If your hard-wired smoke detector has a battery backup, it is important to replace the batteries at least once a year with new ones. This is the only way to ensure the backup system will work in the event of a power failure.
Check your hard-wired smoke detector system at least once every month to ensure it and the batteries are in proper working order. It may be helpful to put a reminder on your calendar.
Hard-wired systems should be replaced every eight to 10 years whether they have a battery backup or not. It it is not already listed by the manufacturer, consider writing the purchase date on the inside of your alarm unit to remind you of when it is time to replace it.
Hard-wired smoke detector systems should only be replaced or installed by a qualified electrician who is familiar with residential electrical wiring codes. The battery installation is easy to handle on your own provided you are comfortable standing on a ladder.