In the year I've owned my house, I've had to replace the upstairs t-stat batteries about 4 times, and never the downstairs. This spoke to me that it was either something wrong with the actual control unit, or the HVAC power wire? Recently it gave me a "bat lo" message in the middle of the night, shutting off the A/C and waking me up. I shook it and it resumed normal operation.

Is there a simple way to test if it's either the unit or wiring? Is it as simple as taking a voltmeter to the wire? What voltage should I be looking for? Sorry if this seems DIY, I just don't have a good understanding about how t-stats are powered and what I should be looking for.

The t-stats for both downstairs and upstairs are Honeywell programmables, at least 12 yrs old.