Power banks will definitely have a long term damaging effect on your phone's batteries, though the damages can be negligible.
A case study is this, a friend bought Infinix Hot Note exactly by March last year while I bought mine June last year, although I am a 3 times more voracious user of my device than him, which is also a big factor to consider. Aside that,
Now, he never used Power Banks on his, while I bought mine together with a power bank and from day 1, I have always used power banks on my device whenever I am not charging through direct power supply.
And here are the outcome; My friend phone's battery is still solid presently and have not really shown any serious sign of weakness but mine on the other hand started giving me sleepless night just at the beginning of this month that made it exactly 1 year I purchased it.
My battery has become so weak that I will now have to watch the battery drain to empty in just 2hours of usage after a full charge.
In fact I have just placed order for a new replacement battery for my device before it gives me HBP. The damaging effect of power banks is mostly as result of this: you know power banks are made of cells,
and long time usage of the power banks will definitely make the cells to grow weaker and the steady state voltage supply and current coming out from the power bank to the phone while charging the phone with power banks will be weak on its part and will not be perfectly suitable for your phones battery, even if it charges your phone to full battery percentage, it is still weak voltage.
Have you ever wondered why your phone battery last longer after charging it with direct power supply but drains faster on usage immediately after you finish charging it with power banks?
As time goes on this weak voltage from the power bank also weakens your phone battery and cut short it's life span.
But this is Naija, how man go do na, if you don't augment your phone charging with power banks, your phone might likely keep you offline most of the times