iPhone randomly crashing and saying battery dead

iPhone 15 pro max(iOS 18.3.1) running as normal with 89% battery. Writing out and email and battery went down to 1%, froze for a couple of seconds then died.

I had a similar issue with the apple watch series 10(OS11.3.1) yesterday where it was pretty much fully charged, on for a couple of hours then randomly died. When I put it back on charge it took around 30mins to fire back up.

Any ideas what maybe causing this?