[Mbhat]

Please help me locate the battery in this hard wired smoke alarm. It is beeping and I just cannot find the battery so that I could replace it.

 

[lenaitch]

Not all hard-wired detectors have backup batteries. The rectangular-ish piece on the right looks like the holder for a 9v battery but I can't see any connectors so it may be that they used the same assembly for models with and without batteries.
It could be beeping because it has failed (I could read any date on it) but hard to know how to interpret beeps without the owner's manual.

If all else fails, replace the unit - better to be safe than sorry.

 

[rjniles]

Its more than 10 years old, replace it.

 

[Mbhat]

What is puzzling is that it is still chirping after I switched off the mains.

 

[Cement_Frank]

Detector is 70s-80s vintage, no backup battery. Detector is shot, probably beeping because it detected an internal failure, go to home depot and get a new BRK smoke alarm with battery for $12.