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 holding the cover in place. Press or slide this latch toward the center of the detector, and the cover will pop open or slide off. Some models have a cover that twists counterclockwise instead.

Once the compartment is open, you will see the battery sitting in a plastic holder. Note which direction the battery is facing — the positive (+) end should be pointing toward the spring inside the holder. Gently pull the battery out by the edge, or press it slightly to release it from the spring clip. Do not force it; if it feels stuck, check that you have opened the cover fully.

Key Takeaways

  • First Alert detectors use either 9-volt or AA batteries depending on the model, so check your old battery or the detector label before buying a replacement.
  • The battery compartment opens from the back or bottom of the unit with a latch, clip, or twist mechanism — check your specific model if you are unsure.
  • Always insert the new battery with the positive (+) end facing the spring, matching the direction the old battery was facing.
  • Test the detector when ready after replacing the battery by pressing the test button for three to five seconds until the alarm sounds.
  • Replace batteries once per year, or as soon as the detector chirps — a chirp every 30 to 60 seconds means the battery is low.

Insert the new battery in the correct direction

Before you open the compartment, know what size battery your detector uses. First Alert models typically use either a 9-volt battery or two AA batteries. Check the label on the back of the detector or look at the old battery you just removed. If you are unsure, bring the old battery to a store and match it to a new one.

Slide or place the new battery into the holder, making sure the positive (+) end faces the spring clip. The spring should press lightly against the positive end. If the battery does not sit flat or the cover will not close, you have likely inserted it backward — remove it and flip it around. Close the compartment by sliding the cover back on or pressing the latch until you hear a click.

Test the detector right away

After closing the battery 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 confirms the battery is installed correctly and the detector is working.

If the alarm does not sound, open the compartment again and check that the battery is facing the correct direction. Make sure the battery is making full contact with both the spring and the metal contact point inside the holder. If the alarm still does not sound after reinserting the battery, the detector itself may need to be replaced.

Understand what a low battery chirp sounds like

A low battery does not sound like the alarm that goes off during the test. Instead, you will hear a single short chirp or beep every 30 to 60 seconds. This chirp comes from the detector itself, not from a separate alarm panel. It is the detector's way of telling you the battery is running low and needs to be replaced soon.

Do not ignore the chirp. Replace the battery within a few days. If you wait too long, the battery will die completely and the detector will stop working. Some people remove the battery to stop the noise, but this leaves your home unprotected. Changing the battery takes less than two minutes and is far safer than living without a working detector.

Know when to replace the battery

First Alert recommends replacing batteries once per year, even if the detector has not chirped. Many people change their batteries on a set date — for example, when daylight saving time begins or ends, or on a birthday. This makes it straightforward to remember and keeps all your detectors on the same schedule.

If your detector chirps before a year has passed, replace the battery right away. A chirp means the battery voltage has dropped below the level needed to power the alarm reliably. Some detectors chirp more frequently as the battery gets weaker — you may hear a chirp every 20 or 30 seconds near the end of the battery's life.

Troubleshoot if the detector keeps chirping after a battery change

If you have replaced the battery and the detector still chirps, the battery may not be making good contact with the metal terminals inside the compartment. Open the cover again and look at the metal contact points — they should be clean and shiny. If they look dirty or corroded, wipe them gently with a dry cloth or a pencil eraser.

Also check that you are using a fresh battery. Batteries can sit on store shelves for months and lose charge slowly. If the new battery is more than a year old, try a different one. If the detector continues to chirp even with a fresh battery and clean contacts, the detector's internal circuitry may be failing and the unit should be replaced.

Frequently Asked Questions

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

Most First Alert detectors are designed for single-use alkaline batteries and do not work reliably with rechargeable batteries. Rechargeable batteries have a different voltage curve and may not trigger the low-battery alarm when they are actually running low. Stick with alkaline 9-volt or AA batteries from a major brand.

What should I do with the old battery?

Do not throw 9-volt batteries in the trash. Take them to a battery recycling center, an electronics store like Best Buy, or a hazardous waste collection event in your area. Many grocery stores and pharmacies also have battery drop-off bins. AA batteries can usually go in regular trash, but recycling is better for the environment.

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

The battery may not be making full contact with the terminals inside the compartment. Open the cover, remove the battery, and look at the metal contact points. Wipe them clean with a dry cloth. Reinsert the battery and make sure it sits flat in the holder. If the chirp continues, try a different new battery.

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

A fresh alkaline battery typically lasts 6 to 12 months in a First Alert detector, depending on how often the detector tests itself and whether it has been triggered by actual smoke. This is why First Alert recommends changing batteries once per year as a standard practice, rather than waiting for the chirp.

Can I use a different brand of battery?

Yes. Any alkaline 9-volt or AA battery from a trusted brand will work in a First Alert detector. Duracell, Energizer, and store brands are all reliable choices. Avoid very cheap batteries from unknown brands, as they may have lower voltage or fail sooner than expected.