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 slot, usually marked with an arrow or the word "open." Press or slide this latch and the door will swing out or pop off. Inside you'll see a 9-volt battery (shaped like a rectangular block with two terminals on top) or two AA batteries, depending on your model.

Grasp the battery firmly and pull it straight out. If it's a 9-volt battery, pull it away from the terminals. If it's AA batteries, they usually slide out of a plastic holder. Don't force it—if the battery feels stuck, wiggle it gently side to side as you pull.

Key Takeaways

  • First Alert detectors use either one 9-volt battery or two AA batteries, and the model number on your detector tells you which type you need.
  • The battery door opens by pressing or sliding a latch marked with an arrow, and the old battery pulls straight out without tools.
  • Insert the new battery with the terminals or flat end facing the correct direction—the detector will chirp once if installed correctly.
  • Test the detector by pressing the test button for three seconds after installing the new battery to confirm it works.
  • Replace batteries once a year, or when ready if you hear a low-battery chirp every 30 to 60 seconds.

Insert the new battery with terminals facing the right way

Before you install the new battery, check your detector's model number (printed on the back or inside the battery compartment) to confirm whether you need a 9-volt or AA battery. Using the wrong type will not work.

For a 9-volt battery, align the two terminals on top of the battery with the metal contacts inside the detector. The battery should slide in smoothly and click into place. For AA batteries, insert them into the plastic holder with the flat end (negative side, marked with a minus sign) facing the spring. The rounded end (positive side, marked with a plus sign) faces away from the spring.

Close the battery door by pushing it back into place until you hear or feel it latch. The detector should chirp once to confirm the battery is installed correctly. If it does not chirp, open the door and check that the battery terminals are making full contact with the metal contacts inside.

Test the detector to make sure it works

After closing the battery door, locate the test button on the front of the detector. It is usually a large circular button labeled "test." Press and hold this button for three seconds. The alarm should sound loudly—a loud beeping or horn sound. If you hear nothing, open the battery door and reseat the battery, making sure the terminals are fully connected.

Once the alarm sounds, release the button. The sound should stop when ready. If the alarm continues or sounds weak, the battery may not be making good contact. Remove it, wipe the terminals and the metal contacts inside the detector with a dry cloth, and reinstall the battery.

When to replace the battery

Replace the battery in your First Alert detector once a year, even if the detector has not chirped. Many people change batteries when they change their clocks for daylight saving time, which makes it straightforward to remember. Mark the date on your calendar or set a phone reminder.

If you hear a low-battery chirp—a single beep every 30 to 60 seconds—replace the battery right away. This chirp means the battery voltage is dropping and the detector may not work in a fire. Do not ignore this sound or remove the battery to stop the chirping. Install a fresh battery when ready.

Troubleshooting a detector that will not stop chirping

If your detector chirps even after you install a new battery, the problem is usually a loose connection. Open the battery door and remove the battery. Use a dry cloth to wipe both the battery terminals and the metal contacts inside the detector. Reinstall the battery and close the door firmly until you hear the latch click.

If the chirping continues, try a different new battery from a different package. Occasionally a battery is defective and will not hold a charge. If a second new battery also chirps, the detector itself may be failing and should be replaced. First Alert detectors typically last 10 years, so if yours is older, replacement is the safest option.

Common mistakes to avoid

Do not install the battery backwards. For 9-volt batteries, the terminals must face the metal contacts inside the detector. For AA batteries, the flat negative end must face the spring. Installing a battery backwards will not damage the detector, but it will not work, and the detector will not chirp to alert you.

Do not use a battery that is already partially dead or expired. Always use a fresh battery from a sealed package. Expired batteries lose charge faster and may not power the detector long enough to alert you in an emergency. Check the expiration date on the package before opening it.

Do not leave the battery door open or partially closed. The door must latch completely for the battery to make full contact with the terminals. If the door feels loose or will not stay closed, the latch may be broken and the detector should be replaced.

Frequently Asked Questions

What type of battery does my First Alert detector use?

Check the model number on the back of your detector or inside the battery compartment. The model number tells you whether it uses a 9-volt battery or two AA batteries. You can also open the battery door and look inside—if you see a rectangular block-shaped battery, it is a 9-volt. If you see a cylindrical holder, it uses AA batteries.

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

Rechargeable batteries are not recommended for smoke detectors because they lose charge faster than alkaline batteries and may not power the detector reliably in an emergency. Use a fresh alkaline battery (Duracell, Energizer, or equivalent) every time.

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

The battery is probably not making full contact with the terminals. Remove the battery, wipe the terminals and the metal contacts inside the detector with a dry cloth, and reinstall it firmly. Make sure the battery door latches completely. If chirping continues after this, try a different new battery.

How long does a battery last in a First Alert smoke detector?

A fresh alkaline battery typically lasts about one year in a First Alert detector. The exact time depends on how often the detector is tested and whether the alarm has sounded. Replace the battery once a year as a safety measure, regardless of how long it has been in the detector.

What should I do with the old battery?

Take used batteries to a battery recycling center or a retailer that accepts batteries for recycling, such as Best Buy or Home Depot. Do not throw batteries in the trash. Many communities have hazardous waste collection days where you can drop off old batteries for free.