Thanks for reaching out here in the community.
The battery for the Nest Learning Thermostat and Nest Thermostat E is built-in and not removable. The battey on the new Nest thermostat is located at the back with 2 triple A batteries that you can change when it's getting low.
You could check out this link and find out more about the battery on your Nest thermostat.
Thanks for your assistance, Patrick_Caezza.
We haven't heard from you in a while so we'll be locking this thread if there is no update within the day. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community.