The 9120B uses a 9-volt battery that slides into a compartment on the back of the unit

The First Alert 9120B is a battery-powered ionization smoke detector. When the low-battery chirp starts—a single beep every 30 to 60 seconds—the 9-volt battery is running down and needs replacement. The battery compartment is on the back of the detector, behind a small plastic door that twists or slides open depending on your model year.

You do not need any tools. The whole job takes less than a minute. The detector will stop chirping once the fresh battery is seated properly, and you can test it when ready with the test button on the front.

Key Takeaways

  • The 9120B takes a standard 9-volt battery, which you can buy at any drugstore, supermarket, or hardware store for $5 to $10.
  • The battery compartment is on the back of the unit and opens by twisting or sliding a plastic door—check your model to see which.
  • Remove the old battery by pulling the connector tab straight out, then push the new battery connector in until it clicks.
  • Press the test button on the front of the detector after installation to confirm the battery is working and the unit is functioning.
  • If the chirping continues after you install a new battery, the detector itself may have reached the end of its life and needs replacement.

What battery the 9120B needs

The First Alert 9120B requires one 9-volt battery. Any standard 9V battery will work—Duracell, Energizer, Amazon Basics, or store brands are all compatible. Alkaline 9V batteries are the most common and least expensive option. You do not need a rechargeable or lithium battery unless you prefer them.

Buy the battery before you start, or have one on hand. The detector will continue to chirp every 30 to 60 seconds until you replace it, and the sound is designed to be annoying so you notice it.

How to open the battery compartment

Look at the back of your 9120B. You will see a small rectangular door or cover. Depending on your model year, this door either twists counterclockwise or slides downward. Most newer units slide; older ones twist. Try sliding first—if it does not move, twist the door counterclockwise until it comes free.

Once the door is open, you will see the battery connector—a plastic clip with two metal prongs. The old battery is already plugged into this connector. Do not force anything. If the door feels stuck, you are probably turning or pulling in the wrong direction.

Removing the old battery and installing the new one

Grasp the plastic connector tab and pull it straight out of the battery. The connector will come free easily. Do not yank on the battery itself—always pull the connector tab.

Take your new 9-volt battery and align the connector so the two metal prongs match the two holes on the battery terminal. Push the connector onto the battery terminal firmly until you hear or feel a click. The connection should be snug and not wobble.

Close the battery compartment door by sliding it up or twisting it clockwise, depending on your model. The door should sit flush with the back of the detector.

Testing the detector after battery replacement

Press and hold the test button on the front of the 9120B for three to five seconds. You should hear a loud alarm sound—this confirms the battery is connected and the detector is working. Release the button and the alarm stops when ready.

If you do not hear an alarm, the battery connector may not be fully seated. Open the compartment again and push the connector onto the battery terminal harder. Test again.

Why the detector still chirps after a new battery

If the low-battery chirp continues after you install a fresh battery and test the detector, the problem is usually not the battery itself. The 9120B has a lifespan of about 10 years from the date of manufacture. If your detector is older than that, the internal components may have degraded, and the unit may be sending a false low-battery signal.

Check the date printed on the back of the detector. If it is more than 10 years old, the detector should be replaced with a new one, even if a new battery does not stop the chirping. If the detector is newer and the chirping persists, contact First Alert customer service with your model number and the date code.

Frequently Asked Questions

Can I use a rechargeable 9-volt battery in the 9120B?

Yes, rechargeable 9V batteries work in the 9120B, but they are not recommended. Rechargeable batteries lose charge faster than alkaline batteries and may not power the detector reliably during an emergency. Stick with standard alkaline 9V batteries for consistent performance.

How often do I need to replace the battery?

A fresh alkaline 9-volt battery typically lasts 6 to 12 months in the 9120B, depending on how often the detector tests itself and sounds alarms. Replace the battery as soon as you hear the low-battery chirp. Do not wait until the detector stops working.

What if I cannot get the battery compartment open?

Make sure you are twisting or sliding in the correct direction. If the door is stuck, try the opposite motion—if you were twisting, try sliding, and vice versa. Do not force it. If it remains stuck, the plastic may be cracked or the door hinge may be damaged, and the detector may need to be replaced.

Will the alarm go off when I test the detector?

Yes, the test button produces a loud alarm sound. This is normal and expected. The alarm stops when ready when you release the button. Test during the day when you are prepared for the noise, and let anyone in your home know you are testing the detector so they are not startled.