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 door or panel — it usually slides or twists open. Press or slide the latch in the direction the arrow points, and the compartment will come free.

Once the compartment is open, you will see the battery sitting in a plastic holder. The battery is typically a 9-volt rectangular battery, though some First Alert models use AA or AAA batteries instead. Note which direction the battery is facing — the positive and negative terminals matter when you put the new one in.

Gently pull the battery out of its holder. If it is stuck, wiggle it slightly rather than forcing it. Do not touch the terminals with bare fingers if you can avoid it, as oils can reduce contact quality.

Key Takeaways

  • First Alert detectors typically use 9-volt batteries, though some models use AA or AAA — check your detector's label or manual to know which type to buy.
  • The battery compartment is usually on the back or bottom of the unit and opens by sliding or twisting a latch in the direction the arrow indicates.
  • Note the battery orientation before removing the old one so you can install the new battery the same way.
  • After inserting the new battery, test the detector by pressing the test button for three seconds — you should hear a loud alarm.
  • Replace the battery every 12 months, or sooner if the detector chirps once every 30 to 40 seconds, which signals low battery.

Insert the new battery in the correct direction

Before opening the package, check the label on your First Alert detector to confirm the battery type. The label is usually on the back or inside the battery compartment. If you cannot find it, the manual that came with the detector will list the correct size.

Take the new battery out of its packaging. Hold it so you can see the positive terminal (marked with a plus sign) and the negative terminal (marked with a minus sign). Insert the battery into the holder in the same direction the old one was facing — positive side first if that is how the old battery was oriented.

Push the battery down gently until it clicks or seats fully in the holder. You should feel a small resistance, then a click as it makes contact. Do not force it — if it does not seat smoothly, remove it and check that you have the polarity correct.

Close the compartment and test the detector

Slide or twist the battery compartment door back into place until you hear or feel it lock. The door should sit flush against the detector body with no gaps.

Press and hold the test button on the front of the detector for three seconds. The test button is usually a large circular button in the center of the unit. When you press it, the alarm should sound loudly — this confirms the battery is making good contact and the detector is working. If you hear nothing or only a faint sound, remove the battery, check the polarity again, and reseat it.

Release the test button and the alarm should stop within a few seconds. If the alarm continues, press the button again to silence it.

Know when to replace the battery

First Alert recommends replacing the battery once per year, even if the detector has not signaled low battery. Many people replace batteries on a set date — for example, when daylight saving time ends in the fall — to make the task easier to remember.

Your detector will alert you when the battery is running low by chirping once every 30 to 40 seconds. This chirp is different from the alarm that sounds when smoke is detected. If you hear this chirp, replace the battery within a few days. The chirping will continue until you install a fresh battery.

Do not ignore the low-battery chirp. A detector with a dead battery provides no protection and defeats the purpose of having the device installed.

Troubleshoot if the detector still does not work

If the detector does not sound when you press the test button, first check that the battery compartment is fully closed. An open or loose compartment will break the electrical connection even if the battery is new and correctly installed.

Remove the battery again and inspect the metal contacts inside the holder. If they look dirty or corroded, wipe them gently with a dry cloth or a pencil eraser. Reinsert the battery and test again.

If the detector still does not respond, the unit itself may have reached the end of its life. Most First Alert smoke 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.

Different battery types in First Alert models

While most First Alert detectors use 9-volt batteries, some models are designed for AA or AAA batteries instead. A few high-end models use lithium batteries that last 10 years and do not need replacement during the detector's life.

Check the label on your specific detector before buying a replacement battery. The label will state the battery type clearly. If you have lost the label or cannot read it, the model number on the back of the detector can be looked up on the First Alert website or in the original manual to find the correct battery size.

Buying the wrong battery size will not fit the compartment and will waste money. Take 30 seconds to confirm the type before you shop.

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. Rechargeable batteries have a different voltage curve and may not trigger the low-battery alert when they are actually depleted, leaving you with a non-functional detector without warning.

What should I do with the old battery?

Take it to a battery recycling center or an electronics retailer that accepts batteries for recycling. Many stores that sell batteries, including Best Buy and Home Depot, have free drop-off programs. Do not throw batteries in the trash.

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

The chirp may continue for a few minutes after battery installation as the detector resets. If it continues beyond five minutes, remove the battery, wait 30 seconds, and reinsert it. If the chirp persists, the detector may have reached the end of its life and needs replacement.

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

A standard 9-volt alkaline battery typically lasts 12 months in a First Alert detector. The actual lifespan depends on how often the detector tests itself and whether the alarm has sounded. Lithium batteries in some models last up to 10 years.

Do I need to turn off the detector before changing the battery?

No. Changing the battery does not require you to turn off the detector. straightforward open the compartment, swap the battery, close the compartment, and test. The detector will remain armed and functional throughout the process.