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I bought a 30000mah powerbank 2 years ago. After 2 years the capacity has been reduced like crazy, you shouldn't keep the powerbank at 100% all the time as it would reduce the life time of lithium battery inside. As a prepper I ignored it as I always want to be prepared for a long term power outage. I only charged and discharged the powerbank for like 30 to 40 times btw, while it's rated 500 charge and discharge times before the capacity reduce to 80%.

I remember when I first got it, I can use it to charge my phone for like a week ! Now I can only use it to charge my phone for around 3 days. When the powerbank capacity becomes too small I would have to throw it away (maybe salvage the circuits board).

Now I have a few more powerbanks, I keep them at 40% to 50% charged if I don't have a plan to use them for a long time, that's the best way to prevent lithium battery degradation.

Edit: My 30000mah powerbank use Li polymer battery (the silver pouch lithium battery) btw, which is not as robust as normal Li ion so if you use Li ion powerbank then you might not notice remarkable battery degradation like mine.