Has your Nest thermostat started giving low voltage warnings and disconnecting from your WiFi? The problem is likely a worn out battery.
Since most thermostats only require low voltage, this usually isn't a problem. But with the Nest, over time, the internal battery goes bad and won't hold a charge.
Google's solution is to add an auxiliary charger, periodically recharge the battery or replace the thermostat.
Adding an auxiliary charger or running a dedicated 'C' wire to maintain serviceability of the thermostat is in convenient.
Periodically recharging the battery is annoying for an otherwise service-free device. Additionally, the recharge is only good for a few days, before requiring recharging again.
Replacing the thermostat is an unexpected expensive for a device that should last indefinitely.
What about replacing the battery? According to Google, the battery isn't replaceable.
Fed up with Google's answers and the Nest thermostat, I replaced it with a different WiFi-enabled smart thermostat.
This video give you simple instructions to replace the battery if you choose. The current cost on Amazon is between $10 and $20 depending on the mAh rating.
Note: You'll search for a Nest thermostat battery - atl 2.1wh 3.7v 284449. What will come up are compatible batteries usually ending in the number 3, but should be compatible.