * Digital display: Shows CO level in PPM (parts per million) * Updates digital reading every 15 seconds allowing you to see if levels change * Peak level memory: Displays the highest CO concentration measured since the last reset * Test/Reset button: Tests CO alarm circuit operation and allows you to immediately silence the alarm * Low battery signal: Alerts user when the batteries need to be replaced * Slide-out battery door cover: Gives immediate access to batteries * Battery safeguard: Makes it difficult to close the cover without batteries in the unit