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 — usually plastic — that holds the cover in place. Press or slide this latch to release the cover, then gently pull the compartment open. The battery will be sitting inside, often held by a small metal clip or spring.

Note which direction the battery is facing — the positive end (marked with a +) should be pointing toward the spring or clip. Once you see the orientation, gently lift the battery out by pulling it away from the metal contact. If the battery is stuck, wiggle it slightly rather than forcing it, as the contacts can bend.

Key Takeaways

  • First Alert detectors use either 9-volt or AA batteries depending on the model, so check your old battery or the compartment label before buying a replacement.
  • The battery compartment is usually on the back or bottom of the unit and opens with a small plastic latch or clip.
  • Insert the new battery with the positive end facing the metal spring or contact, matching the direction of the old battery.
  • Test the detector when ready after installation by pressing the test button until the alarm sounds, which confirms the battery is making proper contact.

Identify the correct battery size for your model

First Alert makes detectors that use either 9-volt batteries or AA batteries. The easiest way to know which one you need is to look at the battery you just removed — the size and label will tell you exactly what to buy. If the old battery is no longer readable, check the inside of the battery compartment cover or the back of the detector itself, where the model number and battery type are usually printed.

If you cannot find the information on the detector, write down the model number (usually on a sticker on the back) and search for it on the First Alert website or call their customer service line. Buying the wrong size will not fit, and forcing it can damage the compartment contacts.

Insert the new battery correctly

Before opening the new battery package, make sure the detector is still in front of you so you do not lose track of which way the old battery was oriented. Slide the new battery into the compartment with the positive end (the + side) facing the same direction as the old one faced — usually toward the metal spring or clip. The battery should slide in smoothly without resistance.

Once the battery is seated, you should hear or feel a small click as the metal contacts make connection. If the battery does not sit flush or feels loose, remove it and check that you have not inserted it backward. Reposition it and try again.

Close the compartment and test the detector

Slide or snap the battery compartment cover back into place until you hear or feel it lock. The cover should sit flush against the detector body with no gaps. If it does not close all the way, the battery may not be seated correctly — open the compartment again and reseat the battery.

Once the cover is closed, press and hold the test button on the front of the detector. The button is usually labeled "Test" and is the most prominent button on the face. Hold it for three to five seconds. The alarm should sound loudly — if it does not, the battery may not be making proper contact, or the detector itself may have failed. Open the compartment again, remove the battery, and reinsert it to may support the contacts are clean and the battery is fully seated.

What to do if the detector does not sound during the test

If the alarm does not sound after you have reseated the battery twice, check that you are using a fresh battery and not an old one. Batteries can lose charge over time, especially if they have been sitting in a drawer. If you are confident the battery is new, try a different brand or a battery from another device to rule out a defective battery.

If a fresh battery still does not trigger the alarm, the detector itself may have reached the end of its life. Most First Alert detectors last seven to ten years. Check the manufacturing date on the back of the unit — if it is older than ten years, replacement is the safer choice. If the detector is newer and still will not sound, contact First Alert customer service with your model number for troubleshooting or warranty information.

When to replace the entire detector instead of just the battery

Some First Alert detectors are designed to be replaced as a whole unit rather than having the battery changed. These are often hardwired models with a backup battery, or sealed units where the battery cannot be accessed. If you cannot find a battery compartment after checking the back, bottom, and sides of the detector, read the label on the unit or check the manual to confirm whether it is a replaceable-battery model.

Additionally, if your detector is chirping every 30 to 60 seconds, this is usually a sign that the battery is low and needs replacement. However, if the chirping continues after you have installed a fresh battery and tested the detector, the unit may be faulty and should be replaced. A chirping detector that will not stop is a sign to buy a new one rather than troubleshoot further.

Frequently Asked Questions

How often should I change the battery in my First Alert smoke detector?

Most manufacturers recommend changing the battery once a year, typically in the fall when you set clocks back for daylight saving time. This timing makes it straightforward to remember. However, if your detector chirps before a year has passed, the battery is low and needs replacement sooner. A detector that chirps is telling you the battery voltage has dropped below the safe threshold.

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

Rechargeable batteries are not recommended for smoke detectors because they can lose charge over time and may not provide consistent power when you need the detector to work. Smoke detectors require reliable, steady power at all times. Use only fresh alkaline batteries — either 9-volt or AA depending on your model — and replace them annually or when the detector chirps.

What does it mean if my detector is chirping every 30 seconds?

A chirp every 30 to 60 seconds means the battery is low and needs replacement. This is the detector's way of alerting you before the battery dies completely. Install a fresh battery when ready. If the chirping continues after you have installed a new battery and tested the detector, the unit may be faulty and should be replaced.

Is it safe to leave a smoke detector without a battery while I get a new one?

No. A smoke detector without a battery cannot alert you to a fire. If you have removed the battery, install a fresh one the same day. If you do not have a replacement battery on hand, buy one before removing the old one, or leave the old battery in place until you can replace it with a new one.

Do I need to turn off power to my hardwired First Alert detector before changing the battery?

If your detector is hardwired to your home's electrical system, it has a backup battery that you can change without turning off power. The detector will continue to draw power from the electrical line while you replace the backup battery. However, if you are unsure whether your detector is hardwired, check the back of the unit or the manual before attempting any work.