My SBS TTCB100002UF powerbank, which charges with 10 W of input power, has two USB ports rated at different outputs: 2.1 A and 1.0 A for output.

The 1.0 A-labelled port also delivered 2 A, just like the 2.1 A port. It could have probably delivered 3 A too, but I have no device that charges 5 V at 3 A.

Now, what is the possible difference between these ports? Is it about priority on simultaneous usage?


1 Answer 1


The only difference in these ports tend to be the resistors on the USB port's data pins. These resistors, likely the Apple Standard, are used to tell the cell phone at what rate it's allowed to charge at. That's it. There is rarely any actual current limiting circuitry in them. Both ports are physically and electrically directly tied to the same 5V source rail. Devices that do not use the data pins to detect or self-limit current, will pull what ever they can.

Caveat: This does not apply to smarter/newer power banks with active communication circuitry, as is required for QuickCharge/QC or other high speed charging standards.

  • \$\begingroup\$ Does current count? Apple standard? Doesn't the phone only take the ampèrage it needs and nothing beyond? I could connect a phone to a 5V 1000A charger, and the phone just takes it's 2A. The charger can technically reach 5000V 1A, but as long as it maintains 5V, only the minimum ampèrage counts. \$\endgroup\$
    – neverMind9
    Jan 17, 2018 at 7:27
  • \$\begingroup\$ No. Phones have variable current control for a decade now. they are not simple resistive loads. Apple standard is the usb resistor connection style and values that dictate what capacity charger is being used. \$\endgroup\$
    – Passerby
    Jan 17, 2018 at 7:55

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