Most battery-powered smoke detectors use either a 9‑volt battery or AA batteries, but the exact size depends on the model and what the manufacturer designed it for.

Quick Scoop

  • The most common sizes are:
    • 9‑volt rectangular batteries (very typical in older or basic smoke alarms).
* **AA** cylindrical batteries, often 2–6 of them in newer or dual‑sensor models.
  • Some newer alarms have a sealed 10‑year lithium battery built in, which you cannot replace; you replace the whole unit when it chirps end‑of‑life.
  • A few models use AAA cells, but this is less common and very brand‑specific.

How to know what your detector takes

Because there is no single universal battery size, the safest move is:

  1. Check the label or sticker on the back of the alarm (you usually have to twist it off its mounting plate).
  1. Look inside the battery compartment ; it normally lists “Use only 9V alkaline” or “Use 2 x AA 1.5V” or similar text.
  1. If you still are not sure, look up the model number in the user manual or on the manufacturer’s website.

Practical tips for replacement

  • Use good‑quality alkaline or lithium batteries specifically recommended for smoke detectors; cheap carbon‑zinc batteries do not last as long.
  • Replace batteries at least once a year , or immediately if you hear chirping.
  • Test the alarm monthly with the test button after changing batteries to confirm it’s working.

In short: most people will need either a 9‑volt or AA battery for their smoke detector, but always confirm by reading the marking on your specific alarm before buying replacements.

TL;DR: What size batteries do smoke detectors take? Typically 9‑volt or AA, with exact type printed on the device or in the manual, and some models using sealed 10‑year lithium packs instead of replaceable batteries.

Information gathered from public forums or data available on the internet and portrayed here.