How are Power Banks and AC Adapters different?

[A80]

I want to make a power bank for my cell phone(cell phone's Li-ion battery capacity = 2070mAh). The AC adapter of the phone has an output rating of 5V dc and 550mA. Hence, it has a constant output. Now a power bank consists of a dc battery. So it's voltage and current will vary with time. So how is it that both the power bank and ac adapter are able to safely charge the cell phone battery when one provides a constant input while for the other it varies?
Also how is the battery capacity(2070mAh) related to its charging?

 

[BobK]

AC adapters do not have a constant output. The can supply up to the rated current at the rated voltage. The actual current drawn is whatever the load needs. They attempt to keep the voltage constant.

A power bank basically does the same thing. They use a 3.7V battery (typically) and a boost regulator to give out 5V as long as they can. Thus they will not vary their output voltage until the battery is low enough that they cannot supply the current, then it will shut off completely.

Bob

 

[A80]

AC adapters do not have a constant output. The can supply up to the rated current at the rated voltage. The actual current drawn is whatever the load needs. They attempt to keep the voltage constant.

A power bank basically does the same thing. They use a 3.7V battery (typically) and a boost regulator to give out 5V as long as they can. Thus they will not vary their output voltage until the battery is low enough that they cannot supply the current, then it will shut off completely.

Bob

I believe that there are various stages of charging for a Li-ion battery like constant current charge, saturation charge, floating charge etc. So are you saying that the (smart)phone controls the current and voltage it requires?

 

[BobK]

Yes. The cell phone expects a 5V supply, and it regulates the current at which the battery is charged, through several stages, as you say. And a power bank just does the same thing as an AC based charger, it provides a constant 5V.

Bob

 

[A80]

Yes. The cell phone expects a 5V supply, and it regulates the current at which the battery is charged, through several stages, as you say. And a power bank just does the same thing as an AC based charger, it provides a constant 5V.

Bob

Thank you very much!