I installed a Nest Thermostat a little over one year ago. For the first year it was fine, then all of a sudden one day it started displaying error messages saying the batteries were dead. It said the battery voltage was something absurd like 0.05V. I took the batteries out and checked them with my multimeter and they were about 1.4V each. I grabbed some new batteries, tested those, they were 1.55V each, and I put those in. It took several days for the low battery voltage error message to go away. The thermostat displayed a measured battery voltage that went up slightly, a fraction of a volt per day, until it stabilized around 2.5V (which was still below actual battery voltage). A month later, all of a sudden one day with no warning once again the thermostat said battery voltage was <0.1V and batteries needed to be replaced. And once again, my multimeter says actual battery voltage is ~3.0V combined. And again today, about another month later, it says the batteries are dead again when in fact they're just fine. What gives? I've done multiple factory resets, so don't tell me to do a factory reset. And yes, it has perfectly fine 24VAC supply. If it didn't, the first battery wouldn't have lasted a year.
