Is there any problem for charging M3 Note using a powerbank?

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2016-12-09 19:28

Two days before my M3 Note battery decreased fast and at it got switched off at night around 2 AM. My charger was at office. But, i had a powerbank with me. I plugged into 2.1A output of that powerbank to my M3 Note. M3 Note charger adapter output voltage is normally 2A.
But, when i woke up in the morning around 6 AM my battery was only 57% charged.

So, my question about is there any problem for charging M3 Note using a powerbank with output voltage 2.1A or 1A?
Otherwise can we use any other chargers having 1A/ 2A/ 2.1A output voltage?
But, why my mobile only charged 57% in 2 hours when the output voltage of powerbank was 2.1A?

Thanks in advance.