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What battery does a carbon monoxide detector use?
Most carbon monoxide detectors take an AA (1.5 V alkaline cylindrical battery). Some use 9V instead. The battery size is printed in the battery compartment and on the label on the back of the alarm.
| Battery | How common | Size | Volts | Capacity |
|---|---|---|---|---|
| AA | Most common | 14.5 mm × 50.5 mm | 1.5 V | ≈ 2,700 mAh |
| 9V | Also used | 48.5 mm × 26.5 mm × 17.5 mm | 9 V | ≈ 550 mAh |
Carbon Monoxide Detector battery: what to know
- Plug-in CO alarms with battery backup usually take AA or 9V; battery-only models usually take AA.
- Combination smoke and CO alarms follow the smoke-alarm rule: most take 9V or AA, and sealed 10-year models cannot be opened.
- CO sensors wear out: most alarms show an end-of-life warning after 7 to 10 years, and a new battery does not fix that.
How to check which battery your carbon monoxide detector takes
The battery size is printed in the battery compartment and on the label on the back of the alarm. Battery codes are printed on the flat face of coin and button cells and along the side of AA, AAA and 9V batteries. Match the code exactly: a cell that looks similar can be a different thickness or voltage; see the AA page for safe swaps.
AA: Lithium AA (FR6, about 3,000–3,500 mAh) lasts longer in cold weather and high-drain devices. NiMH rechargeable AA (HR6) is 1.2 V and 1,900–2,800 mAh.
9V: Lithium 9V (about 1,200 mAh) lasts several times longer in smoke alarms. NiMH 9V is usually 8.4 V and 175–300 mAh, too little for most smoke alarms.