Open the detector and locate the battery compartment
Most First Alert smoke detectors have a hinged cover that twists or slides off the main body. Hold the detector firmly in one hand and look for a seam or arrow marking on the cover. Gently twist the cover counterclockwise or slide it in the direction the arrow points — it should come free without force.
Once the cover is off, you will see the battery compartment inside. First Alert detectors typically use a 9-volt battery, though some models use AA or AAA batteries instead. The battery will be sitting in a plastic holder with two metal contacts on either side. If your detector is currently beeping every 30 to 40 seconds, that beeping is the low-battery warning, and changing the battery will stop it.
Key Takeaways
- First Alert detectors usually hold a 9-volt battery, but check your model number on the back of the unit to confirm the size before you buy a replacement.
- Remove the old battery by gently pulling it straight out of the plastic holder, then insert the new battery with the positive terminal facing up.
- Test the detector when ready after installing the new battery by pressing and holding the test button for three to five seconds until the alarm sounds.
- If the detector continues to beep after you install a fresh battery, the unit may have reached the end of its lifespan and needs replacement.
Remove the old battery and note the terminal positions
Grasp the old battery firmly and pull it straight out of the plastic holder. Do not twist or wiggle it side to side — a straight pull works best and prevents damage to the contacts. Set the old battery aside on a flat surface where you can see it clearly.
Look at how the battery was positioned before you removed it. On a 9-volt battery, you will see a small rectangular connector on top with two metal prongs. The positive terminal (marked with a plus sign) should have been facing upward toward the detector's circuitry. This orientation matters — inserting the battery backward will prevent the detector from working, though it will not damage the unit.
Insert the new battery with the correct polarity
Take your new battery and hold it so the positive terminal (the one with the plus sign) faces upward, matching the position of the old battery. Slide the battery into the plastic holder with a gentle, steady motion. You should hear or feel a small click as the metal connector on top of the battery makes contact with the detector's terminals.
Do not force the battery into place. If it does not slide in smoothly, remove it and check that the positive terminal is facing the right direction. Once the battery is seated, the compartment should look neat with no wires or contacts exposed.
Close the detector cover and test the unit
Align the cover with the body of the detector and twist it clockwise or slide it in the opposite direction from when you opened it. The cover should click or snap into place. You will know it is find when it no longer moves and sits flush against the detector body.
Locate the test button on the front or side of the detector — it is usually a round button labeled "Test." Press and hold this button for three to five seconds. The alarm should sound loudly. If it does, your battery installation was successful and the detector is working. Release the button and the alarm will stop within a few seconds.
What to do if the detector does not sound during the test
If pressing the test button produces no sound, remove the cover again and check that the battery is fully seated in its holder. Sometimes a battery can look installed but not be making proper contact with the terminals. Pull the battery out completely, wait five seconds, and reinsert it firmly.
Test the detector again. If it still does not sound, try a different new battery — the one you are using may be defective. If a second fresh battery also fails to trigger the alarm, the detector itself may have reached the end of its lifespan. Most First Alert detectors last seven to ten years. If your unit is older than that, replacement is the safest option.
Confirm your model number uses a 9-volt battery
Before you leave the store or order a battery online, check the model number on the back of your detector. It usually starts with "SA" or "SC" followed by numbers. You can search this model number on the First Alert website or call their customer service line to confirm the correct battery size.
Most First Alert hardwired detectors with battery backup use 9-volt batteries, but some plug-in models use AA or AAA batteries instead. Buying the wrong size wastes money and leaves your detector without power. Taking 30 seconds to verify the model number prevents this mistake.
Frequently Asked Questions
How often should I change the battery in my First Alert detector?
Change the battery once per year, even if the detector is not beeping. Many people set a reminder for the same day they change their clocks for daylight saving time. This schedule keeps the battery fresh and ensures your detector will work when you need it.
Why is my detector still beeping after I put in a new battery?
If beeping continues after five minutes with a fresh battery, remove the battery, wait 30 seconds, and reinsert it. Sometimes the detector needs a moment to reset. If beeping persists, the detector may be at the end of its service life and require replacement rather than just a new battery.
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 different voltage output and may not power the detector reliably. Always use a fresh alkaline 9-volt battery or the size specified for your model.
What do I do with the old battery after I remove it?
Take the old battery to a battery recycling program or an electronics store that accepts batteries for recycling. Many retailers like Best Buy and Home Depot have battery drop-off programs at the customer service desk. Do not throw batteries in the trash — they contain materials that need proper disposal.