Yes, the Nest thermostat is battery-operated.
There's an internal battery that keeps the thermostat working when the electrical system power goes off. This battery charge can last from one to two hours, depending on its state (how much it's depleted).
The Nest thermostat uses the HVAC wires to get power to operate and charge the battery. Most of the time, you just need to follow the instructions on how to install the Nest thermostat by connecting the existing cooling/heating wires.
The wire that charges the Nest thermostat
In a perfect situation, the thermostat battery is charged by a C wire; however, most installations don't have such wire available, so the power to charge the battery comes from the HVAC system.
I installed a Nest Thermostat myself without using a C wire, and the system is working fine, and it didn't have any issues (yet).
Nest thermostat wiring (Click to enlarge)
I replaced my old LuxPro thermostat, which uses 4 wires and a jumper, and I think I'm one of the lucky guys that didn't run into any problem with such a replacement.
Read the article How to Install Nest Thermostat with 6 Wires to learn more.
The Nest thermostat was designed to work without a C wire, but sometimes, people face power issues or battery drain, in which case Nest recommends using a C wire to fix the problem.
The C wire provides constant power to the thermostat, so it's not necessary to get it from the heating/cooling system.
What happens if the Nest thermostat battery dies?
If the thermostat battery dies, you can't access the thermostat functions, so it is necessary to replace the battery and charge it again.
You can see a flashing red light when the battery is charging by the HVAC system, but there's also a way to charge the thermostat yourself by connecting a phone charger to the thermostat USB port.
If you have any type of emergency, just use this alternative to charge your thermostat battery.
Conclusion
Now you know your Nest thermostat uses an internal battery.
To keep your system up and running, make sure to check if the battery is being charged correctly and use a C wire in case you run into some problem.
I hope this article can help you; please share it with your friends.
