Turn off the alarm, remove the old battery, and install a new one

Most smoke detectors use a 9-volt battery that slides or clicks into a compartment on the unit's body. To change it, press the test button for three seconds to silence any alarm, then locate the battery door — usually on the front or side of the detector. Slide or unclip the door, note which way the battery terminals face, pull out the old battery, and slide a fresh 9-volt battery in with the terminals matching the original direction. Close the door and press test again; a loud beep means the new battery is working.

The entire job takes about two minutes once you have the new battery in hand. Most detectors will chirp or beep every 30 to 60 seconds when the battery is low, which is your signal to change it before the alarm stops working altogether. Some newer models use non-replaceable lithium batteries that last 10 years, so check your detector's label first — if it says "sealed battery" or "10-year battery", you cannot open it and will need to replace the whole unit when the battery dies.

Key Takeaways

  • A 9-volt battery is the standard size for most household smoke detectors and costs between $5 and $10.
  • Press the test button for three seconds before opening the battery compartment to silence any active alarm.
  • Match the battery terminals to the metal contacts inside the compartment — the positive and negative ends must line up correctly.
  • Press test again after installation to confirm the new battery is seated properly and the alarm sounds.
  • If your detector has a sealed or 10-year battery, you must replace the entire unit rather than just the battery.

Locate the battery compartment on your detector

The battery door is usually on the front of the unit, but some models have it on the side or bottom. Look for a small rectangular panel or a clip that holds a cover in place. If the compartment is not obvious, check the back of the detector or the manual that came with it — most manufacturers print a diagram showing where the battery goes.

If you have lost the manual, the model number is printed on the detector itself, usually on the back or inside the battery compartment. You can search that number online to find a PDF of the manual, which will show you exactly where to open the unit and which battery type it uses. Some detectors use AA or AAA batteries instead of 9-volt, so confirming the size before you buy a replacement saves a trip back to the store.

Remove the old battery safely

Once the compartment is open, you will see the old battery held in place by metal contacts or a plastic clip. Grasp the battery by its sides — not the terminals — and pull it straight out. If it is stuck, gently wiggle it side to side while pulling; do not force it or you may damage the contacts inside.

Set the old battery aside. If it is swollen, leaking, or has visible corrosion around the terminals, do not touch the corrosion directly. Wash your hands after handling an old battery, especially if it has leaked. Corroded batteries can damage the detector's contacts, so if you see white or blue-green powder inside the compartment, wipe it away with a dry cloth or cotton swab before installing the new battery.

Install the new battery with terminals aligned correctly

A 9-volt battery has two terminals on top: a larger flat one (negative) and a smaller round one (positive). Inside the detector's compartment, you will see two metal contacts that match these shapes. Slide the new battery in so the positive terminal (the small round one) lines up with the small contact and the negative terminal (the flat one) lines up with the larger contact.

Push the battery in until you hear or feel a click, which means the metal contacts are making a solid connection. If the battery does not click into place, remove it and check that the terminals are facing the right direction. A battery installed backwards will not power the detector, and the alarm will not sound when you test it.

Test the detector to confirm the battery is working

Close the battery compartment door and press the test button on the front of the detector. Hold it down for three seconds. You should hear a loud alarm sound — usually a series of beeps or a continuous tone. If the alarm sounds, the battery is installed correctly and the detector is working.

If you hear nothing, open the compartment again and check that the battery terminals are aligned properly. Remove the battery, wait five seconds, and reinsert it. Press test again. If the detector still does not sound, the battery may be defective or the detector itself may have failed. Try a different new battery first; if that does not work, the detector may need to be replaced.

Know when to replace the entire detector instead

Smoke detectors with sealed or 10-year batteries cannot be opened and cannot have their batteries changed. These units are designed to last 10 years from the date of manufacture, and when the battery dies, you must throw away the entire detector and install a new one. The manufacture date is printed on the back or inside of the unit.

Even detectors with replaceable batteries should be replaced every 10 years, because the sensors that detect smoke degrade over time. If your detector is more than 10 years old, changing the battery is a temporary fix — plan to buy a new detector soon. Hardwired detectors that are connected to your home's electrical system still have a backup battery that you can change using the same method, but the unit itself should be replaced after 10 years as well.

Frequently Asked Questions

What size battery does my smoke detector use?

Most household smoke detectors use a 9-volt battery, but some use AA or AAA. Check the label on the back of your detector or inside the battery compartment — it will say which size you need. If you cannot find the label, search the model number online to find the manual.

Why is my smoke detector beeping every 30 seconds?

A regular beep or chirp every 30 to 60 seconds means the battery is low and needs to be changed. This warning sound will continue until you install a new battery. If the beeping does not stop after you change the battery, the detector may have a different problem and should be tested or replaced.

Can I use a rechargeable battery in my smoke detector?

No. Smoke detectors require a fresh, single-use battery because rechargeable batteries lose charge over time and may not power the alarm when you need it. Always use a new alkaline 9-volt battery or the battery type specified on your detector's label.

What should I do with the old battery?

Take old batteries to a recycling center, electronics store, or hazardous waste collection site in your area. Many retailers that sell batteries, such as Best Buy or Home Depot, accept old batteries for recycling at no charge. Do not throw batteries in the trash.

How often should I change my smoke detector battery?

Change the battery once a year, or whenever the detector beeps to signal a low battery. Many people change their batteries when they set their clocks forward or back for daylight saving time, which makes it straightforward to remember. Test the detector after changing the battery to make sure it is working.