That battery should last 10 years due to the very shallow cycling we did on it. It only gets charged to 3.9v max, the usable capacity is less than half the design capacity to extend the life.
Is Voc listed at 37v? That's a bit high to be honest. It's possible you have no issues with your battery but you're hitting overvoltage lockout on the buck preventing any charging from happening. Did anything change with your furnace? I'd expect a lower number from a 24v transformer (I see 21v on mine, which is on the low side, but above 35v is also unusual).
Nest DVT serial number 1 still going strong on my wall (and I only have RH/W wires, no C).
I would not be at all surprised if the real world tolerances on the AC transformers used for HVAC were rather
loose.
Even a 25% RMS overvoltage
under load would not seem out of the question given the low cost and made in China sourcing I would expect is common. Open circuit voltages could be even farther out, I imagine.
I hope the NEST can cope with 'unusually high'
open circuit AC input voltage without causing the battery to paradoxically run itself flat.
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