Long term, keeping your phone at 80% and having battery backups charged is going to be your best bet, assuming having having said battery backups is reasonable for you. It won't take long for your 100% to suddenly be what 80% was when the phone was new.
If/when a situation happens where you need it, you can charge up to 100% no problem off the backups.
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.
Obviously that'll be true with battery packs too. They're also significantly cheaper, so it's usually fairly reasonable to have multiple and them being at 50% capacity doesn't matter nearly as much.
That requires this knowledge of how batteries work. Saying keep a battery pack and your phone at 100% could leave people in a situation worse than if they just used the battery manager to stop their phone at 85%. 99% of people will plug their battery pack in until it's full, stash it wherever they decide for emergencies, and will find a dead pack when they need it.
True, but if you live in a place with natural disasters, and local officials recommend keeping a go bag, you should make a habit to check that once a year. Charge the batteries, swap expired food, etc.