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

The battery your smoke detector needs depends on which type of detector you own. Hardwired detectors with battery backup almost always use 9-volt batteries. Battery-powered detectors (the kind you install without an electrician) typically use either AA or 9-volt batteries, though some older models use 10-year lithium batteries that cannot be replaced. Check the label on the front or back of your detector to see which size it takes—the manufacturer prints this clearly.

The 9-volt battery is the most common choice across both hardwired and battery-powered models because it fits the standard connector that comes built into most detectors. AA batteries are less common but appear in some compact or wireless models. If you are unsure, do not guess: opening the battery compartment and looking at the size of the slot, or reading the manual that came with the detector, takes 30 seconds and prevents you from buying the wrong battery.

Key Takeaways

  • Check the label on your smoke detector or open the battery compartment to see whether it takes a 9-volt or AA battery.
  • Hardwired detectors with battery backup almost always use 9-volt batteries.
  • Battery-powered detectors use either 9-volt or AA batteries, and some newer models use non-replaceable 10-year lithium batteries.
  • Replace smoke detector batteries at least once per year, or when the detector beeps to signal low battery.

How to find out what battery your detector takes

The fastest way is to look at the front or back of the detector itself. Most manufacturers print the battery type and size in small text on a label. If the label is worn or missing, open the battery compartment. On a 9-volt detector, you will see a rectangular slot with two metal connectors at the top. On an AA detector, you will see a cylindrical slot sized for a round battery.

If you cannot open the compartment or read the label, find the manual. The manual came in the box when you bought the detector, or you can search the manufacturer's website by model number. The model number is usually printed on the back or inside the battery compartment. Entering the model number and the word "manual" into a search engine will usually bring up a PDF you can read.

9-volt batteries: the standard for most detectors

The 9-volt battery is rectangular, about 1.5 inches tall and 0.6 inches wide, with a snap connector on top. It powers most hardwired detectors with battery backup and many standalone battery-powered models. A fresh 9-volt battery typically lasts six months to one year in a smoke detector, depending on the model and how often the detector tests itself.

When you buy a 9-volt battery, any standard brand works—Duracell, Energizer, store brands, and others all fit the same connector. The price varies slightly, but the difference is usually a few cents. Keep at least one spare 9-volt battery in your home so you can replace it when ready when the detector beeps.

AA batteries: less common but still in use

Some battery-powered smoke detectors use AA batteries instead of 9-volt. These are the same AA batteries you use in flashlights, remote controls, and other household devices. An AA battery is cylindrical, about 2.1 inches long, and smaller in diameter than a 9-volt. If your detector takes AA batteries, it usually takes two of them, though some models use only one.

AA batteries last about as long as 9-volt batteries in a smoke detector—typically six months to one year. The advantage is that AA batteries are cheaper and easier to find in most stores. If you have other devices in your home that use AA batteries, buying in bulk can save money.

10-year lithium batteries: non-replaceable and sealed

Some newer smoke detectors come with a 10-year lithium battery that is sealed inside and cannot be removed or replaced. These detectors are designed to last the full 10 years without any battery changes. When the 10 years are up, you throw away the entire detector and buy a new one.

If your detector has a 10-year lithium battery, the label will say so clearly. These detectors are more expensive upfront but eliminate the need to remember to change batteries. However, if the battery fails before 10 years, you cannot fix it—you must replace the whole unit. Some people prefer this trade-off; others prefer the flexibility of changeable batteries.

When and how to replace your smoke detector battery

Replace your smoke detector battery at least once per year. Many people do this on a set date—the first day of spring, or the day they change their clocks for daylight saving time. This makes it straightforward to remember. If your detector beeps or chirps, that is a signal that the battery is low and needs replacement when ready, even if you changed it recently.

To replace the battery, open the compartment on the front or back of the detector. Remove the old battery and note which way it was facing. Insert the new battery the same way—on a 9-volt, the snap connector goes on top; on an AA, the positive end (marked with a plus sign) usually goes toward the spring. Close the compartment and press the test button to confirm the detector works. You should hear a loud beep.

Common mistakes when buying smoke detector batteries

The most common mistake is buying the wrong size. A 9-volt battery will not fit in an AA slot, and an AA battery will not fit in a 9-volt connector. Before you leave the store or order online, double-check that the battery size matches what your detector takes.

Another mistake is buying old stock. Batteries have a manufacturing date printed on them. If a battery has been sitting on a shelf for years, it may have lost charge. Buy from stores with high turnover, and check the date if you can. A battery manufactured within the last year is a safe choice.

A third mistake is installing the battery backward. On a 9-volt, the snap connector only fits one way, so this is hard to do. On an AA battery, check that the positive end (the flat end with a plus sign) faces the spring inside the compartment. If the battery is backward, the detector will not work, and you may not realize it until there is a fire.

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, and a low battery in a smoke detector is a safety issue. Use alkaline 9-volt or AA batteries, which hold their charge longer and are designed for this use.

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

Take old batteries to a recycling center or a store that accepts batteries for recycling—many hardware stores and electronics retailers have collection bins. Do not throw batteries in the trash. Batteries contain materials that can harm the environment if they end up in a landfill.

Why does my smoke detector keep beeping even after I put in a new battery?

The beeping may continue for a few minutes after you install a new battery. If it keeps beeping after 10 minutes, the battery may be installed backward or not fully seated in the compartment. Open the detector again, remove the battery, and reinstall it, making sure it clicks or snaps into place. If it still beeps, the detector itself may be faulty and need replacement.

Do all smoke detectors use the same battery?

No. Hardwired detectors use 9-volt batteries, many battery-powered detectors use 9-volt or AA, and some newer models use sealed 10-year lithium batteries. Always check your specific detector's label or manual to know which battery to buy.

How long do smoke detector batteries actually last?

A fresh alkaline 9-volt or AA battery typically lasts six months to one year in a smoke detector. The exact time depends on how often the detector runs its self-test and the brand of battery. Replace the battery once per year as a routine, or sooner if the detector beeps to signal low battery.