Open the detector and remove the old battery

Most First Alert smoke detectors have a battery door on the front or side. Look for a small latch or clip—usually plastic—that holds the door closed. Press or slide this latch 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, depending on your model.

Grip the battery firmly and pull it straight out. If it is stuck, wiggle it gently side to side as you pull. Do not force it hard enough to damage the plastic contacts inside the detector. Once the old battery is out, set it aside—you can recycle 9-volt batteries at most hardware stores or electronics retailers.

Key Takeaways

  • First Alert detectors use either one 9-volt battery or two AA batteries depending on the model, so check which type yours needs before you buy replacements.
  • Open the battery door by pressing or sliding the latch on the front or side of the detector, then pull the old battery straight out.
  • Insert the new battery with the positive and negative terminals facing the correct direction—the metal contacts inside the detector show which way it goes.
  • Test the detector when ready after installing the new battery by pressing and holding the test button until the alarm sounds.
  • Replace batteries once per year, or sooner if the detector chirps every 30 to 40 seconds, which signals low battery.

Insert the new battery the correct way

Before you install the new battery, look at the metal contacts inside the battery compartment. You will see two terminals—one marked with a plus sign (+) and one marked with a minus sign (−). The positive terminal on your new battery must face the positive contact, and the negative terminal must face the negative contact.

Slide or insert the new battery into the compartment, matching the terminals to the correct contacts. You should hear or feel a small click when the battery seats fully. If the battery does not sit flush or feels loose, remove it and try again—it may be inserted backward or not pushed in far enough.

Close the battery door and test the alarm

Slide or swing the battery door closed until the latch clicks into place. The door should sit flush against the detector body with no gaps. If it does not close all the way, the battery may not be seated correctly—open the door again and reseat the battery.

Once the door is closed, press and hold the test button on the front of the detector. This button is usually round and labeled "Test." Hold it for three to five seconds. The alarm should sound loudly—a loud beeping or chirping noise. If you hear nothing, the battery may be installed backward or the detector may have a different problem. Remove the battery, check the terminal orientation, and try again.

Know when the battery is running low

A low battery does not stop the detector from working, but it sends a warning signal. You will hear a single chirp or beep every 30 to 40 seconds, usually at night when the house is quiet. This chirp is not the alarm itself—it is the detector telling you the battery is almost dead and needs to be replaced soon.

Do not ignore this chirp. Replace the battery within a few days. If you wait too long, the battery will die completely and the detector will not work at all, leaving your home unprotected. Some First Alert models have a low-battery indicator light that flashes instead of or in addition to the chirp.

Replace batteries on a regular schedule

The best practice is to replace smoke detector batteries once per year, even if they have not started chirping. Many people change their batteries when they change their clocks for daylight saving time in the spring and fall—this creates a natural reminder twice per year. You can also pick a date like your birthday or New Year's Day.

If you replace the battery and the detector still chirps after a few minutes, the new battery may be defective or installed incorrectly. Remove it, check the terminal orientation again, and try a fresh battery from a different package. If the chirping continues, the detector itself may be faulty and may need to be replaced.

Troubleshoot a detector that will not stop chirping

If you have installed a new battery and the detector still chirps every 30 to 40 seconds, first make sure the battery is fully inserted and the door is completely closed. Open the door, remove the battery, and reinstall it, paying close attention to the positive and negative terminals. Close the door firmly and wait two minutes.

If the chirping continues, try a different new battery—the one you installed may be defective. If a second new battery also produces the same chirp, the detector's internal circuitry may be failing. At that point, it is time to replace the entire detector. Most First Alert smoke detectors last 8 to 10 years before the sensors wear out.

Frequently Asked Questions

What type of battery does my First Alert detector use?

Most First Alert models use one 9-volt battery, which is a rectangular block with two metal terminals on top. Some models use two AA batteries instead. Check the battery compartment or the back of your detector—the label usually shows which type. If you are not sure, take a photo of the compartment and bring it to a hardware store.

Can I use a rechargeable battery in my First Alert detector?

Rechargeable 9-volt batteries work in First Alert detectors, but they tend to lose charge faster than alkaline batteries and may not last as long between charges. Alkaline 9-volt batteries are cheaper and more reliable for smoke detectors. Stick with a standard alkaline battery unless your detector specifically says rechargeable batteries are safe.

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

The most common cause is the battery installed backward—the positive and negative terminals are reversed. Remove the battery, check the plus and minus signs inside the compartment, and reinstall it the correct way. If it still chirps, try a different new battery, as the first one may be defective.

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

A standard alkaline 9-volt battery typically lasts 6 to 12 months in a First Alert detector, depending on how often the detector tests itself and sends signals. This is why replacing batteries once per year is the standard recommendation. If your detector chirps sooner, the battery may be defective or the detector may be faulty.

What should I do with the old battery?

Do not throw 9-volt batteries in the trash. Take them to a hardware store, electronics retailer, or battery recycling center. Many stores like Home Depot and Best Buy have battery recycling bins near the front entrance. Some communities also have hazardous waste collection days where you can drop off old batteries for free.