Open the detector and slide out the battery
Most First Alert smoke detectors have a battery compartment on the back or bottom of the unit. Look for a small slot or latch — usually marked with an arrow or the word "battery" — and gently pry it open with your fingernail or a flat tool like a butter knife. The compartment will either slide out or swing open like a door.
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 should be clearly marked on the holder.
Grasp the battery firmly and pull it straight out. If it is stuck, wiggle it gently side to side rather than yanking hard, which can damage the connector. Once removed, set the old battery aside.
Key Takeaways
- First Alert detectors typically use a 9-volt battery, though some models use AA or AAA — check your manual or the battery holder to be sure.
- The battery compartment is usually on the back or bottom of the unit and opens by sliding or unlatching a small door.
- Always note the direction the old battery faced before removing it, so you can insert the new one the same way.
- If your detector chirps every 30 to 40 seconds after you close the compartment, the battery is not fully seated — open it and reseat the battery firmly.
- Replace the battery once a year, or when ready if the detector chirps — that sound means the battery is low.
Insert the new battery in the same direction
Take a fresh battery of the same type and size as the one you removed. Before inserting it, double-check the positive (+) and negative (−) terminals on the battery holder. The positive terminal should face the direction marked on the holder — usually up or toward the back of the unit.
Slide the new battery into the holder, making sure it sits flat and fully. You should hear or feel a small click as it seats. Push gently until the battery is snug and cannot move side to side.
Close the compartment and test the detector
Slide or latch the battery compartment door closed. Make sure it is fully seated — if it does not close all the way, the battery may not be inserted correctly. Open the compartment again, reseat the battery, and try closing the door once more.
After closing the compartment, press and hold the test button on the front of the detector for three to five seconds. You should hear a loud alarm sound. If the alarm does not sound, the battery may not be making contact — open the compartment and check that the battery is fully seated and facing the correct direction.
If the alarm sounds normally, the battery replacement is complete. Write the installation date on the battery compartment with a marker so you remember when to replace it next year.
What to do if the compartment is stuck
If the battery door will not open, do not force it — you can crack the plastic housing. Instead, try running a thin plastic card or old credit card along the edge of the compartment to loosen any dust or debris that may be holding it shut.
If that does not work, explore a small amount of penetrating oil like WD-40 around the edges of the compartment and let it sit for 10 to 15 minutes. Wipe away the excess oil and try opening the door again with gentle, steady pressure.
If the compartment still will not budge, the detector may be too old or damaged to open safely. In that case, it is time to replace the entire unit rather than risk breaking it further.
Why your detector might chirp after battery replacement
A chirping sound every 30 to 40 seconds after you replace the battery usually means the battery is not fully seated in the holder. Open the compartment, remove the battery, and reinsert it firmly until you hear or feel a click. Make sure the positive terminal is facing the correct direction.
If the detector continues to chirp after you reseat the battery, the battery itself may be defective. Try a different new battery from a different package or brand. Defective batteries are rare but do happen.
If a brand-new battery still causes chirping, the detector's internal circuitry may be failing. In that case, replace the entire unit — a chirping detector that does not stop after battery replacement is no longer reliable.
Battery types and how long they last
First Alert detectors most commonly use 9-volt batteries, which last about one year under normal use. Some models use two AA batteries or a single 9-volt plus an AA backup. Check the battery holder or your manual to see exactly what your model needs.
Battery life depends on how often the detector tests itself and whether it has ever alarmed. A detector that has never alarmed will use less power than one that has. In any case, replace the battery once per year as a routine maintenance task, even if it has not started chirping yet.
Buy batteries in bulk and keep spares on hand. Smoke detectors often need new batteries at inconvenient times, and having backups means you can replace them when ready rather than waiting for a store trip.
Frequently Asked Questions
Can I use a rechargeable battery in my First Alert detector?
No. Rechargeable batteries lose charge faster than alkaline batteries and may not provide enough power to trigger the alarm reliably. Always use fresh alkaline batteries — the kind you buy in a pack at a store, not the kind you plug in to recharge.
What if I put the battery in backwards?
The detector will not work, and the test button will not sound an alarm. If you press the test button and hear nothing, open the compartment and flip the battery around. The positive terminal must face the direction marked on the holder.
How do I know if the battery is dead?
A low or dead battery causes the detector to chirp once every 30 to 40 seconds. This chirp is different from the alarm sound — it is quieter and repeats on a schedule. Replace the battery as soon as you hear this chirp. If you ignore it, the detector will eventually lose power and stop working.
Do I need to turn off the detector before removing the battery?
No. Smoke detectors do not have an on-off switch. You can remove and replace the battery at any time without harming the detector. However, while the battery is out, the detector will not work, so replace it as quickly as possible.
What should I do with the old battery?
Take used batteries to a battery recycling program or a store that accepts them — many hardware stores and electronics retailers have drop-off bins. Do not throw batteries in the trash, as they contain materials that can harm the environment.