With the EOS M3 / M10 / M100 and CHDK, long-term time lapse can also be performed very comfortably.

There is a feature in the firmware that turns the camera off and back on after the specified number of seconds. That saves a lot of energy. You can also turn off the display.
https://chdk.setepontos.com/index.php?to....msg144521

Here is a test video that runs 2 weeks:
https://youtu.be/5BBKk8aUqN4

This is what it looks like on the camera:
https://youtu.be/snEK4yParrc