Most smoke detectors use either 9-volt or AA batteries

The battery your smoke detector needs depends on the model you own. The two most common types are 9-volt batteries (the rectangular ones with terminals on top) and AA batteries (the standard cylindrical size). Some detectors use two AA batteries instead of one, and a small number use 10-year sealed lithium batteries that you cannot replace. Check the battery compartment or the back of your detector to see which type fits.

The easiest way to know for certain is to open the battery compartment right now and look at what is currently installed, or check the model number on the detector itself and search the manufacturer's website. Most manufacturers list battery requirements on their product pages, and the information takes less than a minute to find.

Key Takeaways

  • Check your detector's battery compartment or the model number to find out whether it uses 9-volt, AA, or sealed lithium batteries.
  • 9-volt and AA batteries are the most common types and are inexpensive to replace at any grocery store or pharmacy.
  • Some detectors have 10-year sealed batteries built in, which means you replace the entire detector instead of just the battery.
  • Replace batteries when the detector chirps or beeps, or once per year as a routine maintenance step.
  • Keep spare batteries on hand so you can replace them when ready when the detector signals a low battery.

How to identify which battery your detector uses

Open the battery compartment on the back or side of your smoke detector. If you see a rectangular battery with two metal terminals on top, you have a 9-volt detector. If you see one or two cylindrical batteries about the size of your thumb, you have an AA detector. If the compartment is sealed and there is no way to open it, your detector has a 10-year lithium battery that cannot be replaced.

If you cannot open the compartment or the detector is mounted high on the ceiling, look at the model number printed on the detector itself. Write down the brand and model number, then visit the manufacturer's website and search for the product manual. The manual will list the battery type in the first few pages, usually under "Specifications" or "Battery Information."

9-volt batteries: when to use them

A 9-volt battery is the standard for many hardwired smoke detectors that have a battery backup, as well as for many wireless or battery-only models. The battery is rectangular, about 1.5 inches tall, and has two metal snap terminals on the top. When you buy a replacement, make sure the label says "9V" or "9-volt."

9-volt batteries are sold at every grocery store, pharmacy, hardware store, and online retailer. A single battery costs between $3 and $8 depending on the brand and where you buy it. Alkaline 9-volt batteries last about six months to a year in a smoke detector, depending on how often the detector tests itself. Some manufacturers make 9-volt batteries specifically for smoke detectors, which may last slightly longer than general-purpose ones.

AA batteries: when to use them

Many battery-only smoke detectors use one or two AA batteries. AA batteries are the standard cylindrical size you probably use in flashlights, remote controls, and other household devices. When you open the compartment, you will see a slot or holder for either one or two batteries, depending on the model.

AA batteries are the cheapest option and are available everywhere. A pack of four AA alkaline batteries costs $3 to $6. They last about one year in a smoke detector. Some detectors specify alkaline batteries only, while others work with alkaline or rechargeable NiMH batteries — check your manual to be sure. Do not use rechargeable batteries unless the manual says they are safe, because some detectors do not work reliably with them.

10-year sealed lithium batteries: what they mean

Some newer smoke detectors come with a 10-year sealed lithium battery built inside. You cannot open the battery compartment or replace the battery yourself. Instead, when the battery reaches the end of its life (usually around 10 years), you throw away the entire detector and buy a new one.

These detectors are convenient because you never have to remember to replace a battery, and the battery will not die unexpectedly. However, they cost more upfront than a detector with replaceable batteries. If you buy one, write the purchase date on the detector with a permanent marker so you know when to replace it. Some models have a light that flashes to show the battery is still working, but not all do.

How to replace the battery safely

Before you replace the battery, press the test button on the detector to make sure it is working. Hold the button for three to five seconds until you hear a loud beep. If the detector does not beep, the battery may already be dead or the detector itself may have failed.

Turn off the power to the detector if it is hardwired (flip the breaker switch in your electrical panel). Open the battery compartment. Remove the old battery and note which way it was facing — the positive and negative ends matter. Insert the new battery the same way the old one was facing. Close the compartment and press the test button again. You should hear a loud beep within a few seconds. If you do not hear anything, try a different new battery, because the first one may have been defective.

When to replace your smoke detector battery

Replace the battery as soon as you hear a chirping or beeping sound coming from the detector. This sound means the battery is low and needs to be replaced within a few days. Do not ignore it — the detector is telling you it will not work if there is a fire.

Even if your detector is not chirping, replace the battery once per year as a routine maintenance step. Many people replace batteries on the same day they change their clocks for daylight saving time, which happens twice per year and is straightforward to remember. If you cannot remember the last time you replaced the battery, replace it now.

Frequently Asked Questions

Can I use rechargeable batteries in my smoke detector?

Check your detector's manual first. Some detectors work fine with rechargeable NiMH batteries, but others do not because they need a consistent voltage. If your manual does not mention rechargeable batteries, use alkaline batteries instead. Rechargeable batteries can lose charge over time even when not in use, which means your detector might not work when you need it.

What happens if I put the battery in backwards?

The detector will not work, but it will not be damaged. The battery straightforward will not make contact with the terminals. Open the compartment, flip the battery around so the positive end faces the correct direction, and close it again. Press the test button to confirm it is working.

Is it okay to leave a smoke detector without a battery while I wait for a new one?

No. A smoke detector without a battery cannot protect you in a fire. Replace the battery when ready, or if you do not have a replacement on hand, buy one at a nearby store before you go to bed that night. Keep spare batteries in your home so you never have to wait.

Do expensive batteries last longer in a smoke detector than cheap ones?

Not significantly. The difference between a budget alkaline battery and a premium brand is usually only a few weeks. What matters more is replacing the battery on schedule — once per year, or whenever the detector chirps. A cheap battery replaced on time protects you better than an expensive battery that you forget about.

Can I use a 9-volt battery in a detector that takes AA batteries?

No. The compartment is designed for AA batteries, and a 9-volt battery will not fit. Using the wrong battery type can damage the detector or cause it to malfunction. Always use the exact battery type listed in your manual or printed on the detector itself.