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What battery does a smoke detector use?
Most smoke detectors take a 9V (9 V alkaline cylindrical battery). Some use AA instead. Open the battery door or twist the alarm off its base: the battery type is printed inside, next to the compartment.
| Battery | How common | Size | Volts | Capacity |
|---|---|---|---|---|
| 9V | Most common | 48.5 mm × 26.5 mm × 17.5 mm | 9 V | ≈ 550 mAh |
| AA | Also used | 14.5 mm × 50.5 mm | 1.5 V | ≈ 2,700 mAh |
Smoke Detector battery: what to know
- Battery-only and hardwired smoke alarms both take a battery; in hardwired alarms it is the backup for power cuts.
- Sealed 10-year alarms have a built-in lithium battery that cannot be replaced: when one chirps at end of life, replace the whole alarm.
- The US Fire Administration and NFPA advise testing alarms monthly and replacing any alarm that is 10 years old.
- A lithium 9V lasts several times longer than an alkaline 9V; rechargeable 9V cells hold too little charge for most alarms.
How to check which battery your smoke detector takes
Open the battery door or twist the alarm off its base: the battery type is printed inside, next to the compartment. 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 9V page for safe swaps.
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.
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.