Smoke Detector Battery Types: 9V, AA, and CR123A Guide

The 3 common smoke detector battery types

9V (nine-volt)

The rectangular battery with snap connectors on top. This is the most common type found in older and mid-range smoke detectors. If your detector was installed before 2015, there's a good chance it uses a 9V. These batteries are cheap, widely available at any grocery or hardware store, and easy to swap. The downside: alkaline 9V batteries typically last only 6-12 months in a smoke detector. Lithium 9V batteries (like Energizer Ultimate Lithium) can last up to 5 years, though they cost significantly more.

AA (double-A)

Many newer smoke detectors use 2 or 3 AA batteries instead of a single 9V. AA batteries have a higher total energy capacity than a 9V, which means longer battery life — typically 1-2 years with alkaline, 3-5 years with lithium. The Nest Protect, for example, uses 6 AA lithium batteries and advertises a 5-year battery life. AA batteries are the most common battery type in the world, so finding replacements is never an issue.

CR123A (3V lithium)

These small cylindrical lithium batteries are used in some newer smart smoke detectors and combo units. They're less common than 9V or AA, but they offer excellent energy density and a long shelf life— typically 3-5 years in a detector. You'll find CR123A batteries at electronics stores and online, though they may not be available at every grocery store. Some X-Sense models and certain wireless interconnected detectors use this type.

How to check which type your detector uses

1. 1. Open the battery compartment. Twist the detector off its mounting bracket and look for a battery door or slide-out tray. The battery currently installed tells you the type.
2. 2. Check the label on the detector. Most detectors have a sticker on the back with the model number and battery type printed. Look for text like "Replace with 9V" or "Uses 2 AA."
3. 3. Look up the model number. If the battery is dead and the label is faded, search the model number online. The manufacturer's website will list the correct battery type.

Brand guide: which batteries they use

First Alert

• SA303CN3, SA320CN — 9V battery
• SA511CN2 — 2 AA batteries
• PR710, SA10YR — Sealed 10-year lithium (non-replaceable)
• Hardwired models (SA4121B, SC7010B) — 9V backup battery

Kidde

• i9050, i9060 — 9V battery
• P4010ACS-W, P4010DCS-W — Sealed 10-year lithium (non-replaceable)
• Hardwired models (i12060, PI2010) — 9V or AA backup battery depending on model

Nest Protect

• Battery model (2nd gen) — 6 Energizer Ultimate Lithium AA batteries
• Wired model (2nd gen) — No backup battery needed (AC powered with internal rechargeable)

X-Sense

• SD01, SC01 — Sealed 10-year lithium (non-replaceable)
• XS01-WX (wireless) — CR123A lithium battery
• XP02-W — Sealed 10-year lithium (non-replaceable)

How often to replace batteries

• Alkaline 9V — every 6-12 months. The NFPA recommends replacing these annually at a minimum. A good habit: swap them when you change your clocks for daylight saving time.
• Alkaline AA — every 1-2 years, depending on the detector's power draw.
• Lithium (any type) — every 2-5 years. Lithium batteries have a much longer shelf life and degrade more slowly. Check the manufacturer's recommendation for your specific model.

Sealed 10-year batteries

An increasing number of smoke detectors come with a sealed, non-replaceable lithium batterydesigned to last the full 10-year lifespan of the detector. When the battery dies, you discard the entire unit and install a new one. This is actually by design — the sensing element in a smoke detector degrades over 10 years, so there's no value in keeping a decade-old detector running on a fresh battery. If your sealed-battery detector is chirping, there is no battery to replace. The whole unit goes.

First, find the chirping detector

None of this battery information helps if you can't figure out which detector is chirping. The high-frequency chirp is notoriously hard to locate by ear because it bounces off walls and ceilings. Before you buy replacement batteries, use WhichBeep to identify the exact detector that needs attention — so you know which battery type to get.