Most smoke detectors use either 9-volt or AA batteries

The battery your smoke detector needs depends on the model you own. Check the label on the back or inside the battery compartment — it will tell you exactly which size. The two most common are 9-volt batteries (the rectangular ones) and AA batteries (the standard cylindrical size). Some older or specialty detectors use 10-year lithium batteries that cannot be replaced, but those are built-in and not something you choose.

If you have lost the manual or cannot read the label clearly, open the battery compartment and look at the space itself. A 9-volt battery is distinctly rectangular and wider than it is tall. An AA battery is much smaller and cylindrical. The compartment will only fit one or the other, so the shape of the space tells you what you need.

Buy the same brand and type you removed — there is no performance difference between brands for standard alkaline batteries, but consistency makes it easier to remember what goes in each detector in your home.

Key Takeaways

  • Check the label inside your smoke detector's battery compartment to find out whether it takes a 9-volt or AA battery.
  • A 9-volt battery is rectangular; an AA battery is small and cylindrical — the compartment shape will match only one.
  • Replace batteries once a year, or when ready if the detector chirps — that chirp means the battery is low.
  • Some detectors have built-in 10-year lithium batteries and do not take replaceable batteries at all.
  • Test your detector monthly by pressing the test button to make sure it still works after you install a new battery.

How to identify which battery your detector uses

The fastest way is to look at the detector itself. Most models print the battery type right on the back or on a label inside the compartment. If the label is faded or missing, remove the old battery and look at its shape. Write down the size before you throw it away, or take a photo on your phone.

If you cannot find the old battery, measure the compartment with a ruler. A 9-volt battery compartment is roughly 1.5 inches wide and 2.5 inches tall. An AA compartment is about 0.5 inches wide and 2 inches tall. You can also search online for your detector's model number — the manual will list the battery type.

When to replace smoke detector batteries

Replace the battery once a year, whether it seems dead or not. Many people change them on a set date — the first day of spring, or the day they change their clocks for daylight saving time. Pick a date you will remember and stick to it.

If your detector chirps or beeps once every 30 to 60 seconds, the battery is low and needs to be replaced right away. That chirp is a warning that the battery voltage has dropped below what the detector needs to work. Do not ignore it — a detector with a dead battery cannot alert you to smoke.

After you install a new battery, press the test button on the detector and hold it for three to five seconds. You should hear a loud alarm. If you do not hear anything, the battery may not be seated correctly, or the detector itself may have failed and need replacement.

Where to buy the right battery

Standard 9-volt and AA alkaline batteries are sold at every grocery store, pharmacy, hardware store, and online retailer. You do not need a specialty battery — a basic alkaline battery from any brand works fine. Avoid buying the cheapest option in bulk unless you use them regularly; batteries lose charge over time even when not in use.

If your detector uses a 10-year lithium battery, you cannot buy a replacement — the entire detector must be replaced when the battery reaches the end of its life. The detector will chirp to warn you when that time is near, usually around year nine or ten.

Detectors with built-in batteries you cannot replace

Some newer smoke detectors come with sealed lithium batteries designed to last 10 years. These detectors do not have a removable battery compartment. When the battery finally dies, you throw away the entire detector and install a new one.

These detectors are convenient because you do not have to remember to change batteries every year. However, they cost more upfront. If you prefer the option to replace just the battery, buy a detector that takes standard 9-volt or AA batteries instead.

Hardwired detectors and backup batteries

If your smoke detector is hardwired into your home's electrical system, it may still have a backup battery — usually a 9-volt — that powers the alarm if the power goes out. Check the detector for a battery compartment even if it is plugged in. If there is one, replace that battery once a year just like you would for a battery-only detector.

If the power goes out and your hardwired detector does not have a backup battery, it cannot alert you to smoke. That is why the backup battery matters. Test it monthly along with the rest of the detector.

Frequently Asked Questions

Can I use rechargeable batteries in my smoke detector?

Rechargeable batteries are not recommended for smoke detectors. They lose charge faster than alkaline batteries, even when not in use, and a low battery in a smoke detector is a safety risk. Stick with standard alkaline 9-volt or AA batteries.

What happens if I put the wrong size battery in?

The battery will not fit in the compartment, or it will not make proper contact with the terminals. The detector will not work. Do not force a battery that does not fit — check the label again and get the correct size.

How long does a battery last in a smoke detector?

A standard alkaline 9-volt or AA battery lasts about one year in a smoke detector. That is why you should replace them annually. If your detector chirps before a year is up, the battery may be defective or the detector may be drawing more power than normal.

Do I need to replace all my detectors on the same day?

It is not required, but it makes the job easier to remember. If you replace them all on the same date each year, you will not forget which ones are due. Write the replacement date on each detector with a permanent marker.

What should I do with old batteries?

Take used batteries to a recycling center or a retailer that accepts them — many hardware stores and electronics shops have battery recycling programs. Do not throw them in the trash. Check your local waste management website to find a drop-off location near you.