I saw many members asking about the charging issue on Mi3 & many members here already answered it.
Here I may explain a little based on my own understanding & experiences.
1. Can I use 5V 2.0A or above charger to charge Mi3?Yes, you may but the maximum input voltage of Mi3 capped to 5V 1.2A. No matter how high the Amp the charger supported but still Mi3 only can charged at 5V 1.2A max.
To those who saw the launching video of Mi3 in China before, may aware that Mi3 is claiming supported fast charging technology & can boost the current charging speed up to 30% faster by purchasing optional higher Amp charger. So our original Mi3 charger is rated at 5V 1.A & battery capacity is 3040mAh, the formula of charging time from 0% to 100% is simple.
3.05Ah(3050mAh)/1.0A = 3 hours 5 mins if you're using 1A charger.
3.05Ah(3050mAh)/1.2A = 2 hours 35 mins if you're using 2A charger. (Maximum charging input voltage is 1.2A for Mi3)
This is what Xiaomi claiming about up to 30% fast charging technology.
However, there is one statement to be cleared here, don't assumed that charging time I posted here is accurate because the charging time is normally higher that what I posted due to below reasons:-
1) The Amp charging of phone will be fluctuated within 10% to 40% due to your phone is on or background running task that may consumes battery. That's why when you're playing games while charging the phone charging speed is slow.
2) The charging speed will be slowed down when phone battery is almost full.
3) Other factors like battery temperature, charging currents losing & etc.
Therefore, don't provide wrong statement to Mi3 users that if using 2A charger can finished charging it 2x the speed where our Mi3 didn't support 2A charging input voltage. However, if you're using Samsung S4, Note 3 & others then you may see the different because they are supported 2A.
2. How to I check the charging input voltage?There are few battery apps available, my personal favourite would be Exilir 2. You may monitoring the charging current based on below screenshots. For example, negative value means the phone is charging & positive value means the phone is discharging.
3. Does it means Mi Powerbank is useless if it supports up to 5V 2.1A output voltage while my Mi3 only supports 5V 1.2A input voltage?Powerbank is designed to suit & support most of the devices in the market & that's why Mi Powerbank is designed to support charging devices up to 5V 2.1A where you may charge your iDevices, Samsung, Xperia & etc.
Don't you want something that can rules them all?
Statement provided above may be wrong & I'm not an electrical literate ppl. I just shared what I know & hope it will be knowledgeable to you.

i think maybe the design of the max 1.2a of mi3 can accept, is because slow charging can make battery last longer? if compare to 2.0a?