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. Some models require you to unscrew a single fastener; others pop open by hand.
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 sits — 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 smoke detector's internal components while the compartment is open.
Key Takeaways
- First Alert detectors usually hold a 9-volt battery, but check your model number to confirm whether it uses 9V, AA, or AAA batteries.
- The battery compartment is typically on the back or bottom of the unit and opens by sliding, twisting, or unscrewing a single fastener.
- Always note the battery orientation — positive terminal up or down — before removing the old battery so you install the new one the same way.
- After inserting the new battery, press the test button to confirm the detector sounds; if it does not beep, remove and reorient the battery.
- Replace batteries once a year or whenever the detector chirps to signal low battery, usually in the fall when you change your clocks.
Insert the new battery in the correct direction
Buy a replacement battery that matches the one you removed. Check the battery compartment or your detector's manual to confirm the size — 9V, AA, or AAA. First Alert detectors are sold with batteries included, but replacement batteries are inexpensive and available at any drugstore or supermarket.
Slide or place the new battery into the holder, making sure the positive terminal (marked with a + sign) faces the direction the old battery faced. The battery holder usually has a small metal spring or contact that the positive terminal must touch. If you insert the battery backward, the detector will not work.
Push the battery all the way into the holder until it sits flush. You should hear or feel a slight click as it seats properly.
Close the compartment and test the detector
Slide or twist the battery compartment door closed until it is fully seated. If your model has a screw, tighten it by hand — do not over-tighten, as this can crack the plastic.
Press and hold the test button on the front of the detector for three to five seconds. The alarm should sound loudly. If it does not beep or chirp, open the compartment again and check that the battery is oriented correctly and fully inserted. A detector that does not sound during the test will not alert you to smoke.
Once the test confirms the detector works, you are done. Write the date on the detector with a permanent marker so you remember when you last changed the battery.
When to replace the battery
First Alert detectors typically need a new battery once a year. Many people change them in the fall when they set their clocks back, which makes the task straightforward to remember.
Your detector will also chirp or beep at regular intervals — usually every 30 to 60 seconds — when the battery is running low. This warning sound is different from the alarm that sounds during a fire. If you hear this chirping, change the battery within a few days. The low-battery warning can be annoying, but it gives you time to buy a replacement before the battery dies completely.
Do not ignore the chirping. A detector with a dead battery provides no protection.
Troubleshooting a detector that will not work after battery replacement
If the detector still does not beep after you insert a new battery and close the compartment, the battery may be oriented backward. Open the compartment again and flip the battery 180 degrees, then test again.
If the detector still does not work, the battery itself may be defective. Remove it and try a different new battery from a different package or brand. Occasionally a battery sits on a shelf too long and loses charge before you use it.
If a second new battery also fails to make the detector sound, the detector itself may have reached the end of its life. Most First Alert detectors last seven to ten years. If your detector is older than that, consider replacing the entire unit rather than troubleshooting further.
Where to buy replacement batteries
Replacement batteries for First Alert detectors are sold at drugstores, supermarkets, hardware stores, and online retailers. A single 9-volt battery typically costs between two and five dollars. AA and AAA batteries cost less, usually under two dollars each.
You do not need to buy a specific brand. Any standard 9-volt, AA, or AAA battery will work in a First Alert detector. Alkaline batteries are the most common and reliable choice for smoke detectors.
If you have multiple detectors in your home, buy enough batteries to replace all of them at once. This ensures every detector in your house is protected at the same time.
Frequently Asked Questions
What size battery does my First Alert detector use?
Most First Alert models use a 9-volt battery, but some use AA or AAA. Check the battery compartment itself — the holder is shaped to fit only one size, so you will see when ready which battery goes in. You can also check your detector's model number on the back or bottom and search the First Alert website for the manual.
Can I use a rechargeable battery in my smoke detector?
Rechargeable batteries are not recommended for smoke detectors. They discharge over time even when not in use, and a detector with a dead battery provides no protection. Alkaline batteries are more reliable for this purpose because they hold their charge longer and give you a low-battery warning before they fail completely.
Why does my detector keep chirping even after I put in a new battery?
The battery may be inserted backward or not fully seated in the holder. Open the compartment, remove the battery, and reinsert it making sure the positive terminal faces the correct direction and the battery clicks into place. If chirping continues after that, try a different new battery in case the first one is defective.
How often should I replace the battery in my smoke detector?
Replace the battery once a year, or whenever the detector chirps to signal low battery. Many people change batteries in the fall when they adjust their clocks, which makes it straightforward to remember. A detector with a fresh battery provides the best protection.
What should I do with the old battery?
Take used batteries to a battery recycling program or a store that accepts them — many retailers like Best Buy and Home Depot have battery recycling bins near the entrance. Do not throw batteries in the trash, as they contain materials that can harm the environment.