The activity monitor shows no heavy applications, but it says finder needs 99.7% of my CPU to run. Is this normal? Is this what is causing my battery drain?
The Finder needing that much CPU is not normal and it is very likely it was causing the unusual drain. The Finder desktop just sitting there, should not use up an entire modern CPU. Chances are it got stuck on something...a network connection, managing a window, maybe generating a preview (although usually that's QuickLook that gets hung up).
If you see unusual Finder CPU usage happen again, ask here again so it can be troubleshooted.