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. Kidde still documents a 9V model — the i9050, which its product page describes as powered by a 9V battery^[2] — and 9V cells are cheap and stocked by any grocery or hardware store. The downside: no manufacturer document read for this page publishes how long a 9V lasts inside a smoke detector, so this page no longer gives a number. Kidde's instruction is the one to work from — replace the batteries in all your alarms at least once a year, or any time an alarm tells you its battery is low^[1]. Lithium 9V cells cost more; what First Alert does document is that well-designed lithium cells are often less affected by a drop in temperature^[8], which is the difference that turns up as a 3 AM chirp in winter.

AA (double-A)

Many detectors take 2 or 3 AA batteries instead of a single 9V, and Kidde's battery-replacement instructions carry a step that only applies to them: with AA models, check that the contacts are not facing the same direction^[1]. Two cells dropped in the same way round is a detector that will not power up. AA is the most common battery format in the world, so replacements are never hard to find.

CR123A (3V lithium)

These small cylindrical lithium batteries turn up in some wireless and smart detectors. They are lithium cells, so they share the cold-weather behavior First Alert describes above^[8]. One detector below takes one, and it is the only CR123A model any manufacturer document read for this page names: X-Sense's XS01-WR, whose specification table gives the power supply as a replaceable 3V CR123A lithium battery^[11]. It also gives the only service life on offer for the format, and X-Sense qualifies it in the same table's note — 5 years, measured in weekly testing and standby, and shorter for a unit that has actually been in alarm^[11]. If your detector's label says CR123A, buy that; do not buy one on the strength of a general guide.

How to check which type your detector uses

1. 1. Open the battery compartment. Kidde's own instruction is to take the alarm down first, then open the battery compartment door^[1]; the battery already in there tells you the type. Kidde also asks you to line the battery's + and − connectors up with the metal plates inside the compartment^[1], so note which way round the old one sat before you pull it.
2. 2. Check the label on the back — but know what is on it. Kidde documents a label attached to the back of the unit and points you at it for the date code^[10], which is how you tell whether the alarm is out of time. No manufacturer document read for this page promises the battery type is printed there too. If it is, use it; if it is not, go to step 3.
3. 3. Look up the model number. If the battery is dead and the label is faded, search the model number on the manufacturer's own site. The product page, the model's own manual, or the manufacturer's own support article gives the battery type — between them, that is where every model below came from. Try all three: for the Nest Protect the support article is the only one of them that states the cell, because Google publishes no live product page for it.

Models the manufacturers document

Every line here was read off a manufacturer's own product page, manual or support article. This list is shorter than it used to be: models whose battery type could not be confirmed on a manufacturer page have been removed rather than guessed at, because sending someone to the store for the wrong cell leaves a detector unpowered.

First Alert

• SA511 — 2 AA batteries^[5]
• SMI110 (10-year battery) — sealed lithium metal cell. First Alert's spec table lists the battery life as 10 years and marks it not replaceable^[7].
• SC7010B (hardwired smoke/CO) — 2 AA batteries as the backup, not a 9V^[6]. Most hardwired alarms of its era take a 9V, which is why this one sends people home with the wrong battery.

Google Nest

• Nest Protect — replaceable AA lithium cells. Google's own instruction for a low-battery chirp is to put lithium AA cells back in, and its support article names the exact product^[12] — open that source for the designation, because this page does not recommend battery brands. Google states no cell count and no battery service life for the unit, so count what comes out of your own compartment rather than buying to a number.

Kidde

• i9050 — 9V battery^[2]
• P4010DCS-W — 10-year sealed lithium battery, non-replaceable^[3]
• P4010ACS-W (hardwired) — 10-year sealed lithium backup, non-replaceable^[4]

X-Sense

• XS01-WR (wireless interlinked) — one replaceable 3V CR123A lithium battery. X-Sense's specification table states a 5-year battery life inside a 10-year operating life^[11], so unlike the sealed units above this one expects you to change the cell rather than bin the detector. X-Sense publishes no replacement schedule of its own for it, so the annual rule below is what applies.

The 9V detector is on its way out

Worth knowing before you stock up on any one cell: both of Kidde's sealed 10-year P4010 models are marked discontinued on their own product pages, and Kidde names the replacement for each — an AA-powered model in place of the battery unit^[3], and a hardwired model with an AA backup in place of the hardwired one^[4]. The 9V i9050 carries the same note, pointing at a successor model^[2]. So the detector you buy to replace a chirping one is more likely to take AA cells than the 9V it came with. Check the new unit's label rather than assuming it matches the old one.

How often to replace batteries

• Any replaceable cell, whatever the chemistry — Kidde publishes one interval and it does not vary by battery type: replace the batteries in all your alarms at least once a year, and any time an alarm tells you its battery is low^[1]. A habit that hits the same interval: swap them when you change your clocks.
• There is no separate schedule per chemistry — this page used to give one, in figures no manufacturer document read for it publishes. The annual rule above is what the manufacturers actually say, whichever cell you buy.
• Check the label on your own detector — where a manufacturer specifies a battery for a given model, that instruction beats any general rule on this page.

Sealed 10-year batteries

An increasing number of smoke detectors come with a sealed, non-replaceable lithium battery designed to last the full 10-year lifespan of the detector — First Alert's SMI110 spec table states both halves outright, a 10-year battery life and no replacing it^[7]. When the battery dies, you discard the entire unit and install a new one. This is by design, and it matches the replacement interval the manufacturers publish for the alarm itself: Kidde replaces smoke and heat alarms at ten years, and carbon monoxide and combination alarms at seven^[9]. There is 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.

Sources

Every beep pattern and troubleshooting step on this page comes from the manufacturer's own documentation or a federal safety regulator. Where a manual contradicted us, we changed the page.

1. [1] Kidde. How Do I Replace My Smoke Alarm's Battery? (retrieved 2026-09-07)
2. [2] Kidde. Battery Operated Smoke Alarm i9050 (retrieved 2026-09-06)
3. [3] Kidde. Wire-Free Interconnected Battery Powered Smoke Alarm P4010DCS-W (retrieved 2026-09-06)
4. [4] Kidde. Wire-Free Interconnected AC Hardwired Smoke Alarm P4010ACS-W (retrieved 2026-09-06)
5. [5] First Alert. User's Manual: Wireless Interconnect Smoke Alarm, Model SA511 (M08-0194-005) (retrieved 2026-09-06)
6. [6] First Alert. Hardwired Combination Photoelectric Smoke and Carbon Monoxide Alarm with Battery Backup (SC7010B) (retrieved 2026-09-06)
7. [7] First Alert. 10-Year Battery Smoke Alarm (SMI110) (retrieved 2026-09-07)
8. [8] First Alert. Why Does Your Smoke Alarm Chirp in the Middle of the Night? (retrieved 2026-09-06)
9. [9] Kidde. When Should I Replace My Alarms? (retrieved 2026-09-07)
10. [10] Kidde. What Causes Intermittent Beeping or Chirping? (retrieved 2026-09-07)
11. [11] X-Sense. User Manual: Wireless Interlinked Smoke Alarm XS01-WR (retrieved 2026-10-02)
12. [12] Google. Nest Protect lights and sounds (retrieved 2026-10-02)