@max-headroom you should get an official factory image for your Nexus 5 https://developers.google.com/android/images#hammerhead and flash it using the instructions at the beginning of that page (https://developers.google.com/android/images) or following one of the many tutorials about it that you can find using your favorite search engine.

Hi @Max-Headroom - I'm sure there'll be someone around who's more qualified than I am to help! I run an N5 too. The battery does seem to drain quite quickly, depending on how aggressively the phone is being used. As for storage: you must have put a lot of files on your phone!