Most smoke detectors use 9-volt or AA batteries, depending on the model

The battery your smoke detector needs depends on which detector you own. The two most common sizes are 9-volt batteries (the rectangular ones) and AA batteries (the cylindrical ones). Some detectors use two AA batteries instead of one 9-volt. A few older or specialty models use 10-year sealed batteries that you cannot replace—those detectors themselves get thrown away when the battery dies.

Check the label on the back or inside the detector cover to see which type yours takes. The label will say "9V" or "AA" or sometimes show a picture of the battery. If you have lost the label, open the battery compartment and look at the size and shape of the battery already inside, or measure the space where the battery goes.

Buy alkaline batteries, not rechargeable ones. Alkaline batteries last longer in smoke detectors and are cheaper. Rechargeable batteries can lose charge while sitting unused, which defeats the purpose of a backup power source.

Key Takeaways

  • Check the label on your detector or look inside the battery compartment to find out whether it takes a 9-volt or AA battery.
  • Always use alkaline batteries, not rechargeable batteries, because they hold their charge longer when the detector is not actively alarming.
  • Replace batteries once a year, or when the detector beeps to warn you the battery is low—do not wait until it stops working.
  • If your detector has a 10-year sealed battery and it dies, you replace the entire detector, not just the battery.

How to identify which battery your detector uses

The fastest way is to look at the detector itself. Open the battery compartment cover—usually a plastic door on the front or side that slides or twists open. You will see either a single 9-volt battery (shaped like a small rectangular block with two metal terminals on top) or one or two AA batteries (shaped like a cylinder, about the size of your pinky finger).

If you cannot open the compartment or the battery is already dead, check the back or bottom of the detector for a label. It will list the battery type and sometimes the brand recommended. If there is no label and you cannot open it, measure the battery 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.

9-volt batteries versus AA batteries

A 9-volt battery is a single rectangular battery with two metal snap terminals on the top. It fits into a clip or connector inside the detector. Most hardwired detectors (the ones plugged into your ceiling) have a 9-volt backup battery. Many battery-only detectors also use 9-volts. A 9-volt battery costs between $3 and $8 depending on the brand.

An AA battery is a standard cylindrical battery. Some detectors use one AA, others use two AA batteries placed end-to-end in a holder. AA batteries are cheaper than 9-volts—usually $1 to $3 each—and easier to find in any store. If your detector uses two AA batteries, replace both at the same time, even if only one is dead.

Both types last roughly one year in a working smoke detector. The actual lifespan depends on how often the detector tests itself and how sensitive it is set. Some detectors test more frequently than others, which drains the battery faster.

When to replace your smoke detector battery

Replace the battery once a year, whether it seems dead or not. A good habit is to change it on a specific date—many people do it when they change their clocks for daylight saving time in the spring or fall. Mark it on your calendar so you do not forget.

Your detector will also tell you when the battery is low. Most detectors beep or chirp once every 30 to 60 seconds when the battery voltage drops. This warning beep is annoying on purpose—it means you should replace the battery within a few days. Do not ignore it and hope it goes away. A detector with a dead battery is not protecting your home.

If your detector is hardwired to your home's electrical system, it has a 9-volt backup battery. Replace that backup battery once a year even though the detector is powered by your house current. The backup battery only works if the power goes out, so you might not notice it has died until you need it.

10-year sealed batteries and when to replace the whole detector

Some newer smoke detectors come with a 10-year sealed lithium battery built inside. You cannot open the compartment or replace this battery yourself. The detector is designed to work for 10 years, then the entire unit gets thrown away and replaced.

These detectors are convenient because you do not have to remember to change batteries. However, they cost more upfront—usually $25 to $50 per detector instead of $10 to $20. If you move or want to replace the detector before 10 years, you cannot salvage the battery.

When the 10-year battery dies, the detector will beep or stop responding to test buttons. At that point, remove the detector from your ceiling or wall and replace it with a new one. Do not try to pry it open or replace the battery yourself—the detector is sealed for safety reasons.

Where to buy replacement batteries

You can buy 9-volt and AA alkaline batteries at any grocery store, pharmacy, hardware store, or online retailer. They are inexpensive and stocked everywhere. Buy a name brand like Duracell or Energizer if you want the longest lifespan, or buy a store brand if you want to save money—both work fine in smoke detectors.

Keep spare batteries in a drawer near your detectors so you can replace them quickly when the warning beep starts. Some people keep a battery in each detector's compartment plus one extra, so they always have a replacement ready.

Frequently Asked Questions

Can I use rechargeable batteries in my smoke detector?

No. Rechargeable batteries lose their charge over time even when not in use, and a smoke detector sits idle for months. A rechargeable battery could be dead when you need it most. Alkaline batteries hold their charge much longer in standby mode, which is why they are the right choice for smoke detectors.

What happens if I put the wrong size battery in my detector?

The battery will not fit, or it will fit loosely and not make proper contact with the terminals. The detector will not power on, or it will power on and when ready show a low-battery warning. Always use the size your detector is designed for. Check the label if you are unsure.

Do I need to replace batteries in a hardwired detector?

Yes. Hardwired detectors are powered by your home's electrical system, but they have a 9-volt backup battery for when the power goes out. Replace that backup battery once a year so it works if you need it. If the power fails and the backup battery is dead, the detector will not alarm.

How do I know if the battery is installed correctly?

After you insert the battery, close the compartment cover and press the test button on the front of the detector. It should beep loudly for a few seconds. If it does not beep or beeps very quietly, the battery may not be making good contact. Open the compartment and check that the battery terminals are clean and the battery is seated firmly in its holder.

Can I use a 9-volt battery in a detector that takes AA batteries?

No. A 9-volt battery is too large and will not fit in an AA compartment. Even if you forced it in, it would damage the detector and create a safety hazard. Always use the exact battery type your detector is designed for.