Most smoke alarms use either 9-volt or AA batteries

The battery your smoke alarm needs depends on the model you own. The two most common sizes are 9-volt batteries (the rectangular ones) and AA batteries (the cylindrical ones about the size of your pinky finger). Some older or specialty models use 10-year lithium batteries that cannot be replaced, but those are built to last the life of the alarm and do not need changing.

Check the label on the back or bottom of your alarm to see which size it takes. The label will say "9V" or "AA" clearly. If you cannot find the label, open the battery compartment—the shape of the slot will tell you when ready. A 9-volt battery is wider and flatter; an AA battery is narrower and taller.

Key Takeaways

  • Your smoke alarm's battery type is printed on the back or bottom of the unit, or you can see it by opening the battery compartment.
  • 9-volt and AA batteries are the two standard sizes; 10-year lithium batteries cannot be replaced and come sealed in the alarm.
  • Replace batteries once per year, or when ready if the alarm chirps—that chirp means the battery is low.
  • Test your alarm after installing a new battery by pressing the test button for three seconds until the alarm sounds.
  • Keep spare batteries on hand so you can replace them right away instead of leaving the alarm without power.

How to find out which battery your alarm uses

The fastest way is to look at the back or bottom of the alarm itself. Most manufacturers print the battery type on a label there. You are looking for either "9V" or "AA"—sometimes written as "1.5V AA" or similar. If the label is worn or missing, you can open the battery compartment by twisting or sliding the cover off the front of the alarm.

Once the compartment is open, you will see the battery slot. A 9-volt slot is rectangular and fairly wide. An AA slot is narrower and cylindrical. If you see two or more slots, your alarm takes multiple AA batteries—usually two or four. If you are still unsure, take a photo of the compartment and search the model number online, or call the manufacturer's customer service line (the number is usually on the back of the unit).

When to replace your smoke alarm battery

Replace the battery once per year, even if the alarm has not chirped. Many fire departments recommend doing this on a set date—some suggest the day you change your clocks for daylight saving time, or your birthday. Pick a date you will remember and stick to it.

If your alarm chirps or beeps every 30 to 60 seconds, that is the low-battery warning. Replace the battery when ready. Do not ignore the chirp—it means the alarm will not work if there is a fire. Some people remove the battery to stop the noise, which defeats the entire purpose of having a smoke alarm. Replace it instead.

If your alarm is more than 10 years old, replace the entire unit rather than just the battery. Smoke alarms lose sensitivity over time and should not be relied on past their expiration date.

Steps to replace a smoke alarm battery

Turn off the alarm before you start. Some models have an on-off switch; others do not. If yours has a switch, flip it to off. If it does not, you can straightforward remove the old battery and install the new one—the alarm will not go off during the swap.

Open the battery compartment by twisting or sliding the cover. The direction varies by model, so try both ways gently. Once it is open, remove the old battery. Note which direction it was facing—the positive and negative ends matter. The positive end (marked with a + sign) should face up or toward the front of the compartment, depending on your model.

Insert the new battery in the same direction as the old one. Push it in firmly until it clicks or seats fully. Close the compartment cover by twisting or sliding it back into place. Turn the alarm back on if you switched it off. Press the test button (usually on the front) and hold it for three seconds. The alarm should sound loudly. If it does not, the battery may not be seated correctly—open the compartment and reseat it.

Where to buy replacement batteries

9-volt and AA batteries are sold at nearly every grocery store, pharmacy, hardware store, and online retailer. They are inexpensive—usually between one and three dollars per battery. Buy a pack of extras and store them in a cool, dry place so you have them on hand when you need them.

Avoid buying the cheapest generic batteries if you can. Name brands like Duracell, Energizer, and Panasonic tend to last longer and are worth the small extra cost. For smoke alarms, you want reliable power, not the lowest price.

Smoke alarms with 10-year batteries

Some newer smoke alarms come with sealed 10-year lithium batteries that cannot be removed or replaced. These alarms are designed to last 10 years and then be thrown away and replaced entirely. The battery is built in and will not run out before the alarm itself becomes unreliable.

If your alarm has a 10-year battery, you will not see a battery compartment. The label will say "10-year battery" or "sealed battery." These alarms are more expensive upfront but save you the trouble of remembering to replace batteries every year. When the 10 years are up, replace the entire unit.

Common mistakes to avoid

Do not mix old and new batteries in the same alarm. If your alarm takes two or more batteries, replace all of them at once, not just the one that is low. A mix of old and new batteries can cause the alarm to malfunction.

Do not install the battery backward. The positive end must face the correct direction. If the alarm does not sound when you test it, open the compartment and flip the battery around.

Do not ignore a chirping alarm. The chirp means the battery is dying and the alarm will not protect you. Replace it right away, even if it is inconvenient.

Do not assume all smoke alarms in your home take the same battery. Check each one individually. Older and newer models may use different sizes.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke alarm?

No. Rechargeable batteries lose charge over time even when not in use, and a smoke alarm needs to work reliably at any moment. Use only new alkaline batteries—the standard disposable kind. Rechargeable batteries can fail silently and leave your alarm powerless.

What should I do with the old battery after I remove it?

Take it to a battery recycling center or a store that accepts batteries for recycling—many hardware stores and electronics retailers have drop-off programs. Do not throw batteries in the trash. Check Earth911.com or call your local waste management office to find a recycling location near you.

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

The battery may not be seated correctly. Open the compartment and push the battery in firmly until you hear or feel a click. If it still chirps, try flipping the battery around—you may have installed it backward. If it continues after that, the alarm itself may be faulty and need replacement.

Do I need to turn off the power to my hardwired smoke alarm before changing the battery?

Hardwired alarms have a backup battery that powers them if the electrical circuit fails. Turn off the breaker to that circuit at your electrical panel before you work on it. The backup battery will keep the alarm powered while you work. After you finish, turn the breaker back on and test the alarm.

How long does a 9-volt battery last in a smoke alarm?

A new 9-volt battery typically lasts 6 to 12 months in a smoke alarm, depending on the brand and the alarm's design. This is why replacing once per year is the standard recommendation. If your alarm chirps sooner than that, the battery may be defective—try a new one from a different brand.