@Zoecristy
Unfortunately, Google Nest does not sell spare or replacement parts for any of its products, and its cameras and doorbells are not designed to be user-serviceable.
We have a Google Nest battery camera that is over 3 years old. I was able to find this YouTube video on replacing the battery for this camera; the comments indicate where he found a replacement battery.
https://www.youtube.com/watch?v=8C_M3xaBwjk
