story of power bank and iphone?

Hello,
I would need a power bank to be portable, and therefore, have power on the vacation. A strange fact that i have discovered is that most of the sellers describe the capacity of their power bank like it can charge for Iphone more than 1,2,3 times and so on. What's the direct perfect correlation between iphone and power bank? Why and If this is a good option, I'm purchasing one on www.koeok.com because it claim that it can charge the iphone more than 20 times. It is amazing!!It is said that iPhone led to the advent of the powerbank when it was launched in 2007, in other word, it means that iphone--a life of dedicated service to the iphone?? I'm interested in the story, welcome everyone to share!! How the power bank change from Concept to Current Day Necessity?

A strange fact that i have discovered is that most of the sellers describe the capacity of their power bank like it can charge for Iphone more than 1,2,3 times and so on.

Battery capacity of phones and powerbanks are rated in milliamp hours (mAh). The makers divide that into equivalent full charges on an iphone just to make it a little easier to understand. Since the current iphone 5s has a battery capacity of 1570 mAh then a powerbank of the same rating could fully recharge your phone battery once, a powerbank with about 3000 mAh could charge it twice, and so on.

What's the direct perfect correlation between iphone and power bank?

It depends on what model iphone you have.

Why and If this is a good option, I'm purchasing one on www.koeok.com because it claim that it can charge the iphone more than 20 times.

Note that a battery with that much capacity is generally quite physically large and heavy. For travel I'd recommend something smaller in the 3000-5000 mAh range.

It is said that iPhone led to the advent of the powerbank when it was launched in 2007, in other word, it means that iphone--a life of dedicated service to the iphone??

Powerbanks were available before the iphone. But became more popular because the battery life of the original smartphones was so dismal that you needed a powerbank as you were always in danger of running out of power.

I would buy a powerbank that could provide around twice the capacity of the gadget you want to charge.

For example,your phonebattery has a capacity of 1500mah, I would get a powerbank of 3000-4000mah max. Those already take 5 hours to charge, the ones with larger capacity can take up to 10-15 hours.