Most smoke detectors use 9-volt or AA batteries

The battery your smoke detector takes depends on the model you own. The two most common sizes are 9-volt (9V) batteries and AA batteries. Some older or specialty detectors use 10-year lithium batteries that cannot be replaced. Check the label on the back or inside the battery compartment of your detector to see which type yours requires — the model number and battery type are printed there.

If you have lost the detector or cannot read the label, open the battery compartment and look at the shape and size of the slot. A 9V battery is rectangular and about the size of a lighter. An AA battery is cylindrical and roughly the length of your thumb. If you see a sealed compartment with no removable battery, your detector has a built-in 10-year battery and the entire unit should be replaced when the battery dies.

Key Takeaways

  • The battery type is printed on a label inside your smoke detector's battery compartment or on the back of the unit.
  • 9-volt batteries are rectangular and fit into a clip-style holder; AA batteries are cylindrical and fit into a spring-loaded slot.
  • Some detectors have sealed 10-year lithium batteries that cannot be replaced, and the whole detector must be discarded when the battery dies.
  • Hardwired detectors with battery backup typically use 9V batteries, while battery-only detectors use either 9V or AA depending on the model.

How to identify which battery your detector uses

Open the battery compartment on your smoke detector. Most detectors have a removable cover on the back or bottom held by a single screw or a clip. Inside, you will see either a rectangular slot with two metal prongs (9V) or a cylindrical slot with two springs (AA). The battery type should also be printed on the label inside the compartment or on the detector itself.

If the compartment is sealed and you cannot open it, your detector has a non-replaceable 10-year lithium battery. These detectors are designed to be thrown away and replaced after 10 years. The date of manufacture is usually printed on the back; add 10 years to find when replacement is due.

9-volt batteries in hardwired and plug-in detectors

Hardwired smoke detectors — those connected to your home's electrical system — almost always have a 9V battery as a backup. This battery powers the detector if the electricity goes out. The 9V battery sits in a clip-style holder and should be tested monthly and replaced once per year, even if the detector does not beep to signal a low battery.

Plug-in detectors that go into a wall outlet also typically use 9V backup batteries. The battery compartment is usually on the back or bottom of the unit. Replace the battery annually or whenever the detector beeps three times in a row, which signals the battery is low.

AA batteries in battery-only detectors

Battery-powered smoke detectors that do not plug in or connect to your home's wiring often use AA batteries instead of 9V. These detectors are common in rental properties, basements, or rooms far from electrical outlets. AA batteries last about one year in a smoke detector, so plan to replace them annually or when you hear the low-battery warning beep.

Some battery-only detectors use two AA batteries instead of one. Check inside the compartment to count how many slots are present. If there are two slots, you will need to replace both batteries at the same time, even if only one is low.

10-year sealed lithium batteries

Many newer smoke detectors come with a sealed 10-year lithium battery that cannot be removed or replaced. These batteries are built into the detector and designed to last the full 10-year lifespan of the unit. When the battery dies, the entire detector must be replaced.

Detectors with 10-year batteries are convenient because you do not have to remember to replace the battery every year. However, they cost more upfront than detectors with replaceable batteries. The manufacture date is printed on the back of the detector; when that date plus 10 years arrives, replace the whole unit. Some detectors will beep once per minute for 30 days before the battery fully dies, giving you time to buy a replacement.

Where to buy replacement batteries

9V and AA batteries are sold at any grocery store, pharmacy, hardware store, or online retailer. A single 9V battery costs between $3 and $8 depending on the brand and whether you buy a single battery or a pack. AA batteries cost $1 to $3 each. Alkaline batteries (the standard type) work fine for smoke detectors; you do not need to buy expensive rechargeable or specialty batteries.

Buy batteries in bulk if you have multiple detectors in your home. A pack of four AA batteries or two 9V batteries is usually cheaper per battery than buying them one at a time. Store extra batteries in a cool, dry place — not in a hot garage or damp basement — so they last longer before use.

Testing and replacing your battery

Test your smoke detector's battery once per month by pressing and holding the test button on the front of the detector for three to five seconds. You should hear a loud alarm. If the detector does not sound or sounds very quiet, the battery is low and should be replaced when ready.

To replace the battery, open the compartment, remove the old battery, and insert the new one in the same direction as the old one was facing. For 9V batteries, the prongs on the battery should slide into the clip. For AA batteries, the positive end (marked with a +) should face the spring. Close the compartment and press the test button again to confirm the detector works. Write the replacement date on the detector with a permanent marker so you remember when to replace it next year.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke detector?

Rechargeable batteries are not recommended for smoke detectors because they lose charge faster than alkaline batteries and may not power the alarm during an emergency. Stick with standard alkaline 9V or AA batteries from a store. They are inexpensive and reliable.

What should I do if my detector beeps but I just replaced the battery?

Make sure the battery is inserted correctly — the positive end should face the spring or clip. If it is installed correctly and the detector still beeps, try a different new battery in case the first one was defective. If a second battery does not stop the beeping, the detector itself may be faulty and should be replaced.

Do hardwired detectors need battery replacement if they are plugged in?

Yes. The 9V backup battery in a hardwired detector should be replaced once per year even though the detector is powered by electricity. The battery only works if the power goes out, so you need to keep it fresh and ready. Test it monthly and replace it annually.

How do I know if my detector has a 10-year battery?

Look for the words "10-year battery" or "sealed battery" printed on the back or packaging of the detector. If you can open the battery compartment and see a removable battery, it is not a 10-year battery. If the compartment is sealed shut and cannot be opened, it has a non-replaceable 10-year lithium battery.

Can I use a 9V battery in a detector that calls for AA batteries?

No. A 9V battery will not fit into an AA slot and forcing it could damage the detector. Always use the exact battery type specified on your detector's label. If you are unsure, measure the compartment or check the model number online.