Most smoke alarms use either 9-volt or AA batteries

The battery your smoke alarm needs depends on the model you own. The two most common types are 9-volt batteries (the rectangular ones) and AA batteries (the cylindrical ones). Some hardwired smoke alarms also have a backup 9-volt battery. A few older or specialty models use 10-year lithium batteries that cannot be replaced — the entire alarm is thrown out when the battery dies.

The easiest way to find out which battery your alarm needs is to open the alarm and look at the battery compartment. The shape and size of the slot will match only one battery type. If you cannot open it or the label is worn off, check the model number on the back or bottom of the alarm and search the manufacturer's website or manual.

Battery type also affects how often you change it. A 9-volt battery typically lasts six months to a year in a smoke alarm. An AA battery lasts about one year. The 10-year lithium batteries last a decade but cost more upfront and mean replacing the entire unit when they fail.

Key Takeaways

  • Most smoke alarms take either a 9-volt battery or AA batteries, and the shape of the battery compartment tells you which one yours needs.
  • A 9-volt battery in a smoke alarm usually lasts six months to a year before the low-battery chirp starts.
  • AA batteries last about one year and are cheaper to buy in bulk than 9-volt batteries.
  • Some newer alarms use 10-year lithium batteries that cannot be replaced, so you discard the whole alarm when the battery dies.
  • Check your alarm's model number on the back or bottom if you are unsure, and look up the manual online.

How to identify which battery your alarm uses

Open the alarm by twisting or unclipping the cover. Most smoke alarms have a straightforward release mechanism — usually a slot or tab on the side. Once the cover comes off, you will see the battery compartment. If the slot is rectangular and about the size of a lighter, it takes a 9-volt battery. If the slot holds two or four cylindrical spaces, it takes AA batteries.

If the alarm is hardwired to your ceiling or wall, it may have both a hardwired power source and a 9-volt backup battery. In that case, you only need to replace the 9-volt battery when it dies — the hardwired connection keeps the alarm running the rest of the time.

Write down the model number while you have the alarm open. You can find it printed on the back or bottom of the unit. If you lose the manual or cannot figure out the battery type from looking, search the model number plus "battery type" on the manufacturer's website. Most brands (First Alert, Kidde, Smoke Gard) post manuals online for free.

9-volt batteries versus AA batteries for smoke alarms

A 9-volt battery is larger and more expensive per unit, but it powers a single alarm for six months to a year. You buy them one or two at a time. AA batteries are smaller and cheaper individually, but many alarms need two or four of them, so the total cost per replacement can be similar. AA batteries also last about one year in a smoke alarm.

The real difference is convenience. If you have multiple alarms and most take AA batteries, you can buy a pack of 20 and stock your home. If you have a mix of 9-volt and AA alarms, you need to keep both types on hand. Some people prefer 9-volt alarms because they use fewer batteries per unit, while others prefer AA because they are easier to find in any store.

Neither type is inherently more reliable. Both will power a smoke alarm equally well. The choice between them usually comes down to which type your alarm was designed to use.

10-year lithium batteries and sealed alarms

Some newer smoke alarms come with a 10-year lithium battery sealed inside. You cannot open the alarm to replace the battery. When the battery dies after ten years, you throw away the entire unit and buy a new one.

These alarms cost more upfront — usually $25 to $40 compared to $10 to $20 for a standard alarm — but you never buy replacement batteries. Over ten years, the math can work out either way depending on how many alarms you own and how often you remember to change batteries. The main advantage is that you cannot forget to change the battery, because there is nothing to change.

The main disadvantage is waste. If the alarm malfunctions before ten years, you cannot just swap the battery; you replace the whole unit. Some people also dislike the idea of throwing away a working alarm just because the battery is dead.

Where to buy replacement batteries

9-volt and AA batteries are sold everywhere: grocery stores, pharmacies, hardware stores, and online retailers. A single 9-volt battery costs $3 to $6. A pack of four AA batteries costs $4 to $8. Buying in bulk online is usually cheaper per battery than buying single units at a store.

Check the expiration date on the package before you buy. Batteries can sit on shelves for years, and an old battery may not last as long as a fresh one. For smoke alarms, you want batteries manufactured within the last year or two.

Keep spare batteries in a drawer or cabinet near your alarms so you can replace them as soon as the low-battery chirp starts. Do not wait — that chirp means the battery is nearly dead and the alarm may not detect smoke.

When to replace your smoke alarm battery

Most smoke alarms chirp once every 30 to 60 seconds when the battery is low. That chirp is your signal to replace the battery within the next few days. Do not ignore it. A chirping alarm means the battery voltage has dropped below the level needed to reliably detect smoke.

If your alarm does not chirp and you cannot remember when you last changed the battery, replace it anyway. A good rule is to change all smoke alarm batteries twice a year — many people do it when they change their clocks for daylight saving time in spring and fall. That way, even if a battery fails silently, you catch it before it becomes a problem.

Write the replacement date on the alarm with a permanent marker so you have a record. Some people also set a phone reminder for six months after they change a battery, so they do not forget.

Frequently Asked Questions

Can I use a rechargeable battery in my smoke alarm?

No. Rechargeable batteries (like NiMH AA or 9-volt batteries) discharge faster than alkaline batteries and may not power a smoke alarm reliably for the full rated time. Smoke alarms are designed for single-use alkaline batteries. Use only new alkaline 9-volt or AA batteries.

What if my smoke alarm takes a battery I cannot find?

If your alarm uses an unusual battery size or type, check the model number and contact the manufacturer directly. Some older alarms use batteries that are no longer common. In that case, it may be cheaper and safer to replace the entire alarm with a new one that uses standard batteries.

Do I need to reset my smoke alarm after changing the battery?

Most smoke alarms do not need to be reset. straightforward insert the new battery, close the cover, and the alarm will resume normal operation. If the alarm continues to chirp after you install a fresh battery, the battery may be defective or the alarm itself may be failing.

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

A 9-volt alkaline battery typically lasts six months to one year in a smoke alarm, depending on the model and how often the alarm tests itself. Some alarms test more frequently and drain the battery faster. Check your manual for the expected battery life for your specific model.

Is it safe to use an old battery from another device in my smoke alarm?

No. Use only new, fresh batteries in smoke alarms. An old battery from a remote control or toy may not have enough voltage to power the alarm reliably. Smoke detection is too important to risk with a used or partially drained battery.