A good rule of thumb is to change.
Fire alarm battery replacement.
Buy a new smoke detector.
Fire safety experts recommend you replace your smoke alarm battery two times a year.
To replace batteries in models with rear loading batteries remove the alarm from the mounting plate by twisting it in the direction of the off arrow on the plate.
Mark the date you changed your battery on the inside of the smoke alarm.
If you have an ac hardwired interconnected alarm you will need to remove the quick connect wire harness from the back.
Replace the backup battery in a hard wire unit yearly whether it beeps or not.