Wow! I thought my 34 month battery (9 V) was pretty good.

Art Begun wrote:

I've had the same batteries in my 3 thermostats for 6 years with no
problem. Pretty amazing when I think about it. I know they still
work because usually once every year, the float gets stuck in the
condensation pump on one of them and that causes the safety to cut
power to the furnace and the thermostat goes on battery till I fix it
(clean out the algae). I have 3 separate HVAC systems. Wouldn't
screw around if I only had 1.