Open the detector and remove the old battery

Most First Alert smoke detectors have a battery door on the front or side. Look for a small latch or clip, usually marked with an arrow showing which direction to slide or press. Push or slide it in the direction the arrow points — most move downward or to the side. The door will pop open or swing out, revealing the battery compartment.

Inside you will see a 9-volt battery (shaped like a rectangular block with two terminals on top) or two AA batteries stacked together, depending on your model. Grip the battery firmly and pull it straight out. If it is stuck, wiggle it gently side to side while pulling. Do not force it; if it does not budge after a few tries, the compartment may be designed differently — check your model number on the back of the detector and look up the manual online.

Key Takeaways

  • First Alert detectors use either a single 9-volt battery or two AA batteries, and the type depends on your specific model number.
  • The battery door opens by sliding or pressing a latch marked with an arrow, usually located on the front or side of the detector.
  • Insert the new battery with the terminals or flat end facing the correct direction — the compartment is usually shaped to prevent you from putting it in backward.
  • Test the detector when ready after installation by pressing and holding the test button until the alarm sounds, then release.
  • Replace the battery once per year, or sooner if the detector chirps every 30 to 40 seconds, which signals low battery.

Insert the new battery the correct way

Buy a replacement battery that matches the type you removed. If you took out a 9-volt, buy a 9-volt. If you removed two AA batteries, buy two AA batteries. Most hardware stores, grocery stores, and pharmacies stock both types near the checkout or in the health and safety aisle.

Slide or insert the new battery into the compartment. The battery will only fit one way — the terminals (the metal bumps on a 9-volt) or the flat end (on an AA) must face the direction shown inside the compartment. You will feel resistance as you push it in; this is normal. Push until the battery sits flat and the compartment closes easily. If you feel strong resistance before the battery is fully seated, stop and flip it around — you may have it backward.

Close the battery door and test the detector

Slide or push the battery door back into place until you hear or feel it click or latch. The door should sit flush with the detector body with no gaps. If it does not close all the way, remove the battery and check that it is seated correctly in the compartment.

Press and hold the test button on the front of the detector for three to five seconds. The alarm should sound — a loud, continuous beep. If it does not sound, the battery may not be making contact. Remove the battery, wipe the terminals with a dry cloth, and reinsert it. Test again. If the alarm still does not sound, the detector itself may have failed and may need to be replaced.

When to replace the battery

First Alert recommends replacing the battery once per year. Many people do this on a set date — the first day of daylight saving time, a birthday, or New Year's Day — so they remember. Write the replacement date on the detector with a permanent marker if you want a visual reminder.

If your detector chirps every 30 to 40 seconds, the battery is low and needs to be replaced when ready, even if you replaced it less than a year ago. This chirp is the detector's warning that it no longer has enough power to sound the full alarm if smoke is detected. Do not ignore it or remove the battery to stop the noise — replace it right away.

Troubleshooting a detector that still will not work

If the detector does not alarm after you install a new battery and test it, try these steps. First, remove the battery again and inspect the terminals inside the compartment. If they are corroded (covered in white, blue, or green crusty buildup), wipe them clean with a dry cloth or a pencil eraser. Reinsert the battery and test.

If the detector still does not work, the unit itself may have reached the end of its life. Most First Alert detectors last 8 to 10 years. Check the date printed on the back of the detector — if it is older than 10 years, replace the entire unit rather than troubleshooting further. If it is newer and still will not work, contact First Alert customer service with your model number, or replace the detector.

Different First Alert models and where to find the battery

First Alert makes many detector models, and battery location varies slightly. On hardwired detectors with battery backup, the battery compartment is usually on the side or bottom. On battery-only detectors, it is almost always on the front. If you cannot find the battery door, look for a seam or line on the detector body where two pieces meet — the door is usually there.

If you have lost the manual or cannot locate the battery compartment, write down the model number (printed on the back or bottom of the detector) and search for "[model number] manual" online. First Alert publishes PDFs for most models, and the manual will show you exactly where the battery goes and which type to use.

Frequently Asked Questions

Can I use a rechargeable battery in my First Alert detector?

No. First Alert detectors are designed for single-use alkaline batteries (9-volt or AA). Rechargeable batteries have a different voltage curve and may not power the detector reliably or trigger the alarm when needed. Always use a fresh alkaline battery of the correct type.

What do I do with the old battery?

Take it to a battery recycling program. Many retailers including Best Buy, Home Depot, and Lowe's accept used batteries for free. You can also search for a local recycling center at Earth911.com or call your city's waste management office. Do not throw batteries in the trash.

Why does my detector chirp even after I put in a new battery?

A chirp that continues after battery replacement usually means the detector has reached the end of its life (typically 8 to 10 years) or the battery terminals are corroded. Try cleaning the terminals with a dry cloth. If the chirping continues, the detector itself needs to be replaced, not just the battery.

How do I know if I have a 9-volt or AA battery detector?

Open the battery door and look inside. A 9-volt battery is a rectangular block about the size of a lighter. AA batteries are cylindrical and smaller, and you will see two of them stacked together. If you are unsure, check the model number on the back of the detector and search for the manual online — it will tell you which type your model uses.

Can I test my detector without pressing the button?

No. The test button is the only safe way to check that the detector works. Do not use real smoke or fire to test it — this is dangerous and can damage the detector. Press and hold the test button for three to five seconds every month to make sure the alarm sounds.