Most smoke detectors use 9-volt batteries, but some use AA or AAA

The battery size depends on the model you own. The most common type is a 9-volt battery (often called a 9V), which is rectangular and about the size of a lighter. Some detectors use AA batteries (two or four of them), and a smaller number use AAA batteries. A few hardwired models have a backup battery that is also 9V.

The easiest way to find out which one you need is to open the battery compartment on your detector and look at the empty slot or the old battery. The shape and label will tell you when ready. If you cannot open it or the label is worn off, check the model number on the back or bottom of the detector and search the manufacturer's website or the product manual.

You can also call the manufacturer's customer service line — the number is usually printed on the detector itself or on the packaging if you still have it. They will confirm the battery type in under a minute.

Key Takeaways

  • The battery compartment on your detector shows you exactly what size to buy — open it and look at the slot or the old battery.
  • 9-volt batteries are the most common type in smoke detectors, but AA and AAA batteries are also used depending on the model.
  • If you cannot open the compartment, the model number on the back of the detector will tell you the battery type when you search the manufacturer's website.
  • Replace batteries once a year, or when ready if the detector chirps — that chirp means the battery is low.

How to identify your battery type without opening the detector

If the battery compartment is stuck or you do not want to open it yet, look at the back or bottom of your detector for a model number. It usually starts with letters and is followed by numbers — for example, "SD501" or "HDC-200." Write this number down.

Go to the manufacturer's website (common brands are First Alert, Kidde, and BRK) and search for that model number. The product page will list the battery type in the specifications section. If you cannot find it online, call the manufacturer's customer service number, which is often on the detector itself or in the product manual if you have it.

This method takes a few minutes but saves you from guessing at the store or buying the wrong size.

Where to buy the right battery

Once you know the size, you can buy the battery at any drugstore, grocery store, hardware store, or online retailer. 9-volt batteries are sold everywhere and cost between $3 and $8 depending on the brand and whether you buy a single battery or a pack. AA and AAA batteries are even cheaper and more widely stocked.

Avoid buying the absolute cheapest option if you can — mid-range brands like Duracell or Energizer tend to last longer in smoke detectors than dollar-store batteries. A smoke detector battery that dies early is worse than one that costs a dollar more, because you might not notice it has failed until you test the detector.

When to replace the battery

Replace the battery once a year, ideally on a date you will remember — many people change them when they set their clocks forward or back for daylight saving time. Mark it on your calendar or set a phone reminder.

If your detector chirps or beeps once every 30 to 60 seconds, the battery is low and needs to be replaced when ready, even if you just changed it. That chirp is the detector telling you the power is running out. Do not ignore it — a detector with a dead battery cannot alert you to smoke.

Some detectors have a test button on the front. Press it for a few seconds once a month to make sure the alarm works. If it does not sound, the battery may be dead or the detector itself may have failed.

Hardwired detectors and backup batteries

If your detector is hardwired into your home's electrical system (common in newer homes and apartments), it will have a backup battery that activates if the power goes out. This backup is almost always a 9-volt battery, even if the detector itself runs on house current.

Replace the backup battery once a year, just like you would a battery-only detector. If the detector chirps, it is telling you the backup battery is low. Do not assume the house power is fine — the backup battery still needs to work in case of a power outage or fire that cuts the power.

What to do with the old battery

Do not throw used batteries in the trash. Most communities have a battery recycling program at the local waste facility, hardware store, or electronics retailer. Call your city or county waste management office to find the nearest drop-off location, or search "battery recycling near me" online.

Some retailers like Best Buy and Home Depot accept batteries for free recycling. Bring your old battery in when you buy the new one and ask if they can take it.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke detector?

Rechargeable batteries are not recommended for smoke detectors. They lose charge over time even when not in use, and a detector with a weak battery might not sound the alarm when you need it. Stick with standard alkaline batteries (Duracell, Energizer, or similar brands).

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

The battery will not fit into the compartment, or it will fit but make poor contact and the detector will not work. Take it out and put in the correct size. If you forced it in and it is stuck, do not pry it — call the manufacturer or take the detector to a hardware store for help removing it.

Do I need to replace all the batteries at the same time if my detector uses multiple?

Yes. If your detector uses two or four AA or AAA batteries, replace all of them at once. If you replace only one, the others may be weaker and the detector could fail sooner than expected.

How long does a battery last in a smoke detector?

Most alkaline batteries last about one year in a smoke detector, which is why manufacturers recommend annual replacement. The exact time depends on the battery brand, the detector model, and how often the detector is tested.

Why does my detector keep chirping even after I replaced the battery?

Make sure the new battery is inserted correctly — the positive and negative ends should match the markings inside the compartment. If it is correct and still chirps, the detector itself may be faulty and need to be replaced. Detectors typically last 8 to 10 years before they stop working.