Power banks are great devices that can come in handy whenever you are out of home.
But there are a few tips you must know before you invest on a power bank:
( 1) Power banks have an efficiency of about 90 per cent. So its better to buy one with a huge capacity of 20K mili amp. It will approximately charge a moto g5 plus for 6 times.
( 2) They also have a life cycle. just like memory devices. Typically a power bank would have 500 life cycles, that means it will charge your phone from 0-100% for 500 times.
( 3) Always go with trusted brands. Do not buy generics.
( 4) Normally you would find two ports on a power bank, one 1A port and one 2.0 A port. Plugging your phone into the 1A port would charge it normally while 2A will charge it faster.
You might call it fast charging but its actually not. Injecting higher current into a phone than what its original adapter is rated for might actually degrade its battery over time.
( 5) If your phone natively supports fast charging, then its better to charge it through the original adapter. It will charge it faster than any power bank while maintaining the current/voltage at an optimum level. Many budget phones these days have fast charging. Like moto g4, moto g turbo, etc.
My words:
I personally dont think its a good idea to buy a power bank. Its better to buy a phone with a big battery. There are many budget phones in the market today that have an amazing battery life. Like Lenovo P2, Moto X play, Moto Z play and they also natively support fast charging. So, buying phones like could be a better choice than to get a power bank. Rest is your choice.