Well this applies to anything with a lipo/ion battery. If you charge your backup battery pack to 100% then store it, it's very probably you'll end up having a drained and fully dead battery when you need it.

Wonder if there are any battery packs designed for long term storage. They could hold 100%(4.2v or whatever) but would internally discharge slowly down to 80% then stop. I bet those huge batteries YouTubers use don't even have that level of BMS. It's trivial software but planned obsolescence that eco friendly capitalist companies would never do.

Here I am with 5 year old RC 5k cycle lipos that still have at least 80% of their manufacturing capacity.