Most smoke detectors use 9-volt or AA batteries

The battery your smoke detector needs depends on the model you own. The two most common types are 9-volt batteries (the rectangular ones with terminals on top) and AA batteries (the standard cylindrical size). Some older or specialty detectors use 10-year sealed lithium batteries that you cannot replace—those units are designed to be thrown away and replaced entirely when the battery dies.

Check the label on your detector's front or back to see which type it takes. The label usually shows a battery symbol or lists the battery size directly. If you cannot find a label, open the battery compartment—the shape of the slot will tell you whether it holds a 9V, AA, or another size.

Hardwired smoke detectors (the kind connected to your home's electrical system) often have a 9-volt backup battery in case the power goes out. That backup battery is the one you will replace when the low-battery chirp starts.

Key Takeaways

  • Check your detector's label or battery compartment to find out whether it takes a 9-volt or AA battery before you buy replacements.
  • Replace the battery as soon as you hear the low-battery chirp—that sound means the detector cannot alert you to smoke.
  • Some detectors use 10-year sealed batteries that cannot be replaced; when those batteries die, you replace the entire unit.
  • Test your detector after installing a new battery by pressing the test button for three to five seconds.

How to replace a 9-volt battery

Open the battery compartment on the front or back of your detector. You will see a rectangular slot with two metal terminals (one marked + and one marked −). Gently pull the old battery straight out—it may be held in by a clip, so do not force it.

Insert the new 9-volt battery by aligning the terminals with the clip and sliding it in until you hear or feel a click. The positive terminal (marked +) should face toward the + marking in the compartment. Close the battery door and press the test button to confirm the detector sounds.

How to replace an AA battery

Open the battery compartment, which is usually a plastic door or panel on the front of the detector. You will see a cylindrical slot with a spring at one end. Remove the old AA battery by gently pushing it toward the spring—the spring will push it out.

Insert the new AA battery with the positive end (marked +) facing away from the spring. The spring should hold it snugly in place. Close the compartment and test the detector by holding down the test button for three to five seconds until the alarm sounds.

When to replace your smoke detector batteries

Replace the battery when ready when you hear a chirping sound every 30 to 60 seconds. That chirp is the detector's way of telling you the battery is too weak to power the alarm if smoke is detected. Do not ignore it or cover the detector—a silent detector cannot protect you.

Even if your detector is not chirping, replace batteries once a year. A good habit is to change them when you change your clocks for daylight saving time (spring forward, fall back). Mark it on your calendar so you do not forget.

If your detector chirps constantly even after you install a new battery, the detector itself may be faulty. Replace the entire unit rather than troubleshooting further.

What type of battery to buy

Buy alkaline batteries, not rechargeable ones. Alkaline 9-volt and AA batteries are inexpensive and widely available at grocery stores, pharmacies, and hardware stores. They hold their charge longer than rechargeable batteries and are more reliable in a device that needs to work at any time.

You do not need premium or "heavy-duty" batteries for smoke detectors. Standard alkaline batteries work fine and cost less. Buy a pack of extras so you have replacements on hand when the chirp starts.

Hardwired detectors with battery backup

If your detector is wired into your home's electrical system, it will have a 9-volt backup battery inside. When the power goes out, that battery keeps the detector working. When the power comes back on, the detector switches back to electrical power and the battery rests.

Replace the backup battery once a year, even if it has not chirped. The battery may be hidden behind a plastic cover or inside the detector body. Check your detector's manual for the exact location, or look for a small compartment on the back or side of the unit.

If your hardwired detector chirps and you replace the backup battery but it keeps chirping, the problem may be with the electrical connection rather than the battery. Turn the breaker off for 30 seconds, then turn it back on. If the chirping continues, call an electrician.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke detector?

Rechargeable batteries are not recommended for smoke detectors. They lose their charge faster than alkaline batteries and may not have enough power when you need the detector to work. Stick with standard alkaline 9-volt or AA batteries.

What should I do with the old battery?

Take old batteries to a recycling center or a store that accepts battery recycling—many hardware stores and electronics retailers have collection bins near the entrance. Do not throw batteries in the trash.

Why does my detector keep chirping after I put in a new battery?

Make sure the battery is inserted correctly and fully seated in the compartment. If it is, the detector itself may be failing. Detectors typically last 8 to 10 years; if yours is older, replace the entire unit rather than the battery.

Do all smoke detectors use the same battery?

No. Most use 9-volt or AA batteries, but some use 10-year sealed lithium batteries or other types. Always check your detector's label before buying batteries. If you have multiple detectors, they may not all take the same size.

How long does a smoke detector battery last?

Alkaline batteries in smoke detectors typically last 6 to 12 months, depending on the detector model and how often it runs its self-test. Replace them yearly to be safe, regardless of whether they have chirped.