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

The battery your smoke detector needs depends on which type you own. The two most common are 9-volt batteries (the rectangular kind with terminals on top) and AA batteries (the cylindrical size you find in remote controls). Some detectors use two AA batteries instead of one 9-volt. A few older or specialty models use 10-year sealed batteries that cannot be replaced, though these are less common in homes built after 2010.

Check the label on the back or bottom of your detector to see which battery it takes. The label will either say "9V" or "AA" or show a picture of the battery shape. If you cannot find the label, open the detector's battery compartment — the shape of the slot will tell you when ready.

Buying the wrong size is a common mistake. A 9-volt battery will not fit into an AA slot, and an AA battery will not work in a 9-volt detector. Keep a spare of the correct size in a drawer near your detectors so you can replace it as soon as the alarm chirps.

Key Takeaways

  • Most household smoke detectors use either one 9-volt battery or two AA batteries, and the correct size is printed on a label inside the detector.
  • A 9-volt battery is rectangular with two metal terminals on top, while AA batteries are cylindrical and smaller.
  • Buying the wrong battery size means the detector will not work, so check your model before you shop.
  • Replace the battery as soon as you hear a chirping sound, which signals low power — do not wait until the alarm stops working.
  • Some newer detectors have sealed 10-year batteries built in and do not take replaceable batteries at all.

How to identify which battery your detector uses

The fastest way is to look at the back or bottom of the detector for a label or sticker. Manufacturers print the battery type there, usually in large text. It will say "9V" or "AA" or show a picture of the battery itself.

If the label is worn off or missing, open the battery compartment. On most detectors, this is a small door or panel that slides or twists open. Look at the shape of the slot inside. A 9-volt slot is rectangular and has two metal contacts. An AA slot is cylindrical and narrower. If you see two slots side by side, your detector takes two AA batteries.

If you still cannot tell, take a photo of the detector model number (usually on the back) and search it online, or bring the detector to a hardware store and show it to staff. They can tell you in seconds.

9-volt batteries versus AA batteries

A 9-volt battery is larger and rectangular, about the size of a lighter. It has two metal terminals (one marked + and one marked −) on the top. One 9-volt battery powers most hardwired detectors that have a battery backup, and many wireless or battery-only detectors. A 9-volt battery typically lasts six months to a year before the detector chirps to signal low power.

An AA battery is smaller and cylindrical, the same size as the batteries in a flashlight or game controller. Some detectors use one AA, but most that take AA batteries use two. AA batteries last longer than 9-volts in smoke detectors — often a year or more — because the detector draws less power from two smaller batteries than from one larger one.

Neither type is "better." The battery your detector takes is determined by its design. What matters is replacing it on schedule. A dead battery means your detector cannot alert you to smoke, so treat the chirp as urgent.

When to replace your smoke detector battery

Replace the battery as soon as you hear a chirping sound coming from the detector. This chirp happens every 30 to 60 seconds and means the battery voltage is too low. It is the detector's way of telling you it will not work if a fire starts.

Do not ignore the chirp or assume it will go away. The detector will keep chirping until you replace the battery. Some people remove the battery to stop the noise, which leaves the detector powerless. Instead, buy the correct replacement battery and install it when ready.

If your detector has a test button (most do), press it once a month to make sure the alarm sounds. If it does not, replace the battery even if it is not chirping yet. A silent detector is as dangerous as no detector at all.

Sealed 10-year batteries in newer detectors

Many smoke detectors sold since 2010 come with a sealed 10-year battery built inside. These batteries cannot be removed or replaced by the user. Instead, you throw away the entire detector after 10 years and buy a new one.

If your detector has a sealed battery, there will be no battery compartment to open. The back or bottom will say something like "10-year battery" or "non-replaceable battery." These detectors are convenient because you do not have to remember to buy batteries, but they cost more upfront than a detector with replaceable batteries.

Even with a sealed battery, the detector may chirp after several years if the battery begins to fail early. If this happens, replace the entire unit. Do not try to open it or remove the battery yourself.

Where to buy replacement batteries

9-volt and AA batteries are sold at every grocery store, pharmacy, hardware store, and online retailer. They are inexpensive — usually between one and three dollars each. Buy them wherever you shop for other household items.

Keep a spare battery in a drawer near your smoke detectors. When the detector chirps, you can replace it when ready instead of running to the store. This is especially important if you have detectors in multiple rooms.

Avoid buying batteries in bulk unless you use them regularly. Batteries lose charge over time, even when not in use. A battery sitting in a drawer for two years may not last as long as a fresh one.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke detector?

Rechargeable batteries are not recommended for smoke detectors. They hold a lower voltage than disposable batteries and may not trigger the alarm reliably. Stick with alkaline 9-volt or AA batteries, which are designed for the job and cost very little.

What if I put the wrong size battery in by mistake?

The detector will not work. A 9-volt battery will not fit into an AA slot, and an AA battery will not make contact in a 9-volt detector. Remove the wrong battery and install the correct size. If you forced a battery in and it is stuck, do not pull hard — take the detector to a hardware store and ask staff to help remove it.

How do I know if the battery is dead or if the detector itself is broken?

Replace the battery first. If the detector stops chirping and the test button produces a loud alarm, the battery was the problem. If it still does not work after a fresh battery, the detector itself may be faulty and should be replaced.

Do I need to turn off the power before replacing the battery in a hardwired detector?

No. Hardwired detectors have a battery backup that works independently of the electrical connection. You can replace the battery without turning off the power. Just open the compartment, remove the old battery, and insert the new one.

Can I use an expired battery if it is still in the package?

Batteries do not have a true expiration date, but they lose charge over time. A battery that expired years ago may not last as long as a fresh one. For a smoke detector, buy new batteries. The cost is low and the safety margin is worth it.