I had purchased this 20000 mi power bank in 2017 And I had great experience with it because I have used few powerbank of my frnds and relatives but I felt they are good but not that fast Charging power bank as I realised with this mi power bank.
So whenever you go to travel and somewhere so you must carry this powerbank in your bags if u r using a smartphone having around 4100mah bettery then This powerbank can charge your phone atleast 4 times and when u need to charge your powerbank then you must use your fast charger or original mi charger to charge your powerbank till 5-6 hour to charge full. After 2 year it will get slow but overall I had great experience with this power bank.