How often should you replace your smoke detector batteries?

Dead batteries are the #1 reason smoke alarms fail in a fire.

You lose the warning entirely. A smouldering fire produces a great deal of smoke before it produces heat or light, and a working alarm typically gives several minutes of warning at the point where escape is still straightforward. Without it, the first sign is often the fire itself.

1. Change every alarm on the same day. Do the whole house in one pass rather than one at a time. Mixed ages are how gaps appear.
2. Open the case and note the type. Twist off or hinge open. Check whether it takes a 9V block or AA cells.
3. Fit a fresh battery and check the date. While the cover is off, look for the manufacture date printed on the back of the unit. That date drives replacement of the alarm itself.
4. Vacuum the vents. Dust and cobwebs in the sensing chamber cause both false alarms and delayed response.
5. Press and hold the test button. Hold until it sounds properly, not just a chirp. On interlinked alarms, confirm every other alarm in the house sounds too.

• An intermittent chirp every 30 to 60 seconds, which is the low battery warning
• No sound at all when the test button is held
• A weak or wavering sound rather than a full alarm

• Removing the battery to stop a chirp or a cooking false alarm and never refitting it, which is a common and genuinely dangerous habit
• Testing with a lighter or a candle instead of the test button
• Changing the battery but never checking the manufacture date, so a 15-year-old alarm gets fresh batteries forever
• Painting over an alarm, which blocks the sensing chamber

• Sealed ten-year lithium units need no battery changes at all, only annual testing and replacement at ten years
• Hard-wired alarms still have a backup battery that needs changing
• Pick a fixed annual anchor such as a clock change so it never drifts

Hard-wired interlinked alarms that fail to trigger each other, or that trip the circuit, need an electrician. Battery swaps do not.