Most units being made today have a battery backup. If the battery compartment is not plainly visible from the outside it could be underneath (also the location of the model number and date code). Most smoke/co alarms are removed from their mounted bases by turning counterclockwise, usually about 1/8 of a turn. If the device is more than 10 years old it (the entire unit) must be replaced per the National Fire Code, NFPA72. Otherwise, you my find the opening for the battery underneath as well. There are a few that do not have a battery at all. To be sure, locate the manufacturer and model number and check the appropriate website.
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Most units being made today have a battery backup. If the battery compartment is not plainly visible from the outside it could be underneath (also the location of the model number and date code). Most smoke/co alarms are removed from their mounted bases by turning counterclockwise, usually about 1/8 of a turn. If the device is more than 10 years old it (the entire unit) must be replaced per the National Fire Code, NFPA72. Otherwise, you my find the opening for the battery underneath as well. There are a few that do not have a battery at all. To be sure, locate the manufacturer and model number and check the appropriate website.