Replace the battery in under two minutes by opening the detector cover, sliding out the old battery, and inserting a new one the same size

Most First Alert smoke detectors use a 9-volt battery that slides into a clip inside the detector. The whole job takes less than two minutes and requires no tools. When your detector chirps every 30 to 40 seconds, that chirp is telling you the battery is low — that is the moment to replace it, not a sign the detector is broken.

The steps are the same whether you have a battery-only model or a hardwired detector with a backup battery. The only difference is that hardwired models keep working on house power while you swap the battery, so there is no rush.

Key Takeaways

  • First Alert detectors use a 9-volt battery that you can buy at any drugstore, grocery store, or hardware store for $5 to $8.
  • The chirping sound means the battery is low and needs replacement within a few days — the detector itself is still working.
  • You open the cover by twisting it counterclockwise or pressing a release tab, depending on your model.
  • The battery slides into a clip marked with a plus and minus sign; match the + and − ends to the markings before pushing it in.
  • After you insert the new battery, the detector will beep once or twice to confirm it is working, then stop.

What battery to buy and where to find it

You need a standard 9-volt battery, often called a 9V battery. First Alert detectors do not use AA, AAA, or any other size. The battery has a rectangular shape with two metal terminals on top — one marked + and one marked −. You can buy 9-volt batteries at any drugstore, supermarket, hardware store, or online. A single battery costs between $5 and $8; a pack of two or four is usually cheaper per battery.

Do not buy a rechargeable 9-volt battery for a smoke detector. Rechargeable batteries drain faster and may not power the detector long enough. Stick with a standard alkaline 9-volt battery. Common brands include Duracell, Energizer, and store brands — they all work the same way in a First Alert detector.

How to open your First Alert detector

First Alert makes two main types of covers. The most common type has a cover that twists off. Hold the detector in one hand and twist the cover counterclockwise (the same direction you turn a jar lid to open it) until it comes free. The cover usually twists about a quarter turn before it stops and lifts away.

Some newer First Alert models have a cover with a release tab or button on the side. Press or slide the tab and the cover pops off. If you are not sure which type you have, look at the edge of the cover for a small arrow or the words "twist" or "press." If you twist and nothing happens, try pressing the edge instead.

Once the cover is off, you will see the battery clip inside. The clip has two metal contacts and is usually labeled with a + and − sign. The old battery is already sitting in this clip.

Remove the old battery and insert the new one

Grip the old battery by its sides (not the metal terminals) and slide it out of the clip toward you. The battery should come free easily — do not force it. If it is stuck, wiggle it gently side to side as you pull.

Take your new 9-volt battery and look at the terminals on top. One is marked + (positive) and one is marked − (negative). Look at the clip inside the detector and match the terminals: the + terminal on the battery goes into the + contact on the clip, and the − terminal goes into the − contact. Slide the battery into the clip with the terminals going in first. You will feel it click or snap into place when it is seated correctly.

Do not force the battery. If it does not slide in smoothly, you have the polarity backwards — flip the battery around and try again. Once it is in, the battery should sit flat against the clip with no gaps.

Close the cover and test the detector

Twist the cover clockwise (the same direction you turn a jar lid to close it) until it is snug. Do not over-tighten — it should stop on its own when seated properly. If your detector has a press-to-release cover, straightforward push the cover back on until it clicks.

Within a few seconds, the detector will beep once or twice to confirm the battery is installed and working. Some models beep when ready; others wait a few seconds. If the detector does not beep at all, the battery may not be seated correctly — open the cover again and check that the terminals are aligned with the clip contacts.

The chirping sound should stop within a few minutes. If it continues, the battery may be low even though it is new (rare but possible), or the detector may have another issue. Try a different new battery first. If the chirping continues after that, the detector itself may need replacement.

When to replace the battery before it runs low

Most fire safety experts recommend replacing smoke detector batteries once a year, even if the detector has not started chirping. A good time to do this is during daylight saving time — when you set your clocks forward or back in spring and fall, also change your smoke detector batteries. This way you know all your detectors have fresh batteries at the same time.

If your detector is hardwired to your house power, it still has a backup battery that needs replacing on the same schedule. The backup battery is what keeps the detector working during a power outage, so it is just as important as the battery in a battery-only model.

Common mistakes to avoid

The most common mistake is installing the battery backwards. If the + and − terminals do not match the clip contacts, the detector will not work and will not beep. If your detector is silent after you close the cover, open it again and flip the battery around.

Another mistake is buying the wrong battery size. First Alert detectors use 9-volt batteries only. If you buy AA or AAA batteries by mistake, they will not fit the clip. Check the old battery before you leave home to shop, or take a photo of it to show a store employee.

Do not use a battery that is leaking, corroded, or damaged. If the old battery has white or blue powder on it or smells odd, throw it away and use a fresh one. A damaged battery can damage the detector.

Frequently Asked Questions

Why does my detector keep chirping even after I put in a new battery?

The battery may not be seated correctly in the clip. Open the cover and check that the + and − terminals line up with the clip contacts. If they do, try a different new battery — it is rare but possible to get a defective battery. If a second new battery still chirps, the detector itself may be faulty and need replacement.

Can I use a rechargeable 9-volt battery?

No. Rechargeable 9-volt batteries drain much faster than alkaline batteries and may not last long enough to protect you. Stick with a standard alkaline 9-volt battery from a drugstore or hardware store.

How long does a 9-volt battery last in a smoke detector?

A new alkaline 9-volt battery typically lasts 6 to 12 months in a First Alert smoke detector, depending on the model and how often the detector tests itself. This is why fire safety experts recommend replacing the battery once a year, even if it has not started chirping yet.

What should I do with the old battery?

Take the old battery to a battery recycling program or a store that accepts batteries for recycling — many drugstores and hardware stores have drop-off bins near the checkout. Do not throw it in the trash. If you cannot find a recycling program nearby, you can store the old battery in a safe place until you have several to recycle together.

Do I need to turn off the detector before changing the battery?

No. Battery-only detectors have no power switch. Hardwired detectors stay powered by house current while you change the backup battery, so they keep working the whole time. You do not need to do anything except open the cover and swap the battery.