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 compartment door closed. Press or slide this latch — the direction varies by model, so check which way it moves — and the door will swing open or slide out.
Inside you will see a 9-volt battery (shaped like a rectangular block with two metal terminals on top) or two AA batteries stacked together. Note which way the battery sits: the positive terminal (marked with a plus sign) should face up or toward the spring. Gently pull the battery straight out. If it is stuck, wiggle it slightly side to side rather than yanking it, which can damage the connector.
Key Takeaways
- First Alert detectors use either one 9-volt battery or two AA batteries, depending on the model — check your detector to know which type to buy.
- Open the battery compartment by pressing or sliding the latch on the back or bottom of the unit, then pull the old battery straight out.
- Insert the new battery with the positive terminal facing the same direction as the old one, then close the compartment door until it clicks.
- Test the detector by pressing the test button for three seconds — you should hear a loud alarm sound within a few seconds.
Insert the new battery the same way the old one faced
Take your new battery — either a fresh 9-volt or two new AA batteries — and position it so the positive terminal (the plus sign) faces the same direction the old battery faced. This matters: if you reverse the polarity, the detector will not work and may not beep to warn you.
Slide the battery into the compartment slowly until it sits flat and the terminals make contact with the metal connectors inside. You should feel a slight resistance as the battery seats. Do not force it; if it does not slide in smoothly, pull it out and check that the positive terminal is facing the right direction.
Close the compartment and test the detector
Push or slide the battery compartment door closed until you hear or feel a click. The latch should hold the door firmly in place. If the door does not close all the way, the battery may not be seated correctly — open it again and reseat the battery.
Once the door is closed, locate the test button on the front or top of the detector. Press and hold it for three seconds. The alarm should sound loudly within a few seconds. If you hear nothing, open the compartment again and check that the battery terminals are making contact with the connectors. Wipe the metal terminals with a dry cloth if they look corroded or dirty, then close the compartment and test again.
Know which battery type your model uses before you buy
First Alert makes detectors that run on either one 9-volt battery or two AA batteries. The model number is usually printed on the back of the unit or inside the battery compartment. You can search this model number on the First Alert website or on the packaging of a replacement battery to confirm which type you need.
If you are unsure, open the compartment and look at the old battery. A 9-volt battery is rectangular and blocky with two metal terminals on top. Two AA batteries are cylindrical and stack on top of each other. Buy the same type as what you removed. Buying the wrong size means a wasted trip back to the store.
Replace batteries once a year or when the detector chirps
First Alert recommends replacing batteries once a year, even if the detector has not beeped to warn you. Many people replace them when the clocks change for daylight saving time, which makes it straightforward to remember. Mark your calendar or set a phone reminder so you do not forget.
If your detector chirps once every 30 to 60 seconds, the battery is low and needs replacement right away. This chirp is the detector's way of telling you the battery is almost dead. Do not ignore it — a detector with a dead battery cannot alert you to smoke or fire. Replace the battery as soon as you hear this chirp.
Troubleshoot if the detector still does not work after battery replacement
If the test button does not produce a loud alarm even after you have installed a fresh battery, first check that the battery is making contact. Open the compartment, remove the battery, and look at the metal connectors inside the detector. If they are corroded or covered in dust, wipe them gently with a dry cloth or a pencil eraser. Reinsert the battery and test again.
If the detector still does not beep, the unit itself may have reached the end of its life. Most First Alert 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. A detector that does not respond to the test button cannot be trusted to alert you in an emergency.
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 only. Rechargeable batteries have a different voltage curve and may not power the detector reliably or may drain too quickly. Always use a fresh alkaline 9-volt or AA battery.
What should I do with the old battery?
Take it to a battery recycling program. Many retailers including Best Buy and Home Depot accept old batteries for free recycling. Do not throw it in the trash. You can also search for a local recycling center on Earth911.com or call your city's waste management office for drop-off locations.
Why does my detector chirp even after I put in a new battery?
The chirp may continue for a few minutes after battery replacement as the detector resets. If it keeps chirping after 10 minutes, remove the battery, wait 30 seconds, and reinsert it. If the chirping does not stop, the detector may be faulty and should be replaced.
How do I know if my detector is a 9-volt or AA battery model?
Open the battery compartment and look at the old battery. A 9-volt is a large rectangular block. Two AA batteries are small cylinders stacked together. You can also check the model number on the back of the unit and search it on the First Alert website.