What replacement battery can I use in my smoke alarm (detector)?
Check your user manual or the nameplate on the back of the alarm, however smoke detectors (smoke alarms), carbon monoxide alarms, heat alarms and water alarms typically use the standard, commonly-available rectangular 9-volt battery type which is available wherever batteries are sold. These batteries are also colloquially named as ‘PP3’, ‘clock radio battery’, ‘square battery’, and ‘Japan 006P’.
Don’t use rechargeable batteries because they may not always provide a consistent charge and this can cause low-battery chirping unnecessarily. Given that alarms don’t use a lot of power and batteries can last for a year, you would get better use from a rechargeable battery in something else. Also, you won’t be left unprotected while having to recharge it outside of the alarm.