Replace the battery while the detector is powered off

Turn off your carbon monoxide detector before you open it. Most detectors have a power switch on the unit itself, or you can flip the breaker that controls that outlet. Wait a few seconds after powering down — this clears any residual charge and keeps you safe while you work.

Once the detector is off, locate the battery compartment. On most models, this is on the back or bottom of the unit. You may need to unscrew a small panel or press a release tab to open it. Check your detector's manual if you cannot find the compartment — the location varies by brand and model.

Remove the old battery by pulling it straight out or sliding it in the direction the arrows on the compartment indicate. Note which direction the positive and negative ends face — you will need to insert the new battery the same way.

Key Takeaways

  • Turn off the detector at the power switch or breaker before opening the battery compartment.
  • Most detectors use a 9-volt battery, but check your manual or the old battery to confirm the size.
  • Insert the new battery with the positive and negative ends facing the same direction as the old one.
  • Power the detector back on and listen for a beep or check the display to confirm it is working.
  • Write the replacement date on the detector with a marker so you know when to change it again.

Identify the correct battery size before you buy

The most common battery for carbon monoxide detectors is a 9-volt battery. However, some detectors use AA or AAA batteries, and a few models use both — one 9-volt plus two AAs. Check the old battery you removed, or look inside the compartment to see how many slots are there and what size they are.

If you cannot find the old battery or see the compartment clearly, open your detector's manual. The manual will list the exact battery type and quantity. If you no longer have the physical manual, search online for the model number — you can usually find a PDF version on the manufacturer's website.

Buy a fresh battery from a hardware store, pharmacy, or supermarket. Avoid batteries that are close to their expiration date. A battery that has sat on a shelf for years may not hold a full charge, and carbon monoxide detection depends on reliable power.

Insert the new battery and test the detector

Slide or press the new battery into the compartment, making sure the positive (+) end and negative (−) end match the orientation of the old battery. Push until you hear or feel a click, which means the battery is seated properly.

Close the battery compartment and find any screws or panels. Then turn the detector back on at the power switch or breaker. Most detectors will beep once or twice when power returns, and some display a green light or message on a digital screen. This beep or light confirms the detector recognizes the new battery.

If the detector does not beep or show a light after 30 seconds, the battery may not be inserted correctly. Power it off again, open the compartment, and check that the battery is pushed all the way in and facing the right direction.

Mark the replacement date on the detector

Use a permanent marker to write the month and year on the detector itself — usually on the back or bottom where it will not be visible. For example, write "1/25" if you replaced the battery in January 2025. This straightforward step saves you from guessing later about when you last changed it.

Most 9-volt batteries last 12 months in a carbon monoxide detector. AA and AAA batteries may last longer, depending on the detector model. Check your manual for the expected battery life, and plan to replace it around the same time each year — many people do it when they change their smoke detector batteries or during daylight saving time.

What to do if the detector beeps after battery replacement

A single beep every 30 to 60 seconds after you replace the battery usually means the battery is low or not making good contact. Power off the detector, remove the battery, and check that the compartment is clean. Wipe the battery terminals with a dry cloth to remove any dust or corrosion, then reinsert the battery and power on again.

If the detector continues to beep after you have reseated the battery, try a different new battery. The first one may have been defective or expired. If a second fresh battery still causes beeping, the detector itself may be failing and should be replaced.

A continuous alarm — a loud, repeating sound — means the detector has sensed carbon monoxide. This is not a battery issue. Leave your home when ready, call 911 from outside, and do not return until emergency responders have cleared the building.

Hardwired detectors with battery backup

Some carbon monoxide detectors are hardwired into your home's electrical system but also have a battery backup. These detectors will beep or flash a light when the backup battery is low, even though the main power is still on. The battery replacement process is the same — power off the detector, locate the compartment, and swap in a fresh battery.

Hardwired detectors often have a test button on the front. After you replace the battery, press and hold this button for a few seconds to run a self-test. The detector should beep or display a message confirming the battery is working. If it does not respond to the test button, the battery may not be installed correctly.

Frequently Asked Questions

Can I replace the battery while the detector is plugged in?

No. Always unplug the detector or turn off the breaker before opening the battery compartment. Even though the risk is small, powering down prevents accidental damage to the detector and keeps you safe while you work.

What happens if I put the battery in backwards?

The detector will not power on, and you may see no light or hear no beep when you turn it back on. If this happens, power off the detector, open the compartment, and flip the battery so the positive end faces the direction marked in the compartment. The detector should respond normally once the battery is oriented correctly.

How often should I replace the battery?

Most 9-volt batteries last about 12 months in a carbon monoxide detector. Check your manual for the expected life of your specific model and battery type. Many people replace batteries once a year on a set date, such as when they change their smoke detector batteries.

Is it safe to use a rechargeable battery in my detector?

No. Carbon monoxide detectors are designed for single-use alkaline batteries. Rechargeable batteries have different voltage and discharge rates, and they may not power the detector reliably. Always use a fresh alkaline battery of the correct size and type.

What should I do with the old battery?

Take it to a battery recycling program or a store that accepts used batteries, such as Best Buy or Home Depot. Many communities have battery collection days or drop-off locations. Do not throw batteries in the trash — they contain materials that should be recycled.