Most fire alarms use 9-volt or AA batteries, depending on the model

The battery your fire alarm needs depends on which type of detector you own. Hardwired smoke and carbon monoxide detectors usually have a 9-volt battery as a backup when the power goes out. Battery-only models — the kind you mount on a ceiling or wall without wiring — typically use either one 9-volt battery or two AA batteries. Check the label on the front or back of your detector to see which one yours takes. The label will say "9V" or "AA" clearly.

Some newer models use 10-year lithium batteries that cannot be replaced; you throw away the whole unit when the battery dies. These are sealed units and cost more upfront but save you from remembering to change batteries every year. If your detector has a label saying "non-replaceable battery" or "10-year battery," this is the type you have.

The battery compartment is usually on the back or bottom of the unit. Open it by sliding or unclipping the cover. If you see a single battery slot, it takes a 9-volt. If you see two slots stacked vertically, it takes AA batteries.

Key Takeaways

  • Hardwired detectors with battery backup use 9-volt batteries; battery-only models use either one 9-volt or two AA batteries.
  • Check the label on your detector to confirm which type it needs before you buy replacements.
  • Some newer models have sealed 10-year lithium batteries that cannot be replaced, so you discard the entire unit when the battery dies.
  • Replace batteries once a year, or when the detector beeps to signal low battery, whichever comes first.
  • Test your detector monthly by pressing the test button to make sure the battery is working and the alarm sounds.

How to identify which battery your detector uses

The easiest way is to look at the back or bottom of the unit. Most manufacturers print the battery type right on the label. You may see "9V Battery Required" or "2 AA Batteries Required." If the label is worn or missing, open the battery compartment and look at the shape of the slot. A 9-volt battery is rectangular and wider at the top. AA batteries are cylindrical and smaller.

If you still cannot tell, write down the brand and model number (usually on the front) and search the manufacturer's website. Kidde, First Alert, and Smoke Guard all have product pages that list battery requirements. You can also call the manufacturer's customer service line, which is usually printed on the back of the unit.

9-volt batteries versus AA batteries

A 9-volt battery is a single rectangular unit with two terminals on top. It costs between $3 and $8 per battery. AA batteries are smaller cylinders that come in packs of two, four, or eight, and cost roughly $0.50 to $1.50 each. If your detector takes two AA batteries, you are buying two separate units.

9-volt batteries last about one year in a fire alarm. AA batteries also last about one year. The difference is convenience: if your detector takes a 9-volt, you replace one battery. If it takes two AA batteries, you replace two. Some people prefer AA because they are cheaper and easier to find in bulk. Others prefer 9-volt because it is one swap instead of two.

Alkaline batteries (the standard kind you buy at any store) work fine for fire alarms. Do not use rechargeable batteries in a fire alarm. Rechargeable batteries lose charge over time even when not in use, and a dead battery in a fire alarm defeats the purpose of having one.

When to replace fire alarm batteries

Replace your batteries once a year, even if the detector has not beeped. A good habit is to change them when you change your clocks for daylight saving time — spring forward in March, fall back in November. This gives you two reminder dates per year and keeps the batteries fresh.

If your detector beeps or chirps once every 30 to 60 seconds, the battery is low and needs to be replaced right away. This beep is the detector telling you the battery voltage has dropped below the safe level. Do not ignore it. Replace the battery within a day or two.

After you install a new battery, press the test button on the front of the detector. Hold it for three to five seconds. You should hear a loud alarm sound. If you hear nothing, the battery may not be installed correctly, or the detector itself may be faulty. Remove the battery, wait 30 seconds, and reinstall it. Test again. If it still does not sound, the detector may need to be replaced.

Hardwired detectors with battery backup

If your detector is wired into your home's electrical system, it runs on house power most of the time. The 9-volt battery is a backup that kicks in if the power goes out. You still need to replace this battery once a year, because it sits unused until an outage happens. A battery that has been sitting for two years may be too weak to power the alarm when you need it.

Hardwired detectors usually have a small red or green light on the front that stays on when power is flowing. If that light goes out, check whether the circuit breaker has tripped. If the breaker is fine but the light is still off, the detector may have a loose wire or a faulty connection. Call an electrician or the fire department's non-emergency line for help.

Where to buy replacement batteries

You can buy 9-volt and AA batteries at any grocery store, pharmacy, hardware store, or big-box retailer. Brands like Duracell, Energizer, and store-brand alkaline batteries all work equally well in fire alarms. Prices vary slightly by brand and retailer, but the difference is usually less than a dollar per battery.

Buy batteries in bulk if you have multiple detectors. A pack of eight AA batteries costs less per battery than buying two at a time. Keep a spare 9-volt or pack of AA batteries in a drawer so you have them on hand when the detector beeps. This prevents the situation where you need to replace a battery but have to leave the house to buy one.

Frequently Asked Questions

Can I use a different battery type than what the manual says?

No. Use only the battery type specified on your detector's label or manual. Using the wrong type can damage the detector or cause it to fail when you need it. A 9-volt detector will not work with AA batteries, and vice versa.

How long do fire alarm batteries last?

Alkaline batteries in a fire alarm last about one year. Replace them once yearly as a routine, or sooner if the detector beeps to signal low battery. Lithium 10-year batteries last ten years before the entire detector must be replaced.

What does it mean when my fire alarm beeps every 30 seconds?

A regular beep or chirp every 30 to 60 seconds means the battery is low and needs to be replaced. This is the detector's way of alerting you before the battery dies completely. Replace it within a day or two. If the beeping continues after you install a new battery, the detector itself may be faulty.

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

No. You can replace the battery while the detector is powered. The battery compartment is isolated from the electrical wiring, so changing it will not shock you or damage the detector. Just open the compartment, remove the old battery, and insert the new one.

Where can I recycle old fire alarm batteries?

Take used 9-volt and AA batteries to a battery recycling program. Many retailers like Best Buy and Home Depot have battery drop-off bins near the entrance. You can also search Earth911.com or call your local waste management office to find a recycling center near you.