What can I do to slow down battery degradation of portable power bank for my Android phone? I just got Inland USB-A 5,200mAh Power Bank w/ LED Indicators for my android phone. It has a one-year warrantee, which  seems to tell that its battery might degrade significantly after one year of uses.
The power bank came fully charged. I don't have  a plan of using it sometime soon, except for something unexpectedly, and will just set it aside for weeks or even months. I was wondering how to slow down battery degradation as much as possible?
For example:

In what percentage of charge shall I leave it, when I set it aside? Emptily, fully charged, or somewhere between?

Does its battery degradation depend on how many times it has been  discharged and then charged, and how often it has been ?
Power banks too have the same type of batteries as Android devices (Li-Ion or Li-Po) so the answers covering points you raised are :

In what percentage of charge shall I leave it, when I set it aside? Emptily, fully charged, or somewhere between?

40% is the best for storage
Quoting from this answer


When the phone is marketed, manufacturers, typically charge the battery to around 40% as you may have noticed. This is good for battery longevity and ensures battery degradation does not occur on storage. See BU-702: How to Store Batteries for better understanding



Does its battery degradation depend on how many times it has been discharged and then charged, and how often it has been ?

Quoting from this answer


Yes. The more you use the battery , more the number of discharge cycles ( which covers both charging and discharging as explained earlier) and drop in battery capacity... (see answer for graphs and picture)


Now, the unasked part, charging while in use, see this answer


charge when the battery drops to 50% and stop charging at 80 or 90%.


You may want to look at hardware solution for the last point.