Originally Posted by
ZERACER
The batteries usually last 5 - 7 years so I wouldn't buy a used one that is very old or you will be changing it again shortly.
I think that is about the time you should be changing the batteries, they may last a few more years but you want to make sure you change them before they start leaking and corroding the PCB.
Or just buy a new one when it fails whenever that is. Get one of the newer ones that chirps