Most recommended power-supply option from dual-port Powerbank

Question

I have a Raspberry Pi 3 model B [1]. It's powered by a DC adapter that outputs 5.1V and 2.5A. Although I consider ocassional manual reboots, I'm planning to use a Powerbank as some kind of UPS, after ensuring that it's capable of taking AC power to get charged and giving DC power to my Pi simultaneously. The powerbank specifications are the following:

» Input: 5V 1A from DC adapter
» Output A: 5V 1A
» Output B: 5V 2.1A

I usually run my Pi without mouse/keyboard/display (connect via SSH), just with Ethernet cable plugged-in and, occasionally, a USB stick to transfer some files.

My question is: Which of the two powerbank output ports is more suitable and adequate for my power consumption needs? I have read about Pi power requirements but I'm not sure which of these two ports would be better.

[1] https://www.raspberrypi.org/products/raspberry-pi-3-model-b/

Related:
Uninterrupted Power Supply needed for Raspberry Pi
UPS for raspberry pi

Answer 1

The 2.1A one will be the best. But it may not work and so you should test.

Many power banks and powered hubs are "too smart" (that is, spec compliant, which is good) and will not provide the extended current , above 500mA, without the proper USB PD requirements being satisfied.

In the vanilla case, pre usb pd this was an adhoc "standard" of a resistor of a fixed value across the data pins. With USB PD spec there is a negotiation happening between the power , device, AND cable to determine maximum current draw . Without that, a power supply may deny charging currents above 500mA and the pi will not boot.

Since the RPI input circuit is USB connector but just 5V power and lacks both the power delivery controller or a resistor across those pins, many USB compliant charging banks will limit the current severely and will not work with the raspberry pi.

I believe there are adapter cables you can use to fake the data line resistor so something to consider if you are having issues