I have some tips on buying power bank. When ever you buy power bank keep bellow tips in your mind.
CAPACITY.The main attribute associated to a power bank is its capacity. Capacity is measured in Milli Ampere Hour ( MAH) . If MAH rating is more power bank has more capacity.
OUTPUT VOLTAGE.Make sure that it can charge your device. I mean make sure its output voltage matches with device. If voltage is less then device voltage then your device can not charge or chage slowly.
PORTABILITY.One of the main reason to buy a power bank is the portability factor. The ability to charge your device while on the move.
WARRANTY.If warranty is not give on power bank then never buy. Because many time charging problame occure.
PRICE.Not all power bank that you knew have a good quality. Pick a good power bank that have warranty or a brand that already know as a good product.
So keep above tips when buying power bank. And output voltage is main think that you have to keep in mind. Generally people cant match it and buy power bank.