Most smoke alarms use 9-volt or AA batteries, depending on the model
The battery type your smoke alarm needs depends on which model you own. The two most common types are 9-volt (9V) batteries and AA batteries. Some older or specialty alarms use 10-year sealed lithium batteries that cannot be replaced. Check the label on the back or inside the battery compartment of your alarm — it will tell you exactly which type to buy.
9-volt batteries are rectangular with terminals on top and fit into a clip-style holder. AA batteries are cylindrical and slightly smaller, and they usually sit in a plastic tray inside the alarm. If you are not sure which one your alarm takes, open the battery compartment and look at the shape and size of the slot or holder.
Hardwired smoke alarms with battery backup also typically use 9-volt batteries as their backup power source. If your alarm is wired into your home's electrical system, the battery only activates if the power goes out.
Key Takeaways
- Check the label inside your smoke alarm's battery compartment to find out whether it takes a 9-volt or AA battery.
- 9-volt batteries are rectangular with two terminals on top, while AA batteries are cylindrical and smaller.
- Some smoke alarms have sealed 10-year lithium batteries built in and cannot be opened to replace the battery.
- Hardwired alarms usually have a 9-volt battery as backup power in case of electrical outage.
- Replace batteries once a year or when the alarm chirps to signal low battery.
How to identify your smoke alarm battery type
The fastest way to know what battery you need is to look at the alarm itself. Most manufacturers print the battery type on a label on the back of the unit or inside the battery compartment door. The label will say "9V" or "AA" clearly.
If the label is worn or missing, open the battery compartment and look at the holder. A 9-volt battery holder is a plastic clip with two metal terminals — one marked positive and one marked negative. An AA battery holder is a plastic tray with spring contacts at each end, usually holding one or two batteries side by side.
If you cannot open the compartment or see no battery slot at all, your alarm likely has a sealed 10-year battery. These alarms are designed to be replaced entirely after 10 years rather than having the battery swapped out.
9-volt batteries: the standard for most alarms
The 9-volt battery is the most common choice for household smoke alarms. It is a single rectangular battery about 1.5 inches tall with a snap connector on top. When you buy a 9V battery, it will fit into the clip holder in your alarm with a straightforward push until you hear a click.
9-volt batteries cost between $3 and $8 per battery depending on the brand and where you buy them. Duracell, Energizer, and store brands all make 9V batteries that work in smoke alarms. Alkaline 9V batteries last about one year in a smoke alarm before the battery runs low.
When the 9V battery gets weak, your alarm will chirp once every 30 to 60 seconds. This is the signal to replace it. Do not ignore the chirp — it means the alarm will not sound if there is a fire.
AA batteries: common in dual-battery and budget models
Some smoke alarms use one or two AA batteries instead of a 9V. AA batteries are smaller and cylindrical, about the size of your thumb. They cost less than 9V batteries — usually $1 to $4 per battery — and last about the same length of time in a smoke alarm.
Alarms that use AA batteries often have a plastic tray that holds one or two batteries side by side. You slide the batteries in and close the compartment. Like 9V batteries, AA batteries will trigger a chirp when they run low.
Some manufacturers use AA batteries in their budget models because they are cheaper to source. Others use them in combination with a 9V as a dual-battery backup system. Check your manual or the compartment label to see how many AA batteries your specific alarm needs.
10-year sealed batteries: no replacement needed
Newer smoke alarms often come with a 10-year sealed lithium battery built into the unit. These batteries cannot be removed or replaced. Instead, you replace the entire alarm after 10 years.
Sealed-battery alarms are convenient because you do not have to remember to change the battery every year. However, they cost more upfront — usually $25 to $50 per alarm compared to $10 to $20 for a battery-powered alarm. If the battery fails before 10 years, you must replace the whole unit.
The alarm will have a label stating the manufacture date. Mark it on your calendar so you know when to replace the unit. Some sealed-battery alarms have a low-battery warning light that blinks even though you cannot replace the battery — this means the alarm is nearing the end of its life.
Hardwired alarms with battery backup
If your smoke alarm is hardwired into your home's electrical system, it still has a battery inside as backup power. This battery activates only if your home loses electricity. Hardwired alarms almost always use a 9-volt battery for this backup.
You replace the 9V battery in a hardwired alarm the same way you would in a battery-powered alarm — open the compartment, remove the old battery, and snap in the new one. The alarm will still chirp when the battery runs low, even though it is powered by electricity most of the time.
Some newer hardwired alarms also have sealed 10-year batteries as backup instead of replaceable 9V batteries. Check your alarm's label to see which type you have.
Where to buy replacement batteries and what to expect
You can buy 9V and AA batteries at any grocery store, pharmacy, hardware store, or online retailer. Common brands include Duracell, Energizer, Panasonic, and store-brand alkaline batteries. All alkaline batteries of the same size work the same way in smoke alarms — the brand does not matter for performance.
Prices vary by location and retailer. A single 9V battery costs $3 to $8 at a drugstore but may cost $2 to $4 if you buy a pack of four at a warehouse club or online. AA batteries are cheaper — $1 to $4 per battery depending on quantity and brand.
Do not buy rechargeable batteries for your smoke alarm. Rechargeable batteries lose charge over time even when not in use, and your alarm may not sound if the battery is too weak. Stick with alkaline batteries — they hold their charge longer and are safer for life-safety devices.
Frequently Asked Questions
Can I use a rechargeable battery in my smoke alarm?
No. Rechargeable batteries lose their charge over time even when sitting unused, and your alarm may fail to sound in a fire. Smoke alarms need alkaline batteries that hold a steady charge for the full year between replacements. Use only alkaline 9V or AA batteries.
What should I do if my smoke alarm keeps chirping after I put in a new battery?
Make sure the battery is fully seated in the holder — push it in until you hear a click. If it still chirps, try a different brand-new battery from a fresh package. If the chirping continues, the alarm itself may be faulty and need replacement. Some alarms also chirp if they are too old or if dust has built up inside.
How often should I replace my smoke alarm batteries?
Replace batteries once a year, ideally on the same date each year — many people do it when they change their clocks for daylight saving time. If your alarm chirps before a year has passed, replace the battery when ready. Do not wait for the chirp to replace an old battery.
Is it safe to mix old and new batteries in an alarm that takes two AA batteries?
No. Always replace both batteries at the same time, even if only one seems weak. Mixing old and new batteries can cause the alarm to malfunction or fail to sound when needed. Buy a fresh pair and install them together.
What if I cannot find the battery type label on my smoke alarm?
Look up the model number on the back of the alarm online, or contact the manufacturer directly with a photo. You can also open the battery compartment and look at the holder shape — 9V holders are clips with two terminals, and AA holders are plastic trays with springs. When in doubt, take the alarm to a hardware store and ask staff to identify it.