Well, my brother has an android one phone, the stock charger output is 1A. Now, he needs to travel a lot in the coming months, so I am buying him a power bank from Mi. However the 10400 mah powerbank does have the output of 2.1 A, which is supposed to adjust automatically adjust output based on the phone/tablet. However, I am not sure if using the 2A charger/Power Bank might be harmful in the long run, so I thought I would ask.
Never had any problems with 2.1A unless one of the devices is defective (or maybe old/incompatible). Just charges faster...
As far as I know, you can supply as much current you want to the phone since the charger is built in to the phone. If the phone can make use of the extra power, it will. Otherwise, it'll only draw what it can use.
You need 5.0V-5.1V, and enough capacity to supply the power drawn by the device without going under-voltage. Having too much power capacity (2.1A vs 1.0A) won't hurt. Not having enough will charge more slowly, and may burn out the charger.
A battery will only draw what it can take, not what the supply/charger can deliver. So what you need to be concerned with is the voltage and the polarity of that voltage. If the connector is correct, the polarity will be correct. So really the voltage is the only thing you need to ensure is correct. What the 2.1A means to your brother is his phone's battery might (depending on charging circuits in the phone) charge faster.
