ADC T2000 Wire Power Vs Battery Indication?

Slightly off the wall question, but is there a way to tell if the thermostat is operating on wall power vs. battery? I installed 2 thermostats last year, including the C-Wire, and at about 16 months, both are triggering battery warnings now. According to the manual, they should last about a year on battery, or “much longer” on wall power since battery is only used during power failure. This has me wondering if I somehow wired the power wrong and I’m running on battery alone.

Is there any way to know?

The easiest way to tell if it is currently operating on AC is to pull the batteries and reattach. Does it power up?

The back-end is reporting them on Battery Power, but that just indicates they were learned in that way. That also means they would be in FLIRS mode and would not act as repeaters.

Yup, I’m on battery. The interesting thing is the house had programmable thermostats when I bought it, so I’ve got power there somehow, but can’t quite tell which. Only 4 wires, and no yellow common wire, but the old thermostats had fan control, so I don’t think it was the “cheat” method of wiring them either. I guess for now, I’ll leave well enough alone, can’t complain about 15 months of battery life. If I get some spare time, I’ll trace the wires to find out.

Only 4 wires, and no yellow common wire

Common, C-Wire is typically blue (though color does not matter) but usually the 5th wire. For a standard HVAC setup, 5 conductors would generally be used