I've done this and I'm willing to share code if our needs are the same.https://www.freelancer.com/users/3242859.html
The way I've implemented it my Canon T2i sleeps on it's own before being woken up to take the video.
The battery can last for days with a low duty cycle, or weeks with an external battery.
I can show you how to make that as well.
Instead of PIR I'm also working on a new method to detect motion using a cheap webcam,
this increases the distance dramatically.
I've got it working with a 2W IR laser to detect motion at 50m in the dark.