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

The battery your smoke detector needs depends on which type of detector you own. The two most common are 9-volt batteries (the rectangular ones with terminals on top) and AA batteries (the cylindrical ones). Some detectors use both — one 9-volt as the main power source and AA batteries as backup. A few older or specialty models use 10-year sealed lithium batteries that cannot be replaced. Check the label on the back or inside the battery compartment of your detector to see which one yours takes.

The label will usually show a picture of the battery or print the size clearly. If you cannot find the label, open the battery compartment — the shape of the slot will tell you what fits. A rectangular slot with two metal terminals means 9-volt. A cylindrical slot means AA (or sometimes AAA, though AA is far more common in smoke detectors).

Key Takeaways

  • Check the label inside your smoke detector's battery compartment to find out whether it takes 9-volt, AA, or a sealed 10-year battery.
  • 9-volt batteries are rectangular with two terminals on top; AA batteries are cylindrical and smaller than 9-volt.
  • Some detectors use two AA batteries instead of one 9-volt, so count the slots in the compartment before you buy.
  • Replace batteries when the detector chirps or beeps — that sound means the battery is low, not that the detector is broken.
  • Sealed 10-year lithium batteries cannot be replaced; when they run out, you replace the entire detector.

How to identify which battery your detector uses

Open the battery compartment on the back or bottom of your detector. Most detectors have a small door or cover that slides or pops off. Inside, you will see either a rectangular slot with two metal prongs (9-volt) or one or more cylindrical slots (AA or AAA).

If your detector is hard-wired to your home's electrical system, it may still have a battery compartment for backup power during outages. These backup batteries are usually 9-volt or AA. Check inside that compartment the same way.

If you see no battery compartment at all and the detector is wired to your house, it may have a sealed 10-year lithium battery built in. These detectors cannot be opened and the battery cannot be replaced. When the battery dies (usually after ten years), you throw away the entire detector and install a new one.

9-volt batteries versus AA batteries

9-volt batteries are rectangular, about 1.5 inches tall, with a snap connector on top. They are common in older smoke detectors and in many battery-powered models sold today. One 9-volt battery powers the detector until it runs low, then the detector beeps to tell you to replace it.

AA batteries are cylindrical and smaller — about the size of your pinky finger. Some detectors use one AA, others use two. If your detector has two slots, you will need two AA batteries. AA batteries last longer than 9-volt batteries in most smoke detectors, so if your detector takes AA, you may not need to replace them as often.

Both types cost between one and three dollars each at any grocery store, pharmacy, or hardware store. Buy alkaline batteries (the standard kind), not rechargeable ones. Rechargeable batteries can fail without warning in a smoke detector, and that risk is not worth the small savings.

What to do when your detector beeps or chirps

A beeping or chirping sound from your smoke detector means the battery is low. This is the detector working correctly — it is telling you to replace the battery. Do not ignore it or assume the detector is broken.

Open the battery compartment, remove the old battery, and insert a new one of the same type and size. Make sure the battery is seated firmly in the slot. Close the compartment. The beeping should stop within a few seconds. If it does not, the battery may not be making good contact — open the compartment again and reseat it.

If the detector continues to beep after you have replaced the battery and reseated it, the detector itself may be faulty. Most detectors last five to ten years. If yours is older than that and still beeping after a fresh battery, consider replacing the entire unit.

Sealed 10-year lithium batteries and when to replace your detector

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

These detectors have a small light or label that shows the manufacture date. Ten years from that date, the battery will run out. The detector will not beep — it will straightforward stop working. Mark the date on your calendar or set a phone reminder so you remember to replace the detector before the battery dies.

The advantage of a sealed battery is that you never have to buy replacement batteries or remember to change them. The disadvantage is that you cannot use the detector beyond ten years, even if it still seems to work fine. If you prefer the option to replace batteries and keep a detector longer, buy a model that takes 9-volt or AA batteries instead.

Where to buy replacement batteries

Alkaline 9-volt and AA batteries are sold everywhere: grocery stores, pharmacies, hardware stores, big-box retailers, and online. You do not need a special brand — any alkaline 9-volt or AA battery will work. Store brands are usually cheaper than name brands and perform the same way.

Buy a few extra batteries and keep them in a drawer at home. That way, when your detector beeps at night or on a weekend, you have a replacement on hand and do not have to run out to a store. Batteries last for years before they expire, so there is no harm in keeping spares.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke detector?

No. Rechargeable batteries can lose charge without warning, and a dead battery in a smoke detector defeats the purpose. Use alkaline batteries only — they are cheap and reliable, and the small extra cost is worth the safety.

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

The battery will not fit into the slot, or it will fit but make poor contact and the detector will not work. Remove it and use the correct size. If you are unsure which size you need, take a photo of the battery compartment and bring it to a hardware store — staff can tell you what fits.

Does it matter what brand of battery I buy?

No. Any alkaline 9-volt or AA battery will work. Store brands are usually cheaper and perform just as well as name brands. Buy whatever is on sale.

How often should I replace the batteries?

Replace them when the detector beeps or chirps. That sound means the battery is low. Most 9-volt batteries last six months to a year in a smoke detector; AA batteries often last longer. If your detector has not beeped in over a year, the battery is probably still good.

What should I do with the old battery?

Take it to a battery recycling program. Many retailers like Best Buy and Home Depot accept used batteries for free. You can also search for a local recycling center on Earth911.com or call your city's waste management office to find out where to drop them off.