Your anwer is quite short.
Of course tlyour statement could be true, but my second thought is:
A Nest Protect is permanently connected to AC power. The 3 lithium cells have a lifespan of at least 5 years.
Moreover, the NEST Protect designers would be quite stupid to sound a firewarning because of a battery-faillure, instead of a warning that says: ‘please replace batteries’.
Go figure: there will be not even ONE test engineer that will let such a stupid design flaw unnoticed!
I had this false alarm problem already in 2 unit (both version 1) only after 2 years of continuous operation.
One of them has been sent back to Nest, and they were so nice to send me a replacement u it (version 2 !!)
The second Nest Protect (verdion 1) failed 1 month ago. To test your ‘replace batteries’ solution, I wwill just put new lithium batteries in that second failed unit, and will for sure report back about the result.