Open the detector and remove the old battery

Most First Alert smoke detectors have a battery compartment on the back or bottom of the unit. Look for a small latch or clip — you'll usually need to slide it or press it to open the door. Some models twist open instead; check your detector's face for arrows showing which direction to turn.

Once the compartment is open, you'll see the battery sitting in a plastic holder. Note which direction it's facing — the positive end (marked with a +) should be pointing toward the spring inside the holder. Pull the battery straight out. If it's stuck, gently wiggle it side to side rather than forcing it.

Key Takeaways

  • First Alert detectors use either 9-volt or AA batteries depending on the model — check the compartment or your manual to know which one to buy.
  • Always note the battery direction before removing the old one so you install the new one the same way.
  • The detector will chirp every 30 to 60 seconds after the battery dies, and this chirping stops only when you install a fresh battery.
  • Replace the battery once a year, or whenever the detector chirps — do not ignore the chirping sound, as it means the detector cannot protect you.
  • Test the detector after installing the new battery by holding down the test button for three to five seconds until the alarm sounds.

Install the new battery in the correct direction

Buy a replacement battery that matches what was in your detector. First Alert models typically use either a 9-volt battery (the rectangular kind) or an AA battery (the smaller cylinder). The compartment will only fit one size, so you cannot install the wrong type by accident.

Slide the new battery into the holder with the positive end facing the spring, matching the direction the old battery was facing. You should hear or feel a small click as it seats. Close the compartment door or twist the back closed until it's snug — it does not need to be forced.

Test the detector to confirm it's working

After you close the compartment, press and hold the test button on the front of the detector. This button is usually labeled "Test" and is located in the center of the unit. Hold it down for three to five seconds. The alarm should sound loudly — this is normal and means the detector is working.

If the alarm does not sound, open the compartment again and check that the battery is seated all the way in and facing the correct direction. Close it and test again. If it still does not work, the battery may be defective; try a different new battery.

Understand why your detector was chirping

A chirping sound every 30 to 60 seconds means the battery is low or dead. This chirp is not the alarm itself — it's a warning that the detector's backup power is gone and it cannot protect you if there's a fire. The chirping will stop as soon as you install a fresh battery.

Some detectors also chirp if they're too old (usually after 10 years) or if there's dust inside blocking the sensor. If the chirping continues after you've installed a new battery and tested the unit, the detector itself may need to be replaced.

Know when to replace the battery on your schedule

Replace the battery in your First Alert detector once a year, even if it hasn't started chirping yet. A good time to do this is during daylight saving time changes — many people remember to change their clocks and their smoke detector batteries at the same time.

Write the replacement date on the detector itself with a permanent marker, or set a phone reminder for the same date each year. This way you're not waiting for the chirp to tell you the battery is failing.

Dispose of the old battery safely

Do not throw a used 9-volt battery in the trash. These batteries can cause fires in garbage trucks and at landfills. Take it to a battery recycling location — most electronics stores, hardware stores, and municipal hazardous waste facilities accept them for free.

AA batteries can usually go in regular trash in most places, but check your local rules. Some communities ask you to recycle all batteries. If you're unsure, your local waste management website will tell you where to take them.

Troubleshoot if the detector still isn't working

If you've installed a new battery, tested the detector, and the alarm still won't sound, the detector itself may be faulty or too old. First Alert detectors typically last 8 to 10 years. If yours is older than that, replacement is the safest option.

If the detector is newer and still won't work after a battery change, contact First Alert customer service or check the manual that came with your unit. They can tell you whether the model has a known issue or whether the detector needs to be sent back.

Frequently Asked Questions

How do I know if my First Alert detector uses a 9-volt or AA battery?

Open the battery compartment and look at the size and shape of the holder. A 9-volt battery is rectangular and about the size of a lighter. An AA battery is cylindrical and smaller. The compartment is designed for only one size, so you cannot fit the wrong type.

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

Make sure the battery is pushed all the way into the holder and facing the correct direction — the positive end should touch the spring. If it's seated correctly and still chirping, the detector may be too old or the sensor may be blocked by dust. Try cleaning the vents with a soft brush, or replace the detector if it's over 10 years old.

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

No. First Alert detectors are designed for single-use alkaline batteries (regular 9-volt or AA batteries). Rechargeable batteries do not provide the steady voltage these detectors need and may cause the unit to malfunction or fail to alarm during a fire.

What should I do if the test button doesn't make the alarm sound?

First, check that the battery is fully seated and facing the right direction. Close the compartment firmly and test again. If the alarm still doesn't sound, try a different new battery — the first one may be defective. If a second battery doesn't work, the detector likely needs to be replaced.

Is it normal for the detector to chirp right after I install a new battery?

No. Once you install a fresh battery and close the compartment, the chirping should stop when ready. If it continues, the battery may not be seated all the way in. Open the compartment, push the battery in firmly, and close it again.