LED Emergency and Exit Lights contain a Ni-Cd battery inside the unit. The first steps when determining the functionality of the exit or emergency light is to perform a push button test. A button is on the bottom or side of the emergency / exit light and when pressed activates the back up battery system. When held down for a minimum of 30 seconds you will be able to determine if the battery is in need of replacement. The problem with this preliminary test is that Nickel Cadmium batteries discharge at a 100% output until the very last moments of the battery's life. To clarify further, when a Ni Cd Battery is drained to 15% life capacity it will still discharge at 100% output like it's fully charged. This is why it's advised to replace the batteries every 3 years to make sure all of the emergency and exit lights in the facility will discharge for at least 90 minutes.